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A dynamic supraclavicular field-matching technique for head-and-neck cancer patients treated with IMRT
Int. J. Radiation Oncology Biol. Phys., Vol. 60, No. 3, pp. 959 –972, 2004 Copyright © 2004 Elsevier Inc. Printed in the USA. All rights reserved 0360-3016/04/$–see front matter
doi:10.1016/j.ijrobp.2004.06.213
PHYSICS CONTRIBUTION
A DYNAMIC SUPRACLAVICULAR FIELD-MATCHING TECHNIQUE FOR HEAD-AND-NECK CANCER PATIENTS TREATED WITH IMRT JUN DUAN, PH.D., SUI SHEN, PH.D., SHARON A. SPENCER, M.D., RAEF S. AHMED, M.D., RICHARD A. POPPLE, PH.D., SUNG-JOON YE, PH.D., AND IVAN A. BREZOVICH, PH.D. Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, AL Purpose: The conventional single-isocenter and half-beam (SIHB) technique for matching supraclavicular fields with head-and-neck (HN) intensity-modulated radiotherapy (IMRT) fields is subject to substantial dose inhomogeneities from imperfect accelerator jaw/MLC calibration. It also limits the isocenter location and restricts the useful field size for IMRT. We propose a dynamic field-matching technique to overcome these limitations. Methods and Materials: The proposed dynamic field-matching technique makes use of wedge junctions for the abutment of supraclavicular and HN IMRT fields. The supraclavicular field was shaped with a multileaf collimator (MLC), which was orientated such that the leaves traveled along the superoinferior direction. The leaves that defined the superior field border moved continuously during treatment from 1.5 cm below to 1.5 cm above the conventional match line to generate a 3-cm-wide wedge-shaped junction. The HN IMRT fields were optimized by taking into account the dose contribution from the supraclavicular field to the junction area, which generates a complementary wedge to produce a smooth junction in the abutment region. This technique was evaluated on a polystyrene phantom and 10 HN cancer patients. Treatment plans were generated for the phantom and the 10 patients. Dose profiles across the abutment region were measured in the phantom on films. For patient plans, dose profiles that passed through the center of the neck lymph nodes were calculated using the proposed technique and the SIHB technique, and dose uniformity in the abutment region was compared. Field mismatches of ⴞ 1 mm and ⴞ 2 mm because of imperfect jaw/MLC calibration were simulated, and the resulting dose inhomogeneities were studied for the two techniques with film measurements and patient plans. Threedimensional volumetric doses were analyzed, and equivalent uniform doses (EUD) were computed. The effect of field mismatches on EUD was compared for the two match techniques. Results: For a perfect jaw/MLC calibration, dose profiles for the 10 patients in the 3-cm match zone had an average inhomogeneity range of ⴚ1.6% to ⴙ1.6% using the dynamic-matching technique and ⴚ3.7% to ⴙ3.8% according to the SIHB technique. Measurements showed that dose inhomogeneities that resulted from 1-mm and 2-mm jaw/MLC calibration errors were reduced from as large as 27% and 45% with the SIHB technique to less than 2% and 5.7% with the dynamic technique, respectively. For ⴚ1-mm, ⴚ2-mm, ⴙ1-mm, and ⴙ2-mm jaw/MLC calibration errors, respectively, treatment plans for the 10 patients yielded average dose inhomogeneities of ⴚ5.9%, ⴚ3.0%, ⴙ2.7%, and ⴙ5.8% with the dynamic technique as compared to ⴚ22.8%, ⴚ11.1%, ⴙ9.8%, and ⴙ22.1% with the SIHB technique. Calculation based on a dose–volume histogram (DVH) showed that the SIHB technique resulted in larger changes in EUD of the PTV in the junction area than did the dynamic technique. Conclusion: Compared with the conventional SIHB technique, the dynamic field-matching technique provides superior dose homogeneity in the abutment region between the supraclavicular and HN IMRT fields. The dynamic feathering mechanism substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibration. In addition, isocenter location in the dynamic field-matching technique can be chosen for reproducible patient setup and for adequate IMRT field size rather than being dictated by the match position. It also allows angling of the supraclavicular field to reduce the volume of healthy lung irradiated, which is impractical with the SIHB technique. In principle, this technique should be applicable to any treatment site that requires the abutment of static and intensity-modulated fields. © 2004 Elsevier Inc. Dynamic field matching, Head-and-neck cancer, Intensity-modulated radiotherapy, Supraclavicular field, Dose inhomogeneity.
INTRODUCTION Definitive radiation therapy for squamous cell carcinoma (SCC) of the head and neck has traditionally consisted of
the use of large field arrangements designed to deliver a homogenous dose to the primary tumor and intervening lymphatics. The control of head-and-neck (HN) cancer treated with radiotherapy depends upon the delivery of
Reprint requests to: Jun Duan, Ph.D., Department of Radiation Oncology, University of Alabama Birmingham, 619 South 19th Street, Birmingham, AL 35233. Tel: (205) 934-4763; Fax: (205)
975-2546; E-mail: [email protected] Received Dec 16, 2003 and in revised form May 7, 2004. Accepted for publication Jun 21, 2004. 959
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adequate doses to the regions of gross disease and to regions of increased risk (1–5). Standard radiotherapy portal arrangement has typically consisted of parallel-opposed lateral fields and a low-neck or supraclavicular field. This arrangement requires the abutment of supraclavicular and lateral HN fields. The conventional field-matching technique makes use of a single isocenter and half beams (SIHB) (6, 7). This technique takes advantage of the nondivergent beam on central axis. The half beams are usually shaped by use of independent jaws of a linear accelerator collimator. In principle, two abutting fields that comprise complementary halves of a radiation beam form a smooth junction. However, the presumption is that the device used to shape the half beam is perfectly calibrated. Therefore, accurate calibration of the accelerator jaws is critical. In practice, jaw calibration error is allowed within a tolerance. The American Association of Physicists in Medicine Task Group Report 40 recommended an accuracy of field size and jaw symmetry calibration to be within 2 mm (8). Hence, for an accelerator operating within the guideline, two abutting fields can overlap or underlap up to 2 mm. Because of the steep dose gradient at the field borders on the match line, this misalignment can generate clinically significant dose inhomogeneity at the field junctions (9, 10). To minimize the impact of such dose inhomogeneity, a match-line shift (also known as “feathering”) technique has been traditionally used, i.e., moving the location of the field junction by 5 mm, for example, a few times during the treatment course to reduce the dose inhomogeneity at a certain location. A physical match-line wedge can also be used to substantially reduce the overdose or underdose that results from incorrect field positioning (11). This match-line wedge technique has been extended by use of dynamic collimation to generate a dynamic match-line wedge (12). In recent years, conformal radiation techniques have evolved that may allow irradiation of targets in the head and neck defined on planning CT scans, while sparing substantial portion of the major salivary glands. These techniques include three-dimensional conformal radiotherapy (3DCRT) using beams eye views, static segmental intensity modulation, and dynamic intensity modulation (13–17). These techniques have demonstrated that adequate irradiation of the targets while sparing major salivary glands is feasible in patients with HN cancer. Early clinical experience has demonstrated substantial sparing of saliva flows and suggests an improvement of tumor control and xerostomia (1, 16 –19) compared with use of conventional radiation techniques. Although the gross primary and subclinical disease in the low-neck nodes can be included in the intensitymodulated radiotherapy (IMRT) field, many physicians prefer to treat potential subclinical disease in the entire supraclavicular area. In such cases, the conventional static supraclavicular field is frequently used for treating the lowneck and supraclavicular lymph nodes. Although use of the supraclavicular field is more expeditious, field matching is necessary to abut the supraclavicular field with HN IMRT fields.
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Field matching poses a new challenge in IMRT. Several authors have investigated the techniques for matching IMRT fields. The dynamic “feathering” technique has been used for merging the subfields of a large IMRT field to minimize field-matching errors (20 –22). In this technique, adjacent IMRT fields were gradually blended together in a overlapping area of 2 to 3 cm. A similar technique has been used in matching electron fields with photon fields for improved dose distributions in the abutment regions (23– 25). Matching a static supraclavicular field with HN IMRT fields presents a special challenge in that although the static field has a clearly defined field border, IMRT fields often do not have a sharp, well-defined, nondivergent penumbra at their inferior border. Techniques for matching a static supraclavicular field with HN IMRT fields have not been well documented. In our institution, the SIHB technique has been initially adopted for matching supraclavicular and intensitymodulated HN fields. To generate a sharp penumbra at the inferior border of the HN fields, the intensity-modulated fields were optimized with the planning target volume (PTV) of upper-neck lymph nodes extended approximately 2 cm below the match line. After optimization, the inferior jaw was closed to the central axis for dose calculation, such that the dose gradient at the inferior border is artificially sharpened, thus forming a relatively smooth abutment. However, two major limitations are associated with this technique. First, as inherent to the SIHB technique, substantial dose inhomogeneities can arise from imperfectly calibrated jaws/MLC and cause severe underdose or overdose. Although the inhomogeneity may be spread over a small range as a result of daily setup errors, this area will be constantly “cold” or “hot” throughout the treatment because the inhomogeneity is the result of a systematic error in collimator calibration. In addition, because of the random nature of the setup error, uncertainties exist on the extent of reduction in inhomogeneity by the spread. Second, only half of the full field is available for IMRT fields, which in some cases is not sufficiently large to cover the entire target. In this study, we demonstrate an MLC-based dynamic technique for matching the supraclavicular field with HN IMRT fields to overcome these limitations. METHODS AND MATERIALS Dynamic field-matching method In the SIHB technique, the dose inhomogeneity across the match line generated by potential gap or overlap between asymetric jaws and/or MLC is proportional to the dose gradient at the field edges that constitute the match line. If the dose gradients are reduced to allow the supraclavicular field and the HN IMRT fields to merge gradually, the magnitude of such dose inhomogeneity can be significantly reduced. Figure 1 shows schematically the principle of the field matching by use of the dynamic-matching technique as opposed to the SIHB technique. In the dynamicmatching technique, the dose at the superior border of the supraclavicular field is reduced gradually by the continuous
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Fig. 1. The dynamic field-matching technique (a) compared with the single isocenter and half beam (SIHB) technique (b), and dose inhomogeneities in the abutment region generated by field mismatches caused by a gap or overlap between the abutting fields.
opening up or closing down of the superior border during radiation delivery, which produces a tapered junction. The IMRT fields that overlap with the supraclavicular field over the junction area are optimized by taking into consideration the dose contribution from the wedge-shaped supraclavicular junction, thus producing a complementary wedge at their inferior border. In place of the conventional match line, a match zone is formed. The two sets of fields, which merge gradually, combine to deliver a homogeneous dose in the match zone. As demonstrated in Fig. 1, dose inhomogeneity caused by imperfect jaw calibration can be greatly reduced because it is spread throughout the match zone. The
magnitude of dose inhomogeneity is determined by the magnitude of jaw/MLC calibration error, ␦, and the width of the match zone, w. For a given calibration error, the magnitude of the dose inhomogeneity can be controlled by adjusting the width of the match zone. The magnitude of the dose inhomogeneity is approximately ␦/w, which helps in determining the width of the match zone for a given jaw/ MLC calibration error and acceptable level of dose inhomogeneity. The width of the match zone used in this study is 3 cm, which corresponds to dose inhomogeneity approximately 3% and 6% for 1-mm and 2-mm overlap or underlap of the fields, respectively.
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superior border opened up by 3 mm from the previous one and delivered 10% of the total dose. A portion of the superior border that is intended to block the radiation beam from exposing the larynx and/or the spinal cord remained unchanged. The dynamic MLC leaf sequence file was converted to a fluence map (Fig. 2b) by the TPS before dose distribution of the supraclavicular field was calculated. Importing a leaf sequence file instead of a fluence map into the TPS guarantees the desired leaf motion and eliminates the possibility of multiple subfields if the length of the supraclavicular field exceeds the leaf span. For the IMRT fields, the PTV of the upper-neck lymph nodes was extended to 1.5 cm below the conventional match line. The IMRT plan was optimized to cover the extended PTV by inclusion of the dose contribution from the supraclavicular field, thereby forming another wedge junction to complement that of the supraclavicular field. No restriction applies to the collimator angles of the IMRT fields. Setting collimator angle at 90° can smooth the interleaf leakage and the tongue-and-groove effects through various gantry positions (22) but may increase the number of subfields for elongated targets. To avoid field overlap beyond the match zone, the inferior borders of the IMRT fields, which were adjusted automatically by TPS to fit the dynamic MLC fields, were manually closed down to 1.5 cm inferior to the match line by use of the collimator jaw before doses were calculated. Dynamic multileaf collimating IMRT (DMLC-IMRT) delivery mode was used in this study. In this study, the following convention was used in regard to calibration errors of independent jaws and/or MLC: A positive error refers to an overlap, whereas a negative error refers to a gap between two abutting fields. Fig. 2. (a) Supraclavicular field shaped by a multileaf collimator. During treatment, the superior border (except the portion that shields the larynx) moves continuously from 1.5 cm below to 1.5 cm above the conventional match line. (b) Fluence map of the dynamic supraclavicular field.
Treatment planning for dynamic match A commercial treatment planning system (TPS) (Eclipse; Varian Medical Systems, Palo Alto, CA) was used to generate treatment plans. Based on the conventional static supraclavicular field (shaped by either cerobend block or MLC), a static supraclavicular field was shaped with the MLC oriented such that the leaves moved in the superoinferior direction (Fig. 2a). To generate the match-line wedge junction of the supraclavicular field, the static MLC leaf sequence file of the supracavicular field was converted to a dynamic MLC leaf squence such that the leaves that shape the superior border of the field move continuously from 1.5 cm below to 1.5 cm above the conventional match line during treatment. Because our TPS does not allow a dynamic leaf sequence manually created directly within the TPS, software developed in-house or commercially available (Shaper by Varian) was used to convert the static field to a dynamic field of 10 segments. In each segment, the
Phantom measurement study To validate the dynamic field-matching technique, a polystyrene phantom was used for measurement. The phantom consists of a lightproof polystyrene box and 6-mmthick polystyrene plates that can be placed inside the box along the axial, sagittal, or coronal plane. The phantom was imaged with a CT simulator (AcQSim; Philips Medical Systems, Andover, MA). Five structures were outlined on the CT images, which simulated 2 targets (a primary PTV and a secondary nodal PTV) and 3 organs-at-risk (OAR) (left and right parotids and spinal cord). Figure 3 shows a 3D view of the phantom rendered from CT images. Treatment plans were prepared for the phantom using the dynamic and the SIHB field-matching techniques. The plans were delivered to the phantom, and Kodak EDR2 film was used to measure the 2D dose distributions on a coronal plane across the midplane of the neck PTV, as shown in Fig. 3. Before delivery, the systematic error of collimator calibration was determined by use of a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half-beam fields. A film was first irradiated with the two half-beam fields without couch movement to determine the direction of the couch movement necessary to correct for the calibration error. The source-to-film distance was set equal to the
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Fig. 3. Three-dimensional view of the polystyrene phantom rendered from CT images. Shown are simulated primary planning target volume (PTV) (yellow), secondary nodal PTV (red), parotids (cyan), and spinal cord (purple). The position of the film is indicated by the blue plane.
source-axis-distance of the linear accelerator, 100 cm. The couch was then moved systematically between exposures of the 2 half beams until a smooth junction was obtained. The distance by which the treatment couch had to be moved was considered as the collimator calibration error. The movement of the treatment couch was controlled by the Crane (NOMOS, Sewickley, PA), which allowed couch movements with 0.01-mm precision. The practical accuracy of the Crane was determined to be within 0.03 mm by comparing the reading of the Crane caliper and that of a caliper directly attached to the couch. When the plans were delivered to the phantom, this systematic collimator calibration error was corrected for by countermoving the couch between the deliveries of supraclavicular field and HN IMRT fields. Each of the phantom plans was delivered under the conditions of 0-mm, ⫾1-mm, and ⫾ 2-mm collimator calibration error. The films were digitized and optical density was converted to radiation dose on a Radiation Therapy Film Dosimetry System (Radiologic Imaging Technology, Colorado Springs, CO). Line-dose profiles across the center of the right neck PTV were obtained by software developed in-house. *RTOG protocols are available online at www.rtog.org.
Patient plan study For examination of the feasibility of the dynamic technique, treatment plans were prepared for 10 HN cancer patients. For the purpose of this study, Radiation Therapy Oncology Group (RTOG) Protocol H0022* was followed for dose prescription and OAR tolerances. Simultaneous integrated boost technique is used in this protocol, which requires the primary PTV to receive 66 Gy while the lymph node PTV receives 54 Gy in 30 fractions. Lymph node PTV was defined according to the protocol for the superior portion from 1.5 cm below the conventional match line and above. Prescription of the supraclavicular field adopted the conventional method (i.e., to 3-cm depth). The PTV for the inferior portion is not delineated but is defaulted to the volume that receives 100% of the prescription dose from the supraclavicular field. Seven or 9 equally spaced gantry positions that included one for the anteroposterior beam were used for the HN IMRT fields. For comparison, similar plans were generated by use of the conventional SIHB technique with the same beam geometry and dose constraints. To allow study of the impact of imperfectly calibrated jaws/MLC, calibration errors were simulated by
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moving the isocenter of the supraclavicular field in the superoinferior direction. For each plan pair, dose distributions were calculated for 0-mm, ⫾1-mm, and ⫾2-mm collimator calibration errors. Dose profiles through the center of the PTV in the right neck along the superoinferior direction were calculated by the TPS and compared between the 2 techniques. To facilitate 3D volumetric dose analysis of the junction area, lymph node PTV in the 3-cm abutment region was created and named junction PTV (JPTV). A dose–volume histogram (DVH) of JPTV was calculated for each plan. Equivalent uniform doses (EUD) were computed based on the assumption of uniform cell distribution across JPTV according to the following formula (26):
关 兺 (SF ) 兴 ⁄ ln(SF )
N EUD(GY) ⫽ Dref · ln 1 ⁄ N
2
Di⁄Dref
2
i⫽1
where Di is the dose in the ith partial volume, Dref is 1.8 Gy in the current study, SF2 is the survival fraction of 2 Gy, and a previously reported value, SF2 ⫽ 0.33, was used in the computation (27). RESULTS Phantom measurement study Before any measurement was conducted, collimator calibration errors were determined. Figure 4a shows a film image exposed by a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half beams. The dark stripe at the junction indicates overlap between the 2 half beams caused by an imperfect calibration of the 2 independent jaws. Figure 4b shows the dose profile across the center of the field (dashed line in Fig. 4a). Also shown in Fig. 4b are dose profiles of 2 of the film images used to determine collimator calibration errors; that is, images with ⫺0.65-mm and ⫺1-mm table shifts between the exposures of the 2 half beams. The negative sign indicates that the table movement shifted the 2 half beams away from each other. To display the curves clearly, the dose axis was shifted by ⫹10% and ⫺10% for 0-mm and ⫺1-mm table movements, respectively. The ⫺0.65-mm table movement produced a smooth abutment, which represented the collimator calibration error. This 0.65-mm jaw calibration error was corrected in the subsequent delivery of phantom plans by countermoving the couch between the supraclavicular field and the HN IMRT fields. Figure 5 shows measured dose profiles with and without jaw/MLC calibration errors for the dynamic and the SIHB technique. Without jaw/MLC calibration errors, the dynamic technique produced a smooth field abutment (Fig. 5a). In comparison, the SIHB technique, shown in Fig 5b, produced a small but acceptable dose inhomogeneity at the field junction. When simulated jaw/MLC calibration errors were introduced, as shown in Fig. 5c for the dynamic technique and in Fig. 5d for the SIHB technique, measured dose profiles exhibited drastically different magnitudes of
Fig. 4. (a) Film image exposed by a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half beams. The dark stripe at the junction indicates an overlap between the 2 half beams caused by calibration errors of the independent jaws. (b) Dose profiles across the center of the 10-cm ⫻ 10-cm field, which is shown as the dashed line in (a), for 0-mm, ⫺0.65-mm, and ⫺1.0-mm table movements between the exposure of the 2 complementary 5-cm ⫻ 10-cm half beams that were used to determine jaw calibration error. The negative sign indicates that the table movement separated the 2 half beams from each other. To enhance clarity, the curves corresponding to 0-mm and ⫺1-mm table movements are shifted along the dose axis by ⫹10% and ⫺10%, respectively.
dose inhomogeneities in the abutment region between the two techniques. In the dynamic technique, dose inhomogeneities in the match zone were ⫺5.7% for ⫺2-mm, ⫺2.0% for ⫺1-mm, ⫹1.9% for ⫹1-mm, and ⫹5.2% for ⫹2-mm jaw/MLC calibration error (Fig. 5c), respectively. In contrast, corresponding dose inhomogeneities were substantially higher in the SIHB technique, i.e., ⫺45%, ⫺23%, ⫹27%, and ⫹38%, respectively (Fig. 5d). Note that a small
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Fig. 5. Dose profile measured by film compared with that calculated by treatment planning system (TPS) without collimator calibration error for the dynamic matching technique. Smooth field abutment was produced by the dynamic-matching technique. For comparison, those of the single isocenetr and half beam (SIHB) technique are shown in (b). Measured dose profiles with ⫾ 1-mm and ⫾ 2-mm collimator calibration errors show dose inhomogeneities in the match zone for the dynamic (c) and the SIHB (d) techniques.
portion of each measured profile is missing because of the film size (12.5 cm ⫻ 13 cm), which was limited by the phantom. The ripples on the measured profiles of the dynamic match technique (Figs. 5a and 5c) were caused by MLC interleaf transmission. Patient plan study For the dynamic match technique, dose distributions on the coronal plane that passes through the center of the nodal PTV of an HN cancer patient are shown in Fig. 6. Individual doses of the supraclavicular field (Fig. 6a) and HN IMRT fields (Fig. 6b) demonstrate complementary wedges in the match zone, which yields a smooth composite dose distribution in the abutment region (Fig. 6c). Line-dose profiles in the nodal PTV across the field junction were calculated by the TPS for both techniques. An example is shown in Fig. 7a for the dynamic and in Fig. 7b
for the SIHB technique. Dose profiles of the supraclavicular field and the IMRT fields are also shown. In the dynamicmatch technique, they demonstrate two complementary wedges in the 3-cm match zone. Compared with the SIHB technique, the dose homogeneity in the junction zone for the dynamic-match technique was noticeably improved. Table 1 compares the line-dose uniformity within the 3-cm match zone of the two field-matching techniques for the 10 patients in the study. For these patients, dose inhomogeneities ranged from ⫺0.4% to ⫹2.5% in the dynamic-match technique and from ⫺5.2% to ⫹6.8% in the SIHB-match technique for perfectly calibrated jaws/MLC. The average range of dose inhomogeneities was ⫺1.6% to ⫹1.6% for the dynamic technique and ⫺3.7% to ⫹3.8%, for the SIHB technique. In the case of imperfectly calibrated collimator and/or MLC leaves, dose inhomogeneities caused by ⫾ 1-mm
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Fig. 6. Dose distributions in a coronal plane passing through the center of the nodal planning target volume of a head-and-neck cancer patient. The contributions by the supraclavicular field (a) and the HN intensity-modulated radiation therapy fields (b) show complementary doses in the match zone, which yields a smooth composite dose in the abutment region (c).
and ⫾ 2-mm field mismatch are shown in Fig. 7c for the dynamic and in Fig. 7d for the SIHB techniques. Dose inhomogeneity of the dynamic technique, which was spread over the 3-cm match zone, was drastically reduced compared with that of the SIHB technique. Table 2 compares dose inhomogeneities caused by imperfectly
calibrated collimator jaws for the two match techniques. For the 10 patients, the mean dose inhomogeneity ⫾ standard deviation for ⫺2-mm, ⫺1-mm, ⫹1-mm, and ⫹2-mm jaw/MLC calibration errors, respectively, were ⫺5.9% ⫾ 0.4%, ⫺3.0% ⫾ 0.1%, 2.7% ⫾ 0.1%, and 5.8% ⫾ 0.1% for the dynamic technique and ⫺22.8% ⫾
Fig. 7. Typical line-dose profiles across the center nodal planning target volume of a patient calculated by the treatment planning system, under the assumption of perfect collimator calibration, for (a) the dynamic matching and (b) the single isoscenter and SIHB matching techniques. In the dynamic matching technique, dose profiles of the component supraclavicular field and intensity-modulated radiation therapy (IMRT) fields present 2 complementary wedges in the 3-cm match zone. For ⫾ 1-mm and ⫾ 2-mm collimator calibration, dose inhomogeneity of the dynamic matching technique (c), which was absorbed over the 3-cm match zone, was dramatically reduced in comparison with that of the SIHB technique (d).
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Table 1. Improved dose homogeneity in the abutment region for the 10 HN patients planned with the dynamic field-matching technique compared with the SIHB technique Patient number
Dynamic match
SIHB match
1 2 3 4 5 6 7 8 9 10 Average
⫺1.9%–2.1% ⫺1.3%–1.5% ⫺0.4%–0.6% ⫺2.4%–2.0% ⫺1.2%–0.9% ⫺2.1%–2.5% ⫺1.9%–2.4% ⫺1.5%–1.4% ⫺2.2%–1.6% ⫺1.5%–0.9% ⫺1.6%–1.6%
⫺2.5%–6.8% ⫺4.5%–2.5% ⫺3.2%–3.1% ⫺4.1%–1.7% ⫺2.8%–3.0% ⫺3.6%–6.8% ⫺2.7%–1.6% ⫺4.8%–3.8% ⫺5.2%–5.6% ⫺3.2%–3.2% ⫺3.7%–3.8%
0.6%, ⫺11.1% ⫾ 0.7%, 9.8% ⫾ 0.3%, and 22.1% ⫾ 0.5% for the SIHB technique. Three-dimensional volumetric doses were analyzed for the 10 patients. Three-dimensional dose distributions of the two match methods were compared for a ⫺2-mm match error in 3 orthogonal planes and a 3D view. As Fig. 8 shows, the SIHB technique creates substantially cold areas in the abutment region. Whereas the dynamic technique demonstrates a smooth junction, the 3D view of the SIHB technique shows a large gap in the 54-Gy isodose surface. The DVHs of JPTV for Patient 1 were compared by both matching techniques with ⫺2-mm to ⫹2-mm match errors (Fig. 9). EUDs were calculated on the basis of these DVHs. In Patient 1, changes in the EUD of JPTV caused by ⫺2-mm, ⫺1-mm, ⫹1-mm, and ⫹2-mm match errors are ⫺5.8%, ⫺3.1%, 2.3%, and 5.1% for the dynamic technique and ⫺7.0%, ⫺4.0%, 2.8%, and 5.4% for the SIHB technique, respectively. The changes in EUD are summarized in Table
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3 for the 10 patients. The effect of dose inhomogeneity that resulted from field mismatch on the EUD is less in all cases with the dynamic technique than with the SIHB technique.
DISCUSSION The present study has demonstrated that the dynamic field-matching technique has significant dosimetric advantages over the SIHB technique. It produced more uniform dose within the abutment region. More importantly, it suppressed dose inhomogeneities resulting from jaw/MLC calibration inaccuracy by spreading them over the 3-cm-wide match zone via a dynamic feathering mechanism. The magnitude of dose inhomogeneities is substantially reduced as compared with the SIHB technique. A survey of linear accelerators in our institution showed that collimator calibration inaccuracies ranged from ⫾ 0.5 mm to ⫾ 1.5 mm. With use of the dynamic field-matching technique, dose inhomogeneities that result from such inaccuracies can be readily limited to less than 5% with a 3-cm match zone. More that 30 patients have been treated successfully in our clinic using this technique. As mentioned earlier, the SIHB technique requires a common isocenter for the supraclavicular and the HN IMRT fields. The half beams limit the isocenter location to the match line, which is usually not ideal for reproducible daily patient setup. In the case of nasopharyngeal cancer, in which the disease can extend to the base of the skull, the maximum field size of half beams (20 cm) may not be sufficient to cover the entire target. These limitations are overcome with the dynamic technique, which requires neither a common isocenter nor half beams for the abutting fields. However, we recommend the use of a common isocenter to eliminate setup errors associated with isocenter
Table 2. Dose inhomogeneities resulting from field mismatches caused by jaw/MLC calibration errors in 10 patient plans Jaw/MLC calibration error ⫺2 mm
⫺1 mm
⫹1 mm
⫹2 mm
Patient number
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
1 2 3 4 5 6 7 8 9 10 Mean SD
⫺6.1% ⫺4.8% ⫺5.7% ⫺5.8% ⫺6.1% ⫺6.0% ⫺6.1% ⫺6.4% ⫺6.0% ⫺6.0% ⫺5.9% 0.4%
⫺21.6% ⫺20.2% ⫺21.1% ⫺24.5% ⫺25.6% ⫺20.5% ⫺24.7% ⫺23.4% ⫺21.5% ⫺24.8% ⫺22.8% 0.6%
⫺3.0% ⫺2.3% ⫺2.7% ⫺2.9% ⫺3.1% ⫺3.0% ⫺3.1% ⫺3.2% ⫺3.2% ⫺3.0% ⫺3.0% 0.1%
⫺10.8% ⫺9.1% ⫺13.2% ⫺12.3% ⫺12.9% ⫺10.2% ⫺12.3% ⫺11.7% ⫺5.8% ⫺12.4% ⫺11.1% 0.7%
2.4% 2.9% 2.8% 2.3% 2.5% 2.4% 2.5% 2.6% 3.3% 3.0% 2.7% 0.1%
8.6% 11.6% 10.2% 9.8% 10.2% 8.2% 9.9% 9.4% 11.2% 9.2% 9.8% 0.3%
5.8% 5.4% 5.7% 5.5% 5.8% 5.7% 5.8% 6.1% 6.3% 5.9% 5.8% 0.1%
20.5% 23.1% 20.8% 23.3% 24.2% 19.4% 23.4% 22.2% 23.2% 21.2% 22.1% 0.5%
Abbreviations: MLC ⫽ multileaf collimator; SD ⫽ standard deviation; SIHB ⫽ single isocenter and half beam. Note: Dose inhomogeneities are substantially reduced by use of dynamic compared with SIHB technique. The mean values and SD of the 10 patient plans are also listed.
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Fig. 8. Three-dimensional dose distributions in three orthogonal planes and a 3D view for ⫺2-mm field match error in the dynamic (a) and the single isocenter and half beam (SIHB) (b) field-matching technique. The position of the cuts are indicated by the dashed lines. The axial plane passes through the match line. The SIHB technique generated substantially greater dose inhomogeneity in the abutment region than did the dynamic technique.
A Dynamic Supraclavicular and HN IMRT Fields Matching Technique
Fig. 9. Dose volume histograms for ⫺2-mm to ⫹2-mm field mismatch errors with the dynamic (a) and the single isocenter and half beam (b) field-matching technique.
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shift. Because the isocenter position is not limited to the match line, it still can be set at a more favorable location, provided that it is within 20 cm from the inferior border of the supraclavicular field. The location of the match line remains unchanged relative to the patient, which will be inferior to the isocenter. This arrangement allows isocenter placement at more stable location for reproducible daily patient setup and for larger HN IMRT fields. For MLCs that have thinner leaves in the central portion of the field, such as Varian Millennium 120 MLC (0.5-cm-thick leaves in the central 20 cm and 1-cm-thick leaves in the peripheral 10 cm of the field), the flexibility of isocenter location allows the users to take advantage of the higher resolution central leaves for the modulated HN fields. For example, for nasopharyngeal cancer, small, complex optical structures are usually beyond the central 20 cm of the field in the SIHB technique. The dynamic technique allows the isocenter to be set to a location such that the 0.5-mm leaves are used to provide an improved dose distribution for that area (28). Without a cord block, the spinal cord in the supraclavicular field often could receive a dose exceeding its tolerance. Shaping the supraclavicular field with an MLC allows central leaves to be closed to serve as a cord block. However, in cases of low-lying disease in which mediastinal nodes are treated, a complete cord block is not warranted. In these cases, a partial transmission block can be readily incorporated into the dynamic supraclavicular field. By closing the central leaves to block the spinal cord after, for example, 80% of the dose has been delivered, the dose to the spinal cord can be kept within tolerance while the mediastinum is treated. The SIHB technique requires the supraclavicular field be a straight anteroposterior beam to match the superior HN fields (Fig. 10a). This requirement does not apply to the dynamic-match technique, because the modulated HN fields
Table 3. Percentage changes in EUD of junction PTV resulting from field mismatches caused by jaw/MLC calibration errors Jaw/MLC calibration error ⫺2 mm
⫺1 mm
⫹1 mm
⫹2 mm
Patient number
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
1 2 3 4 5 6 7 8 9 10 Mean SD
⫺5.8% ⫺4.9% ⫺5.4% ⫺5.3% ⫺5.6% ⫺5.1% ⫺4.6% ⫺5.1% ⫺5.4% ⫺3.4% ⫺5.1% 0.7%
⫺7.0% ⫺6.1% ⫺6.3% ⫺6.2% ⫺7.6% ⫺7.3% ⫺6.6% ⫺7.1% ⫺8.9% ⫺7.2% ⫺7.0% 0.8%
⫺3.1% ⫺2.2% ⫺2.7% ⫺2.8% ⫺2.7% ⫺2.7% ⫺2.4% ⫺2.7% ⫺2.7% ⫺1.6% ⫺2.6% 0.4%
⫺4.0% ⫺2.7% ⫺3.2% ⫺3.0% ⫺3.1% ⫺4.3% ⫺3.3% ⫺3.8% ⫺3.5% ⫺4.8% ⫺3.5% 0.8%
2.3% 2.7% 2.6% 2.3% 2.7% 2.4% 2.1% 2.1% 2.7% 1.8% 2.4% 0.3%
2.8% 3.4% 3.4% 4.1% 4.8% 3.2% 3.2% 2.9% 3.3% 3.5% 3.5% 0.6%
5.1% 5.4% 5.1% 4.8% 5.3% 4.9% 4.3% 4.3% 5.4% 3.5% 4.8% 0.6%
5.4% 6.6% 6.1% 7.6% 9.1% 5.7% 6.2% 5.8% 7.6% 7.0% 6.7% 1.1%
Abbreviations: EUD ⫽ equivalent uniform dose; MLC ⫽ multileaf collimator; PTV ⫽ planning target volume; SD ⫽ standard deviation; SIHB ⫽ single isocenter and half beam.
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Fig. 10. A supraclavicular field slightly tilted toward the feet in the dynamic technique (a) significantly reduces the volume of healthy lung being irradiated as compared with the straight anterior-posterior field in the single isocenter and half beam technique (b). The dose–volume histograms of lung (c) show that the volume of lung receiving 20 Gy or higher is reduced by 47% in this case.
are optimized to complement the dose contributed by the supraclavicular field. This flexibility in beam direction provides an additional advantage for the dynamic-match technique. As shown in Fig. 10b, if the treatment couch is rotated 90°, the gantry can be slightly tilted toward the feet while sufficient coverage of the supraclavicular area is maintained. In this way, a significant volume of healthy lung is spared from high radiation dose. As shown in the dose–volume histogram (Fig. 10c), the volume that received 20 Gy or more was reduced by 47% in this case. However, the tradeoff of this feature is the need for manual setup of the couch rotation, which increases treatment time and introduces additional uncertainties caused by the couch rotation. We observed that measured dose profiles show greater dose inhomogeneities in the SIHB technique than that calculated by TPS. Figure 11 shows the measured dose profiles
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Fig. 11. Comparison of dose profiles between film measurements and treatment planning system (TPS) calculation for ⫾2-mm jaw/ multileaf collimator calibration errors. The agreement is reasonably good for the dynamic-match technique (a). However, the TPS underestimates the dose inhomogeneities caused by field mismatch for the single isocenter and half beam technique (b).
compared with the calculated dose profiles for ⫾ 2-mm jaw/MLC calibration errors. The profiles for the dynamic technique agree reasonably well between measured and calculated ones. However, those for the SIHB technique show that the TPS underestimated the dose inhomogeneities that resulted from jaw/MLC calibration errors. The underestimation has two causes. First, the penumbra modeling for the TPS used in this study is based on measured beam profiles of full fields. Therefore, approximations are made in the calculation of half-beam penumbra. Second, the highest resolution of the dose grid provided by the TPS is 1.25 mm, which leads to broadened width but reduced height of the inhomogeneity peaks. Nevertheless, because even the underestimated dose inhomogeneity was clinically significant,
A Dynamic Supraclavicular and HN IMRT Fields Matching Technique
it would not affect the general conclusion deduced from these results. The EUDs were calculated from the DVHs, which were calculated by the TPS for the PTV within the 3-cm area in the abutment region. The inhomgeneities that result from field mismatches in the SIHB technique are concentrated within about 1 cm around the match line. The influence of field mismatch on the EDUs for the PTV within this 1-cm range was substantially greater with the SIHB technique than with the proposed technique. For example, with a ⫺2-mm mismatch, the EUDs were reduced by 16.1% for Patient 1 with use of the SIHB technique as compared to 5.9% with the dynamic technique. In addition, considering the underestimation by the TPS in the magnitude of inhomogeneity in the SIHB technique, the actual changes of EUDs would be even greater with use of the SIHB technique than with use of the dynamic technique. Compared with the planning of a conventional static supraclavicular field, the proposed technique requires extra 2 to 4 minutes to generate the dynamic supraclavicular field when our in-house software is used, or 5 to 10 minutes if the field is generated manually by application of MLC-shaping software such as Shaper. However, our method eliminates the need for match-line shifts associated with the SIHB technique, which saves time in both treatment planning and delivery. The number of treatment fractions for the supraclavicular fields and HN IMRT fields must be identical when the dynamic-matching technique is used, because IMRT plans are usually optimized on the basis of the total doses of the treatment course. Otherwise, the fractional dose
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in the matching zone will deviate from the fractionation scheme significantly, even though the total dose remains the same. However, in most HN cases, the secondary nodal PTV usually receives lower dose than the primary PTV. This difficulty can be managed in either of two ways: (1) by splitting the HN IMRT treatment into 2 parts with the first part treated simultaneously with the supraclavicular field by use of the dynamic- matching technique and the second part to finish the remaining dose to the primary PTV or (2) by modifying the doses per fraction for the primary and nodal PTV to equalize the number of fractions. CONCLUSION The proposed dynamic field-matching technique for supraclavicular and HN IMRT fields is a simple, practical technique that can be readily implemented in most clinical settings. Compared with the conventional SIHB technique, the proposed technique provides more homogeneous dose in the abutment region and, more importantly, substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibrations. It provides more flexibility in the choice of isocenter locations, which allows an isocenter at a more favorable location for reproducible patient setup, as well as larger available HN IMRT fields. In addition, it permits a flexible gantry angle of the supraclavicular field, which can reduce the volume of healthy lung being irradiated. In principle, this technique can be applied to any treatment site that requires the abutment of IMRT and conventional static fields.
REFERENCES 1. Barkley HT Jr, Fletcher GH, Jesse RH, et al. Management of cervical lymph node metastases in squamous cell carcinoma of the tonsillar fossa, base of tongue, supraglottic larynx, and hypopharynx. Am J Surg 1972;124:462– 467. 2. Lindberg R. Distribution of cervical lymph node metastases from squamous cell carcinoma of the upper respiratory and digestive tracts. Cancer 1972;29:1446 –1449. 3. Lindberg R, Jesse RH. Treatment of cervical lymph node metastasis from primary lesions of the oropharynx, supraglottic larynx and hypopharynx. Am J Roentgenol Radium Ther Nucl Med 1968;102:132–137. 4. Mendenhall WM, Million RR, Cassisi NJ. Elective neck irradiation in squamous-cell carcinoma of the head and neck. Head Neck Surg 1980;3:15–20. 5. Mendenhall WM, Million RR. Elective neck irradiation for squamous cell carcinoma of the head and neck: Analysis of time-dose factors and causes of failure. Int J Radiat Oncol Biol Phys 1986;12:741–746. 6. Chiang TC, Culbert H, Wyman B, et al. The half field technique of radiation therapy for the cancers of head and neck. Int J Radiat Oncol Biol Phys 1979;5:1899 –1901. 7. Sohn JW, Schell MC, Dass KK, et al. Uniform irradiation of the craniospinal axis with a penumbra modifier and an asymmetric collimator. Int J Radiat Oncol Biol Phys 1994;29:187–190. 8. Kutcher GJ, Coia L, Gillin M, et al. Comprehensive QA for radiation oncology: report of AAPM Radiation Therapy Committee Task Group 40. Med Phys 1994;21:581– 618. 9. Bukovitz AG, Deutsch M, Slayton R. Orthogonal fields: vari-
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17. Sultanem K, Shu HK, Xia P, et al. Three-dimensional intensity-modulated radiotherapy in the treatment of nasopharyngeal carcinoma: The University of California-San Francisco experience. Int J Radiat Oncol Biol Phys 2000;48:711–722. 18. Eisbruch A, Kim HM, Terrell JE, et al. Xerostomia and its predictors following parotid-sparing irradiation of head-andneck cancer. Int J Radiat Oncol Biol Phys 2001;50:695–704. 19. Henson BS, Inglehart MR, Eisbruch A, et al. Preserved salivary output and xerostomia-related quality of life in head and neck cancer patients receiving parotid-sparing radiotherapy. Oral Oncol 2001;37:84 –93. 20. Wu Q, Arnfield M, Tong S, et al. Dynamic splitting of large intensity-modulated fields. Phys Med Biol 2000;45:1731–1740. 21. Hong L, Alektiar K, Chui C, et al. IMRT of large fields: Whole-abdomen irradiation. Int J Radiat Oncol Biol Phys 2002;54:278 –289. 22. Wu Q, Mohan R, Morris M, et al. Simultaneous integrated boost intensity-modulated radiotherapy for locally advanced head-and-neck squamous cell carcinomas. I: Dosimetric results. Int J Radiat Oncol Biol Phys 2003;56:573–585. 23. Li JG, Xing L, Boyer AL, et al. Matching photon and electron
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fields with dynamic intensity modulation. Med Phys 1999;26:2379 –2384. Li JG, Williams SS, Goffinet DR, et al. Breast-conserving radiation therapy using combined electron and intensity-modulated radiotherapy technique. Radiother Oncol 2000;56:65– 71. Dogan N, Leybovich LB, Sethi A, et al. Improvement of dose distributions in abutment regions of intensity modulated radiation therapy and electron fields. Med Phys 2002;29:38 – 44. Niemierko A. Reporting and analyzing dose distributions: A concept of equivalent uniform dose. Med Phys 1997;24:103– 110. Brock WA, Baker FL, Peters LJ. Radiosensitivity of human head and neck squamous cell carcinomas in primary culture and its potential as a predictive assay of tumor radiocurability. Int J Radiat Biol 1989;56:751–760. Fiveash JB, Murshed H, Duan J, et al. Effect of multileaf collimator leaf width on physical dose distributions in the treatment of CNS and head and neck neoplasms with intensity modulated radiation therapy. Med Phys 2002;29:1116 –1119.
doi:10.1016/j.ijrobp.2004.06.213
PHYSICS CONTRIBUTION
A DYNAMIC SUPRACLAVICULAR FIELD-MATCHING TECHNIQUE FOR HEAD-AND-NECK CANCER PATIENTS TREATED WITH IMRT JUN DUAN, PH.D., SUI SHEN, PH.D., SHARON A. SPENCER, M.D., RAEF S. AHMED, M.D., RICHARD A. POPPLE, PH.D., SUNG-JOON YE, PH.D., AND IVAN A. BREZOVICH, PH.D. Department of Radiation Oncology, University of Alabama at Birmingham, Birmingham, AL Purpose: The conventional single-isocenter and half-beam (SIHB) technique for matching supraclavicular fields with head-and-neck (HN) intensity-modulated radiotherapy (IMRT) fields is subject to substantial dose inhomogeneities from imperfect accelerator jaw/MLC calibration. It also limits the isocenter location and restricts the useful field size for IMRT. We propose a dynamic field-matching technique to overcome these limitations. Methods and Materials: The proposed dynamic field-matching technique makes use of wedge junctions for the abutment of supraclavicular and HN IMRT fields. The supraclavicular field was shaped with a multileaf collimator (MLC), which was orientated such that the leaves traveled along the superoinferior direction. The leaves that defined the superior field border moved continuously during treatment from 1.5 cm below to 1.5 cm above the conventional match line to generate a 3-cm-wide wedge-shaped junction. The HN IMRT fields were optimized by taking into account the dose contribution from the supraclavicular field to the junction area, which generates a complementary wedge to produce a smooth junction in the abutment region. This technique was evaluated on a polystyrene phantom and 10 HN cancer patients. Treatment plans were generated for the phantom and the 10 patients. Dose profiles across the abutment region were measured in the phantom on films. For patient plans, dose profiles that passed through the center of the neck lymph nodes were calculated using the proposed technique and the SIHB technique, and dose uniformity in the abutment region was compared. Field mismatches of ⴞ 1 mm and ⴞ 2 mm because of imperfect jaw/MLC calibration were simulated, and the resulting dose inhomogeneities were studied for the two techniques with film measurements and patient plans. Threedimensional volumetric doses were analyzed, and equivalent uniform doses (EUD) were computed. The effect of field mismatches on EUD was compared for the two match techniques. Results: For a perfect jaw/MLC calibration, dose profiles for the 10 patients in the 3-cm match zone had an average inhomogeneity range of ⴚ1.6% to ⴙ1.6% using the dynamic-matching technique and ⴚ3.7% to ⴙ3.8% according to the SIHB technique. Measurements showed that dose inhomogeneities that resulted from 1-mm and 2-mm jaw/MLC calibration errors were reduced from as large as 27% and 45% with the SIHB technique to less than 2% and 5.7% with the dynamic technique, respectively. For ⴚ1-mm, ⴚ2-mm, ⴙ1-mm, and ⴙ2-mm jaw/MLC calibration errors, respectively, treatment plans for the 10 patients yielded average dose inhomogeneities of ⴚ5.9%, ⴚ3.0%, ⴙ2.7%, and ⴙ5.8% with the dynamic technique as compared to ⴚ22.8%, ⴚ11.1%, ⴙ9.8%, and ⴙ22.1% with the SIHB technique. Calculation based on a dose–volume histogram (DVH) showed that the SIHB technique resulted in larger changes in EUD of the PTV in the junction area than did the dynamic technique. Conclusion: Compared with the conventional SIHB technique, the dynamic field-matching technique provides superior dose homogeneity in the abutment region between the supraclavicular and HN IMRT fields. The dynamic feathering mechanism substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibration. In addition, isocenter location in the dynamic field-matching technique can be chosen for reproducible patient setup and for adequate IMRT field size rather than being dictated by the match position. It also allows angling of the supraclavicular field to reduce the volume of healthy lung irradiated, which is impractical with the SIHB technique. In principle, this technique should be applicable to any treatment site that requires the abutment of static and intensity-modulated fields. © 2004 Elsevier Inc. Dynamic field matching, Head-and-neck cancer, Intensity-modulated radiotherapy, Supraclavicular field, Dose inhomogeneity.
INTRODUCTION Definitive radiation therapy for squamous cell carcinoma (SCC) of the head and neck has traditionally consisted of
the use of large field arrangements designed to deliver a homogenous dose to the primary tumor and intervening lymphatics. The control of head-and-neck (HN) cancer treated with radiotherapy depends upon the delivery of
Reprint requests to: Jun Duan, Ph.D., Department of Radiation Oncology, University of Alabama Birmingham, 619 South 19th Street, Birmingham, AL 35233. Tel: (205) 934-4763; Fax: (205)
975-2546; E-mail: [email protected] Received Dec 16, 2003 and in revised form May 7, 2004. Accepted for publication Jun 21, 2004. 959
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adequate doses to the regions of gross disease and to regions of increased risk (1–5). Standard radiotherapy portal arrangement has typically consisted of parallel-opposed lateral fields and a low-neck or supraclavicular field. This arrangement requires the abutment of supraclavicular and lateral HN fields. The conventional field-matching technique makes use of a single isocenter and half beams (SIHB) (6, 7). This technique takes advantage of the nondivergent beam on central axis. The half beams are usually shaped by use of independent jaws of a linear accelerator collimator. In principle, two abutting fields that comprise complementary halves of a radiation beam form a smooth junction. However, the presumption is that the device used to shape the half beam is perfectly calibrated. Therefore, accurate calibration of the accelerator jaws is critical. In practice, jaw calibration error is allowed within a tolerance. The American Association of Physicists in Medicine Task Group Report 40 recommended an accuracy of field size and jaw symmetry calibration to be within 2 mm (8). Hence, for an accelerator operating within the guideline, two abutting fields can overlap or underlap up to 2 mm. Because of the steep dose gradient at the field borders on the match line, this misalignment can generate clinically significant dose inhomogeneity at the field junctions (9, 10). To minimize the impact of such dose inhomogeneity, a match-line shift (also known as “feathering”) technique has been traditionally used, i.e., moving the location of the field junction by 5 mm, for example, a few times during the treatment course to reduce the dose inhomogeneity at a certain location. A physical match-line wedge can also be used to substantially reduce the overdose or underdose that results from incorrect field positioning (11). This match-line wedge technique has been extended by use of dynamic collimation to generate a dynamic match-line wedge (12). In recent years, conformal radiation techniques have evolved that may allow irradiation of targets in the head and neck defined on planning CT scans, while sparing substantial portion of the major salivary glands. These techniques include three-dimensional conformal radiotherapy (3DCRT) using beams eye views, static segmental intensity modulation, and dynamic intensity modulation (13–17). These techniques have demonstrated that adequate irradiation of the targets while sparing major salivary glands is feasible in patients with HN cancer. Early clinical experience has demonstrated substantial sparing of saliva flows and suggests an improvement of tumor control and xerostomia (1, 16 –19) compared with use of conventional radiation techniques. Although the gross primary and subclinical disease in the low-neck nodes can be included in the intensitymodulated radiotherapy (IMRT) field, many physicians prefer to treat potential subclinical disease in the entire supraclavicular area. In such cases, the conventional static supraclavicular field is frequently used for treating the lowneck and supraclavicular lymph nodes. Although use of the supraclavicular field is more expeditious, field matching is necessary to abut the supraclavicular field with HN IMRT fields.
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Field matching poses a new challenge in IMRT. Several authors have investigated the techniques for matching IMRT fields. The dynamic “feathering” technique has been used for merging the subfields of a large IMRT field to minimize field-matching errors (20 –22). In this technique, adjacent IMRT fields were gradually blended together in a overlapping area of 2 to 3 cm. A similar technique has been used in matching electron fields with photon fields for improved dose distributions in the abutment regions (23– 25). Matching a static supraclavicular field with HN IMRT fields presents a special challenge in that although the static field has a clearly defined field border, IMRT fields often do not have a sharp, well-defined, nondivergent penumbra at their inferior border. Techniques for matching a static supraclavicular field with HN IMRT fields have not been well documented. In our institution, the SIHB technique has been initially adopted for matching supraclavicular and intensitymodulated HN fields. To generate a sharp penumbra at the inferior border of the HN fields, the intensity-modulated fields were optimized with the planning target volume (PTV) of upper-neck lymph nodes extended approximately 2 cm below the match line. After optimization, the inferior jaw was closed to the central axis for dose calculation, such that the dose gradient at the inferior border is artificially sharpened, thus forming a relatively smooth abutment. However, two major limitations are associated with this technique. First, as inherent to the SIHB technique, substantial dose inhomogeneities can arise from imperfectly calibrated jaws/MLC and cause severe underdose or overdose. Although the inhomogeneity may be spread over a small range as a result of daily setup errors, this area will be constantly “cold” or “hot” throughout the treatment because the inhomogeneity is the result of a systematic error in collimator calibration. In addition, because of the random nature of the setup error, uncertainties exist on the extent of reduction in inhomogeneity by the spread. Second, only half of the full field is available for IMRT fields, which in some cases is not sufficiently large to cover the entire target. In this study, we demonstrate an MLC-based dynamic technique for matching the supraclavicular field with HN IMRT fields to overcome these limitations. METHODS AND MATERIALS Dynamic field-matching method In the SIHB technique, the dose inhomogeneity across the match line generated by potential gap or overlap between asymetric jaws and/or MLC is proportional to the dose gradient at the field edges that constitute the match line. If the dose gradients are reduced to allow the supraclavicular field and the HN IMRT fields to merge gradually, the magnitude of such dose inhomogeneity can be significantly reduced. Figure 1 shows schematically the principle of the field matching by use of the dynamic-matching technique as opposed to the SIHB technique. In the dynamicmatching technique, the dose at the superior border of the supraclavicular field is reduced gradually by the continuous
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Fig. 1. The dynamic field-matching technique (a) compared with the single isocenter and half beam (SIHB) technique (b), and dose inhomogeneities in the abutment region generated by field mismatches caused by a gap or overlap between the abutting fields.
opening up or closing down of the superior border during radiation delivery, which produces a tapered junction. The IMRT fields that overlap with the supraclavicular field over the junction area are optimized by taking into consideration the dose contribution from the wedge-shaped supraclavicular junction, thus producing a complementary wedge at their inferior border. In place of the conventional match line, a match zone is formed. The two sets of fields, which merge gradually, combine to deliver a homogeneous dose in the match zone. As demonstrated in Fig. 1, dose inhomogeneity caused by imperfect jaw calibration can be greatly reduced because it is spread throughout the match zone. The
magnitude of dose inhomogeneity is determined by the magnitude of jaw/MLC calibration error, ␦, and the width of the match zone, w. For a given calibration error, the magnitude of the dose inhomogeneity can be controlled by adjusting the width of the match zone. The magnitude of the dose inhomogeneity is approximately ␦/w, which helps in determining the width of the match zone for a given jaw/ MLC calibration error and acceptable level of dose inhomogeneity. The width of the match zone used in this study is 3 cm, which corresponds to dose inhomogeneity approximately 3% and 6% for 1-mm and 2-mm overlap or underlap of the fields, respectively.
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superior border opened up by 3 mm from the previous one and delivered 10% of the total dose. A portion of the superior border that is intended to block the radiation beam from exposing the larynx and/or the spinal cord remained unchanged. The dynamic MLC leaf sequence file was converted to a fluence map (Fig. 2b) by the TPS before dose distribution of the supraclavicular field was calculated. Importing a leaf sequence file instead of a fluence map into the TPS guarantees the desired leaf motion and eliminates the possibility of multiple subfields if the length of the supraclavicular field exceeds the leaf span. For the IMRT fields, the PTV of the upper-neck lymph nodes was extended to 1.5 cm below the conventional match line. The IMRT plan was optimized to cover the extended PTV by inclusion of the dose contribution from the supraclavicular field, thereby forming another wedge junction to complement that of the supraclavicular field. No restriction applies to the collimator angles of the IMRT fields. Setting collimator angle at 90° can smooth the interleaf leakage and the tongue-and-groove effects through various gantry positions (22) but may increase the number of subfields for elongated targets. To avoid field overlap beyond the match zone, the inferior borders of the IMRT fields, which were adjusted automatically by TPS to fit the dynamic MLC fields, were manually closed down to 1.5 cm inferior to the match line by use of the collimator jaw before doses were calculated. Dynamic multileaf collimating IMRT (DMLC-IMRT) delivery mode was used in this study. In this study, the following convention was used in regard to calibration errors of independent jaws and/or MLC: A positive error refers to an overlap, whereas a negative error refers to a gap between two abutting fields. Fig. 2. (a) Supraclavicular field shaped by a multileaf collimator. During treatment, the superior border (except the portion that shields the larynx) moves continuously from 1.5 cm below to 1.5 cm above the conventional match line. (b) Fluence map of the dynamic supraclavicular field.
Treatment planning for dynamic match A commercial treatment planning system (TPS) (Eclipse; Varian Medical Systems, Palo Alto, CA) was used to generate treatment plans. Based on the conventional static supraclavicular field (shaped by either cerobend block or MLC), a static supraclavicular field was shaped with the MLC oriented such that the leaves moved in the superoinferior direction (Fig. 2a). To generate the match-line wedge junction of the supraclavicular field, the static MLC leaf sequence file of the supracavicular field was converted to a dynamic MLC leaf squence such that the leaves that shape the superior border of the field move continuously from 1.5 cm below to 1.5 cm above the conventional match line during treatment. Because our TPS does not allow a dynamic leaf sequence manually created directly within the TPS, software developed in-house or commercially available (Shaper by Varian) was used to convert the static field to a dynamic field of 10 segments. In each segment, the
Phantom measurement study To validate the dynamic field-matching technique, a polystyrene phantom was used for measurement. The phantom consists of a lightproof polystyrene box and 6-mmthick polystyrene plates that can be placed inside the box along the axial, sagittal, or coronal plane. The phantom was imaged with a CT simulator (AcQSim; Philips Medical Systems, Andover, MA). Five structures were outlined on the CT images, which simulated 2 targets (a primary PTV and a secondary nodal PTV) and 3 organs-at-risk (OAR) (left and right parotids and spinal cord). Figure 3 shows a 3D view of the phantom rendered from CT images. Treatment plans were prepared for the phantom using the dynamic and the SIHB field-matching techniques. The plans were delivered to the phantom, and Kodak EDR2 film was used to measure the 2D dose distributions on a coronal plane across the midplane of the neck PTV, as shown in Fig. 3. Before delivery, the systematic error of collimator calibration was determined by use of a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half-beam fields. A film was first irradiated with the two half-beam fields without couch movement to determine the direction of the couch movement necessary to correct for the calibration error. The source-to-film distance was set equal to the
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Fig. 3. Three-dimensional view of the polystyrene phantom rendered from CT images. Shown are simulated primary planning target volume (PTV) (yellow), secondary nodal PTV (red), parotids (cyan), and spinal cord (purple). The position of the film is indicated by the blue plane.
source-axis-distance of the linear accelerator, 100 cm. The couch was then moved systematically between exposures of the 2 half beams until a smooth junction was obtained. The distance by which the treatment couch had to be moved was considered as the collimator calibration error. The movement of the treatment couch was controlled by the Crane (NOMOS, Sewickley, PA), which allowed couch movements with 0.01-mm precision. The practical accuracy of the Crane was determined to be within 0.03 mm by comparing the reading of the Crane caliper and that of a caliper directly attached to the couch. When the plans were delivered to the phantom, this systematic collimator calibration error was corrected for by countermoving the couch between the deliveries of supraclavicular field and HN IMRT fields. Each of the phantom plans was delivered under the conditions of 0-mm, ⫾1-mm, and ⫾ 2-mm collimator calibration error. The films were digitized and optical density was converted to radiation dose on a Radiation Therapy Film Dosimetry System (Radiologic Imaging Technology, Colorado Springs, CO). Line-dose profiles across the center of the right neck PTV were obtained by software developed in-house. *RTOG protocols are available online at www.rtog.org.
Patient plan study For examination of the feasibility of the dynamic technique, treatment plans were prepared for 10 HN cancer patients. For the purpose of this study, Radiation Therapy Oncology Group (RTOG) Protocol H0022* was followed for dose prescription and OAR tolerances. Simultaneous integrated boost technique is used in this protocol, which requires the primary PTV to receive 66 Gy while the lymph node PTV receives 54 Gy in 30 fractions. Lymph node PTV was defined according to the protocol for the superior portion from 1.5 cm below the conventional match line and above. Prescription of the supraclavicular field adopted the conventional method (i.e., to 3-cm depth). The PTV for the inferior portion is not delineated but is defaulted to the volume that receives 100% of the prescription dose from the supraclavicular field. Seven or 9 equally spaced gantry positions that included one for the anteroposterior beam were used for the HN IMRT fields. For comparison, similar plans were generated by use of the conventional SIHB technique with the same beam geometry and dose constraints. To allow study of the impact of imperfectly calibrated jaws/MLC, calibration errors were simulated by
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moving the isocenter of the supraclavicular field in the superoinferior direction. For each plan pair, dose distributions were calculated for 0-mm, ⫾1-mm, and ⫾2-mm collimator calibration errors. Dose profiles through the center of the PTV in the right neck along the superoinferior direction were calculated by the TPS and compared between the 2 techniques. To facilitate 3D volumetric dose analysis of the junction area, lymph node PTV in the 3-cm abutment region was created and named junction PTV (JPTV). A dose–volume histogram (DVH) of JPTV was calculated for each plan. Equivalent uniform doses (EUD) were computed based on the assumption of uniform cell distribution across JPTV according to the following formula (26):
关 兺 (SF ) 兴 ⁄ ln(SF )
N EUD(GY) ⫽ Dref · ln 1 ⁄ N
2
Di⁄Dref
2
i⫽1
where Di is the dose in the ith partial volume, Dref is 1.8 Gy in the current study, SF2 is the survival fraction of 2 Gy, and a previously reported value, SF2 ⫽ 0.33, was used in the computation (27). RESULTS Phantom measurement study Before any measurement was conducted, collimator calibration errors were determined. Figure 4a shows a film image exposed by a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half beams. The dark stripe at the junction indicates overlap between the 2 half beams caused by an imperfect calibration of the 2 independent jaws. Figure 4b shows the dose profile across the center of the field (dashed line in Fig. 4a). Also shown in Fig. 4b are dose profiles of 2 of the film images used to determine collimator calibration errors; that is, images with ⫺0.65-mm and ⫺1-mm table shifts between the exposures of the 2 half beams. The negative sign indicates that the table movement shifted the 2 half beams away from each other. To display the curves clearly, the dose axis was shifted by ⫹10% and ⫺10% for 0-mm and ⫺1-mm table movements, respectively. The ⫺0.65-mm table movement produced a smooth abutment, which represented the collimator calibration error. This 0.65-mm jaw calibration error was corrected in the subsequent delivery of phantom plans by countermoving the couch between the supraclavicular field and the HN IMRT fields. Figure 5 shows measured dose profiles with and without jaw/MLC calibration errors for the dynamic and the SIHB technique. Without jaw/MLC calibration errors, the dynamic technique produced a smooth field abutment (Fig. 5a). In comparison, the SIHB technique, shown in Fig 5b, produced a small but acceptable dose inhomogeneity at the field junction. When simulated jaw/MLC calibration errors were introduced, as shown in Fig. 5c for the dynamic technique and in Fig. 5d for the SIHB technique, measured dose profiles exhibited drastically different magnitudes of
Fig. 4. (a) Film image exposed by a 10-cm ⫻ 10-cm field composed of 2 complementary 5-cm ⫻ 10-cm half beams. The dark stripe at the junction indicates an overlap between the 2 half beams caused by calibration errors of the independent jaws. (b) Dose profiles across the center of the 10-cm ⫻ 10-cm field, which is shown as the dashed line in (a), for 0-mm, ⫺0.65-mm, and ⫺1.0-mm table movements between the exposure of the 2 complementary 5-cm ⫻ 10-cm half beams that were used to determine jaw calibration error. The negative sign indicates that the table movement separated the 2 half beams from each other. To enhance clarity, the curves corresponding to 0-mm and ⫺1-mm table movements are shifted along the dose axis by ⫹10% and ⫺10%, respectively.
dose inhomogeneities in the abutment region between the two techniques. In the dynamic technique, dose inhomogeneities in the match zone were ⫺5.7% for ⫺2-mm, ⫺2.0% for ⫺1-mm, ⫹1.9% for ⫹1-mm, and ⫹5.2% for ⫹2-mm jaw/MLC calibration error (Fig. 5c), respectively. In contrast, corresponding dose inhomogeneities were substantially higher in the SIHB technique, i.e., ⫺45%, ⫺23%, ⫹27%, and ⫹38%, respectively (Fig. 5d). Note that a small
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Fig. 5. Dose profile measured by film compared with that calculated by treatment planning system (TPS) without collimator calibration error for the dynamic matching technique. Smooth field abutment was produced by the dynamic-matching technique. For comparison, those of the single isocenetr and half beam (SIHB) technique are shown in (b). Measured dose profiles with ⫾ 1-mm and ⫾ 2-mm collimator calibration errors show dose inhomogeneities in the match zone for the dynamic (c) and the SIHB (d) techniques.
portion of each measured profile is missing because of the film size (12.5 cm ⫻ 13 cm), which was limited by the phantom. The ripples on the measured profiles of the dynamic match technique (Figs. 5a and 5c) were caused by MLC interleaf transmission. Patient plan study For the dynamic match technique, dose distributions on the coronal plane that passes through the center of the nodal PTV of an HN cancer patient are shown in Fig. 6. Individual doses of the supraclavicular field (Fig. 6a) and HN IMRT fields (Fig. 6b) demonstrate complementary wedges in the match zone, which yields a smooth composite dose distribution in the abutment region (Fig. 6c). Line-dose profiles in the nodal PTV across the field junction were calculated by the TPS for both techniques. An example is shown in Fig. 7a for the dynamic and in Fig. 7b
for the SIHB technique. Dose profiles of the supraclavicular field and the IMRT fields are also shown. In the dynamicmatch technique, they demonstrate two complementary wedges in the 3-cm match zone. Compared with the SIHB technique, the dose homogeneity in the junction zone for the dynamic-match technique was noticeably improved. Table 1 compares the line-dose uniformity within the 3-cm match zone of the two field-matching techniques for the 10 patients in the study. For these patients, dose inhomogeneities ranged from ⫺0.4% to ⫹2.5% in the dynamic-match technique and from ⫺5.2% to ⫹6.8% in the SIHB-match technique for perfectly calibrated jaws/MLC. The average range of dose inhomogeneities was ⫺1.6% to ⫹1.6% for the dynamic technique and ⫺3.7% to ⫹3.8%, for the SIHB technique. In the case of imperfectly calibrated collimator and/or MLC leaves, dose inhomogeneities caused by ⫾ 1-mm
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Fig. 6. Dose distributions in a coronal plane passing through the center of the nodal planning target volume of a head-and-neck cancer patient. The contributions by the supraclavicular field (a) and the HN intensity-modulated radiation therapy fields (b) show complementary doses in the match zone, which yields a smooth composite dose in the abutment region (c).
and ⫾ 2-mm field mismatch are shown in Fig. 7c for the dynamic and in Fig. 7d for the SIHB techniques. Dose inhomogeneity of the dynamic technique, which was spread over the 3-cm match zone, was drastically reduced compared with that of the SIHB technique. Table 2 compares dose inhomogeneities caused by imperfectly
calibrated collimator jaws for the two match techniques. For the 10 patients, the mean dose inhomogeneity ⫾ standard deviation for ⫺2-mm, ⫺1-mm, ⫹1-mm, and ⫹2-mm jaw/MLC calibration errors, respectively, were ⫺5.9% ⫾ 0.4%, ⫺3.0% ⫾ 0.1%, 2.7% ⫾ 0.1%, and 5.8% ⫾ 0.1% for the dynamic technique and ⫺22.8% ⫾
Fig. 7. Typical line-dose profiles across the center nodal planning target volume of a patient calculated by the treatment planning system, under the assumption of perfect collimator calibration, for (a) the dynamic matching and (b) the single isoscenter and SIHB matching techniques. In the dynamic matching technique, dose profiles of the component supraclavicular field and intensity-modulated radiation therapy (IMRT) fields present 2 complementary wedges in the 3-cm match zone. For ⫾ 1-mm and ⫾ 2-mm collimator calibration, dose inhomogeneity of the dynamic matching technique (c), which was absorbed over the 3-cm match zone, was dramatically reduced in comparison with that of the SIHB technique (d).
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Table 1. Improved dose homogeneity in the abutment region for the 10 HN patients planned with the dynamic field-matching technique compared with the SIHB technique Patient number
Dynamic match
SIHB match
1 2 3 4 5 6 7 8 9 10 Average
⫺1.9%–2.1% ⫺1.3%–1.5% ⫺0.4%–0.6% ⫺2.4%–2.0% ⫺1.2%–0.9% ⫺2.1%–2.5% ⫺1.9%–2.4% ⫺1.5%–1.4% ⫺2.2%–1.6% ⫺1.5%–0.9% ⫺1.6%–1.6%
⫺2.5%–6.8% ⫺4.5%–2.5% ⫺3.2%–3.1% ⫺4.1%–1.7% ⫺2.8%–3.0% ⫺3.6%–6.8% ⫺2.7%–1.6% ⫺4.8%–3.8% ⫺5.2%–5.6% ⫺3.2%–3.2% ⫺3.7%–3.8%
0.6%, ⫺11.1% ⫾ 0.7%, 9.8% ⫾ 0.3%, and 22.1% ⫾ 0.5% for the SIHB technique. Three-dimensional volumetric doses were analyzed for the 10 patients. Three-dimensional dose distributions of the two match methods were compared for a ⫺2-mm match error in 3 orthogonal planes and a 3D view. As Fig. 8 shows, the SIHB technique creates substantially cold areas in the abutment region. Whereas the dynamic technique demonstrates a smooth junction, the 3D view of the SIHB technique shows a large gap in the 54-Gy isodose surface. The DVHs of JPTV for Patient 1 were compared by both matching techniques with ⫺2-mm to ⫹2-mm match errors (Fig. 9). EUDs were calculated on the basis of these DVHs. In Patient 1, changes in the EUD of JPTV caused by ⫺2-mm, ⫺1-mm, ⫹1-mm, and ⫹2-mm match errors are ⫺5.8%, ⫺3.1%, 2.3%, and 5.1% for the dynamic technique and ⫺7.0%, ⫺4.0%, 2.8%, and 5.4% for the SIHB technique, respectively. The changes in EUD are summarized in Table
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3 for the 10 patients. The effect of dose inhomogeneity that resulted from field mismatch on the EUD is less in all cases with the dynamic technique than with the SIHB technique.
DISCUSSION The present study has demonstrated that the dynamic field-matching technique has significant dosimetric advantages over the SIHB technique. It produced more uniform dose within the abutment region. More importantly, it suppressed dose inhomogeneities resulting from jaw/MLC calibration inaccuracy by spreading them over the 3-cm-wide match zone via a dynamic feathering mechanism. The magnitude of dose inhomogeneities is substantially reduced as compared with the SIHB technique. A survey of linear accelerators in our institution showed that collimator calibration inaccuracies ranged from ⫾ 0.5 mm to ⫾ 1.5 mm. With use of the dynamic field-matching technique, dose inhomogeneities that result from such inaccuracies can be readily limited to less than 5% with a 3-cm match zone. More that 30 patients have been treated successfully in our clinic using this technique. As mentioned earlier, the SIHB technique requires a common isocenter for the supraclavicular and the HN IMRT fields. The half beams limit the isocenter location to the match line, which is usually not ideal for reproducible daily patient setup. In the case of nasopharyngeal cancer, in which the disease can extend to the base of the skull, the maximum field size of half beams (20 cm) may not be sufficient to cover the entire target. These limitations are overcome with the dynamic technique, which requires neither a common isocenter nor half beams for the abutting fields. However, we recommend the use of a common isocenter to eliminate setup errors associated with isocenter
Table 2. Dose inhomogeneities resulting from field mismatches caused by jaw/MLC calibration errors in 10 patient plans Jaw/MLC calibration error ⫺2 mm
⫺1 mm
⫹1 mm
⫹2 mm
Patient number
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
1 2 3 4 5 6 7 8 9 10 Mean SD
⫺6.1% ⫺4.8% ⫺5.7% ⫺5.8% ⫺6.1% ⫺6.0% ⫺6.1% ⫺6.4% ⫺6.0% ⫺6.0% ⫺5.9% 0.4%
⫺21.6% ⫺20.2% ⫺21.1% ⫺24.5% ⫺25.6% ⫺20.5% ⫺24.7% ⫺23.4% ⫺21.5% ⫺24.8% ⫺22.8% 0.6%
⫺3.0% ⫺2.3% ⫺2.7% ⫺2.9% ⫺3.1% ⫺3.0% ⫺3.1% ⫺3.2% ⫺3.2% ⫺3.0% ⫺3.0% 0.1%
⫺10.8% ⫺9.1% ⫺13.2% ⫺12.3% ⫺12.9% ⫺10.2% ⫺12.3% ⫺11.7% ⫺5.8% ⫺12.4% ⫺11.1% 0.7%
2.4% 2.9% 2.8% 2.3% 2.5% 2.4% 2.5% 2.6% 3.3% 3.0% 2.7% 0.1%
8.6% 11.6% 10.2% 9.8% 10.2% 8.2% 9.9% 9.4% 11.2% 9.2% 9.8% 0.3%
5.8% 5.4% 5.7% 5.5% 5.8% 5.7% 5.8% 6.1% 6.3% 5.9% 5.8% 0.1%
20.5% 23.1% 20.8% 23.3% 24.2% 19.4% 23.4% 22.2% 23.2% 21.2% 22.1% 0.5%
Abbreviations: MLC ⫽ multileaf collimator; SD ⫽ standard deviation; SIHB ⫽ single isocenter and half beam. Note: Dose inhomogeneities are substantially reduced by use of dynamic compared with SIHB technique. The mean values and SD of the 10 patient plans are also listed.
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Fig. 8. Three-dimensional dose distributions in three orthogonal planes and a 3D view for ⫺2-mm field match error in the dynamic (a) and the single isocenter and half beam (SIHB) (b) field-matching technique. The position of the cuts are indicated by the dashed lines. The axial plane passes through the match line. The SIHB technique generated substantially greater dose inhomogeneity in the abutment region than did the dynamic technique.
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Fig. 9. Dose volume histograms for ⫺2-mm to ⫹2-mm field mismatch errors with the dynamic (a) and the single isocenter and half beam (b) field-matching technique.
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shift. Because the isocenter position is not limited to the match line, it still can be set at a more favorable location, provided that it is within 20 cm from the inferior border of the supraclavicular field. The location of the match line remains unchanged relative to the patient, which will be inferior to the isocenter. This arrangement allows isocenter placement at more stable location for reproducible daily patient setup and for larger HN IMRT fields. For MLCs that have thinner leaves in the central portion of the field, such as Varian Millennium 120 MLC (0.5-cm-thick leaves in the central 20 cm and 1-cm-thick leaves in the peripheral 10 cm of the field), the flexibility of isocenter location allows the users to take advantage of the higher resolution central leaves for the modulated HN fields. For example, for nasopharyngeal cancer, small, complex optical structures are usually beyond the central 20 cm of the field in the SIHB technique. The dynamic technique allows the isocenter to be set to a location such that the 0.5-mm leaves are used to provide an improved dose distribution for that area (28). Without a cord block, the spinal cord in the supraclavicular field often could receive a dose exceeding its tolerance. Shaping the supraclavicular field with an MLC allows central leaves to be closed to serve as a cord block. However, in cases of low-lying disease in which mediastinal nodes are treated, a complete cord block is not warranted. In these cases, a partial transmission block can be readily incorporated into the dynamic supraclavicular field. By closing the central leaves to block the spinal cord after, for example, 80% of the dose has been delivered, the dose to the spinal cord can be kept within tolerance while the mediastinum is treated. The SIHB technique requires the supraclavicular field be a straight anteroposterior beam to match the superior HN fields (Fig. 10a). This requirement does not apply to the dynamic-match technique, because the modulated HN fields
Table 3. Percentage changes in EUD of junction PTV resulting from field mismatches caused by jaw/MLC calibration errors Jaw/MLC calibration error ⫺2 mm
⫺1 mm
⫹1 mm
⫹2 mm
Patient number
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
Dynamic
SIHB
1 2 3 4 5 6 7 8 9 10 Mean SD
⫺5.8% ⫺4.9% ⫺5.4% ⫺5.3% ⫺5.6% ⫺5.1% ⫺4.6% ⫺5.1% ⫺5.4% ⫺3.4% ⫺5.1% 0.7%
⫺7.0% ⫺6.1% ⫺6.3% ⫺6.2% ⫺7.6% ⫺7.3% ⫺6.6% ⫺7.1% ⫺8.9% ⫺7.2% ⫺7.0% 0.8%
⫺3.1% ⫺2.2% ⫺2.7% ⫺2.8% ⫺2.7% ⫺2.7% ⫺2.4% ⫺2.7% ⫺2.7% ⫺1.6% ⫺2.6% 0.4%
⫺4.0% ⫺2.7% ⫺3.2% ⫺3.0% ⫺3.1% ⫺4.3% ⫺3.3% ⫺3.8% ⫺3.5% ⫺4.8% ⫺3.5% 0.8%
2.3% 2.7% 2.6% 2.3% 2.7% 2.4% 2.1% 2.1% 2.7% 1.8% 2.4% 0.3%
2.8% 3.4% 3.4% 4.1% 4.8% 3.2% 3.2% 2.9% 3.3% 3.5% 3.5% 0.6%
5.1% 5.4% 5.1% 4.8% 5.3% 4.9% 4.3% 4.3% 5.4% 3.5% 4.8% 0.6%
5.4% 6.6% 6.1% 7.6% 9.1% 5.7% 6.2% 5.8% 7.6% 7.0% 6.7% 1.1%
Abbreviations: EUD ⫽ equivalent uniform dose; MLC ⫽ multileaf collimator; PTV ⫽ planning target volume; SD ⫽ standard deviation; SIHB ⫽ single isocenter and half beam.
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Fig. 10. A supraclavicular field slightly tilted toward the feet in the dynamic technique (a) significantly reduces the volume of healthy lung being irradiated as compared with the straight anterior-posterior field in the single isocenter and half beam technique (b). The dose–volume histograms of lung (c) show that the volume of lung receiving 20 Gy or higher is reduced by 47% in this case.
are optimized to complement the dose contributed by the supraclavicular field. This flexibility in beam direction provides an additional advantage for the dynamic-match technique. As shown in Fig. 10b, if the treatment couch is rotated 90°, the gantry can be slightly tilted toward the feet while sufficient coverage of the supraclavicular area is maintained. In this way, a significant volume of healthy lung is spared from high radiation dose. As shown in the dose–volume histogram (Fig. 10c), the volume that received 20 Gy or more was reduced by 47% in this case. However, the tradeoff of this feature is the need for manual setup of the couch rotation, which increases treatment time and introduces additional uncertainties caused by the couch rotation. We observed that measured dose profiles show greater dose inhomogeneities in the SIHB technique than that calculated by TPS. Figure 11 shows the measured dose profiles
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Fig. 11. Comparison of dose profiles between film measurements and treatment planning system (TPS) calculation for ⫾2-mm jaw/ multileaf collimator calibration errors. The agreement is reasonably good for the dynamic-match technique (a). However, the TPS underestimates the dose inhomogeneities caused by field mismatch for the single isocenter and half beam technique (b).
compared with the calculated dose profiles for ⫾ 2-mm jaw/MLC calibration errors. The profiles for the dynamic technique agree reasonably well between measured and calculated ones. However, those for the SIHB technique show that the TPS underestimated the dose inhomogeneities that resulted from jaw/MLC calibration errors. The underestimation has two causes. First, the penumbra modeling for the TPS used in this study is based on measured beam profiles of full fields. Therefore, approximations are made in the calculation of half-beam penumbra. Second, the highest resolution of the dose grid provided by the TPS is 1.25 mm, which leads to broadened width but reduced height of the inhomogeneity peaks. Nevertheless, because even the underestimated dose inhomogeneity was clinically significant,
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it would not affect the general conclusion deduced from these results. The EUDs were calculated from the DVHs, which were calculated by the TPS for the PTV within the 3-cm area in the abutment region. The inhomgeneities that result from field mismatches in the SIHB technique are concentrated within about 1 cm around the match line. The influence of field mismatch on the EDUs for the PTV within this 1-cm range was substantially greater with the SIHB technique than with the proposed technique. For example, with a ⫺2-mm mismatch, the EUDs were reduced by 16.1% for Patient 1 with use of the SIHB technique as compared to 5.9% with the dynamic technique. In addition, considering the underestimation by the TPS in the magnitude of inhomogeneity in the SIHB technique, the actual changes of EUDs would be even greater with use of the SIHB technique than with use of the dynamic technique. Compared with the planning of a conventional static supraclavicular field, the proposed technique requires extra 2 to 4 minutes to generate the dynamic supraclavicular field when our in-house software is used, or 5 to 10 minutes if the field is generated manually by application of MLC-shaping software such as Shaper. However, our method eliminates the need for match-line shifts associated with the SIHB technique, which saves time in both treatment planning and delivery. The number of treatment fractions for the supraclavicular fields and HN IMRT fields must be identical when the dynamic-matching technique is used, because IMRT plans are usually optimized on the basis of the total doses of the treatment course. Otherwise, the fractional dose
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in the matching zone will deviate from the fractionation scheme significantly, even though the total dose remains the same. However, in most HN cases, the secondary nodal PTV usually receives lower dose than the primary PTV. This difficulty can be managed in either of two ways: (1) by splitting the HN IMRT treatment into 2 parts with the first part treated simultaneously with the supraclavicular field by use of the dynamic- matching technique and the second part to finish the remaining dose to the primary PTV or (2) by modifying the doses per fraction for the primary and nodal PTV to equalize the number of fractions. CONCLUSION The proposed dynamic field-matching technique for supraclavicular and HN IMRT fields is a simple, practical technique that can be readily implemented in most clinical settings. Compared with the conventional SIHB technique, the proposed technique provides more homogeneous dose in the abutment region and, more importantly, substantially reduces dose inhomogeneities that result from imperfect jaw/MLC calibrations. It provides more flexibility in the choice of isocenter locations, which allows an isocenter at a more favorable location for reproducible patient setup, as well as larger available HN IMRT fields. In addition, it permits a flexible gantry angle of the supraclavicular field, which can reduce the volume of healthy lung being irradiated. In principle, this technique can be applied to any treatment site that requires the abutment of IMRT and conventional static fields.
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