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Impacts of Multileaf Collimators Leaf Width on Intensity-Modulated Radiotherapy Planning for Nasopharyngeal Carcinoma: Analysis of Two Commercial Elekta Devices
Medical Dosimetry, Vol. 36, No. 2, pp. 153-159, 2011 Copyright © 2011 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/11/$–see front matter
doi:10.1016/j.meddos.2010.02.007
IMPACTS OF MULTILEAF COLLIMATORS LEAF WIDTH ON INTENSITY-MODULATED RADIOTHERAPY PLANNING FOR NASOPHARYNGEAL CARCINOMA: ANALYSIS OF TWO COMMERCIAL ELEKTA DEVICES SHICHAO WANG, B.S., YOULING GONG, M.D., PH.D., QINGFENG XU, M.S., SEN BAI, PH.D., YOU LU, M.D., QINGFENG JIANG, and NIANYONG CHEN, M.D., PH.D. Radiation and Physics Center; Department of Thoracic Oncology and Radiation Oncology; State Key Laboratory of Biotherapy; and Department of Head & Neck Oncology and Radiation Oncology, West China Hospital, Sichuan University, Chengdu, Sichuan Province, PR China (Received 7 July 2009; accepted 18 February 2010)
Abstract—We compared the impacts of multileaf collimator (MLC) widths (standard MLC width of 10 mm [SMLC] and micro-MLC width of 4 mm [MMLC]) on intensity-modulated radiotherapy (IMRT) planning for nasopharyngeal carcinoma (NPC). Ten patients with NPC were recruited in this study. In each patient’s case, plans were generated with the same machine setup parameter and optimizing methods in a treatment planning system according to 2 commercial Elekta MLC devices. All of the parameters were collected from dose-volume histograms of paired plans and evaluated. The average conformity index (CI) and homogeneous index (HI) for the planning gross target volume in IMRT plans with MMLC were 0.790 ⴞ 0.036 and 1.062 ⴞ 0.011, respectively. Data in plans with SMLC were 0.754 ⴞ 0.038 and 1.070 ⴞ 0.010, respectively. The differences were statistically significant (p < 0.05). Compared with CI and HI for planning target volume in paired plans, data with MMLC obviously were better than those with SMLC (CI: 0.858 ⴞ 0.026 vs. 0.850 ⴞ 0.021, p < 0.05; and HI: 1.185 ⴞ 0.011 vs. 1.195 ⴞ 0.011, p < 0.05). However, there was no statistical significance between evaluated parameters (Dmean, Dmax, D5, gEUD, or NTCP) for organs at risk (OARs) in the 2 paired IMRT plans. According to these two kinds of Elekta MLC devices, IMRT plans with the MMLC have significant advantages in dose coverage for the targets, with more efficiency in treatment for NPC but fail to improve dose sparing of the OARs. © 2011 American Association of Medical Dosimetrists. Key Words: Multileaf collimators, Leaf width, Intensity-modulated radiotherapy, Nasopharyngeal carcinoma.
INTRODUCTION
powerful tool to improve the quality of the delivered dose distribution in head and neck radiotherapy both for its radiation of planning target volume (PTV) and the sparing of OARs.6,7 The introduction of the multileaf collimator (MLC) has streamlined the radiotherapy process, although the target conformity is limited by the discrete step size of the leaves. For some kinds of tumors, the anatomy location between targets and OARs is very close and complicated; MLC leaf widths may affect the dose distribution of targets and OARs because of the differences of every single MLC to conform to the outline of targets, which have a close relationship with the leaf width of the MLC. Recently, a few researchers evaluated impacts of MLC leaf width on treatment planning for several kinds of tumors, and the results are somewhat controversial.8 –12 So far, few studies investigated the impact of MLC leaf width on IMRT planning for NPC, although 1 case and 3 cases have been studied by Monk et al. and Jin et al., respectively.9,11 Although Cheung et al. studied the treatment planning for NPC and concluded that a 3-mm MLC is more suitable for tumor volumes very close to the critical normal structures than a 10-mm MLC, the technique they evaluated was the 3D-CRT.13
Nasopharyngeal carcinoma (NPC) is one of the most common head and neck cancers in Asia and Africa, but the incidences are rare in Western countries.1 Patients with NPC are typically treated with radiation therapy rather than surgery because of the anatomically challenging and difficult location and a demonstrated favorable response to radiotherapy and chemotherapy.2– 4 It has been considered that definitive radiotherapy with or without chemotherapy is the standard treatment of NPC.5 For patients with NPC, it is crucial for treatment success to minimize the dose to the surrounding organs at risk (OARs) while maintaining adequate radiation dose coverage to the target volume. In the recent 2 decades, IMRT has been stated to have advantages not only in physical dosimetry, but also in clinical practice compared with 3D conformal radiotherapy (3D-CRT). As Longobardia et al. and Xia et al. summarized, intensitymodulated radiotherapy (IMRT) was recognized as a The first two authors contributed equally to this work. Reprint requests to: Youling Gong, M.D., Ph.D., Department of Thoracic Oncology and Radiation Oncology, Tumor Center, West China Hospital, Sichuan University, Chengdu, Sichuan Province, PR China. E-mail: [email protected] 153
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Here, for the first time, we conducted a study to compare the impacts of MLC leaf width in IMRT planning for NPC. All plans were generated with our therapy planning system (TPS, Philips Pinnacle3 version 8.0 m, Milpitas, CA, USA) commissioned for the MLC device on the Elekta Precise Treatment System (Elekta Oncology System, Stockholm, Sweden) and the MLC device on the Elekta Synergy Treatment System.
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whole group was 43 years (range, 35–56). All patients were staged according to the 1997 UICC/AJCC staging system.18 The number of patients in Stage II was 2 (20%), Stage III 7 (70%), and Stage IVa 1 (10%). The T and N stage distribution in the present study is shown in Table 1. Permission to conduct the study was granted by the Research Ethics Board of the University Health Network. Clinically, 6 patients accepted treatment with the SMLC device and 4 patients accepted treatment with the MMLC device.
METHODS AND MATERIALS Collimators The standard MLC (SMLC) we analyzed is the Elekta MLCi, which is equipped in the Elekta Precise Treatment System. The SMLC is fully integrated in the linear accelerator both physically within the standard collimator housing and electrically within the control system. The maximum clearance between the isocenter and collimator is 45 cm. The diameter and the height of the SMLC are 62 and 42 cm, respectively. The leaf width of the SMLC is 10 mm at the isocenter. It has 80 leaves (40 pairs), with a travel range of 32.5 cm in the y-direction (12.5 cm beyond the central axis) covering a full 40 ⫻ 40-cm field. Preparation and treatment of non-MLC and MLC prescriptions is controlled via a single control console. There are no separate dedicated displays for the SMLC. The conventional collimators are in the x-direction. The backup diaphragms in the y-direction ensure a minimum X-ray leakage specification.14 There is a minimum leaf gap across banks. The micro-MLC (MMLC) we analyzed is another commercial system newly installed in our center, the Elekta Beam Modulator (Elekta Oncology Systems, Crawley, UK), which is equipped on the Elekta Synergy Treatment System.15,16 It has 80 individually controlled leaves that have a travel range of ⬎21 cm for each leaf. Each of the leaves is capable of interdigitation and projects a width of 0.4 cm at the isocenter. The true rectangular field size is 16 ⫻ 21 cm. The leaf bank is positioned approximately 39 cm from the target. There are no movable backup diaphragms and therefore the field is defined only by the open leaves.15 Leaf movements are achieved by individual motors using a rack and pinion drive to the leaf. The position readout is controlled by a high-resolution optical system. The speed of leaf movement is 0.2–3.0 cm per second.17 As a result, there would be a variation in penumbra as the leaves move to different positions with respect to the central axis. To counter this effect, the leaf ends are rounded. It is possible to create islands of radiation because it is not necessary to have all open leaves defined in a contiguous block.15
Target delineation and dose prescription All of the patients were immobilized with head and neck thermoplastic masks. Each of the patients in this study underwent a spiral computer tomography (CT, Siemens Sensation 4) with 3-mm-slice thickness in the supine position. Three radio-opaque ball markers made of lead and fixed on a tiny self-sticker paper were placed coplanarly on the surface of the mask to mark the CT reference center; 2 were on the lateral sides and 1 was on the anterior. All of the CT images of the patients acquired were transferred to and registered in the TPS with the same method and a standard of couch removal and laser center localization. All target delineations in this study were in accordance with ICRU 50 and 62 reports.19,20 The gross tumor volume (GTV) included all known gross disease (primary tumor ⫹ grossly enlarged lymph nodes) as determined by the imaging, clinical, and endoscopic findings. A planning gross target volume (Pgtv) was created based on the GTV with an additional 3-mm margin (GTV ⫹ 3 mm), allowing for any setup variability to prevent the gross tumor from being missed by the high-dose zone. The clinical target volume (CTV) was defined as the GTV ⫹ 1-cm margin (0.5-cm margin posteriorly) to encompass any microscopic extension, together with the regional lymphatics. Regional lymphatics were outlined according to the recommendation by Nowak et al.21 Surgical neck levels Ib (submandibular nodes) to V were electively irradiated. Submental region (level Ia) was included if the submandibular nodes or oral cavity were grossly involved. The PTV was the CTV ⫹ 3-mm margin, based on the observation of the average setup variability of our institution. Critical normal structures were contoured as OARs, including brainstem, spinal cord, parotid glands, optic nerves/chiasm, eyeballs, temporomandibular joints, and inner ears. A 3-mm planning organ-at-risk volume
Table 1. T- and N-stage distribution of the patients in this study (n ⫽ 10) T-Stage
Patient data This study was conducted between March 2007 and October 2008. Ten patients with pathologically confirmed NPC were recruited in the present study. There were 8 males and 2 females, and the median age for the
N-Stage
1
2
3
4
0 1 2 3
0 0 0 0
0 2 1 0
2 3 1 0
0 1 0 0
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Fig. 1. Case-by-case comparison of MUs between IMRT plans with MMLC and SMLC (10 and 4 mm ⫽ MLC leaf widths of 10 and 4 mm, respectively).
and evaluated using the TPS mentioned previously. The plans were generated using nine coplanar beams whose angels were 200°, 240°, 280°, 320°, 0°, 40°, 80°, 120°, and 160°, respectively. In plan generation, the direct machine parameter optimization (DMPO, Raysearch Laboratories, Stockholm, Sweden) method was applied. Such algorithms are clearly superior to those with sequencing after a complete fluence optimization process.22 The plan optimizing based on the 2 MLC devices was generated with the same dose constraints and optimizing parameters. The maximum iterations of plan optimizing were 40, and the maximum number of all segments in 1 plan was restricted to 100. There is no limitation to the minimum monitor units (MUs) per segment.
(PRV)-margin was added to the spinal cord and brainstem during optimization to allow for possible positional errors. The planned treatment for each patient consisted of 68.8 Gy to be delivered to Pgtv in 32 fractions, 60.8 Gy for PTV in 32 fractions, and the prescription dose covered at least 95% of the volume of Pgtv and PTV, respectively. The maximum tolerance doses to the critical normal structures were as follows: brain stem 54 Gy, spinal cord 45 Gy, optic nerve and chiasm 54 Gy, inner ears 54 Gy, and parotid gland (V26 ⬍50%). Because identical lower neck portals were generated in the IMRT plans with the different MLC devices, the dose-volume histograms (DVHs) of the tumor targets and normal structures in the lower neck and supraclavicular regions were not compared.
Evaluation of DVH-based parameters The conformity index (CI) for Pgtv was calculated as per the formula: CI ⫽ Pgtvref/Pgtv ⫻ Pgtvref/Vref.8 The Pgtvref is the overlap volume between the Pgtv and volume of prescription isodose surface. The Vref is tissue volume that is enclosed by the prescription isodose surface also outside of Pgtv. The prescription isodose was 100% isodose to Pgtv. The same method was applied in analysis of PTV. The higher the CI is, the more conformal the plan is. The homogeneous index (HI) for the targets was defined as follows: HI ⫽ D5/D95, where D5 and D95 are the doses received by the 5% and 95% volumes of Pgtv and PTV. The more D5 and D95 approach each other, the steeper the target’s curve in DVHs. The generalized equivalent uniform dose (gEUD) and normal tissue complication probability (NTCP) for OARs were calculated by the TPS automatically.
Treatment planning and optimizing In the process of treatment planning setup and optimizing, all of the inverse IMRT plans were generated
Statistical analysis The statistical analysis was performed using SPSS software (version 13.0, Chicago, IL). All data were an-
Fig. 2. Transverse sections of the representative IMRT plans of 1 patient with the irradiation isodose curves (10 and 4 mm ⫽ MLC leaf widths of 10 and 4 mm, respectively).
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Table 2. Comparisons of the DVH-based parameters for targets in the present study (n ⫽ 10) MLC Leaf Width Targets Pgtv Dmax Dmin Dmean PTV Dmean
10 mm
4 mm
p-Value
7570.0 ⫾ 90.1 5976.8 ⫾ 327.4 7173.7 ⫾ 28.4
7531.8 ⫾ 83.9 6132.2 ⫾ 247.0 7166.7 ⫾ 25.3
0.34 0.25 0.57
6600.9 ⫾ 65.8
6592.7 ⫾ 60.9
0.78
Abbreviations: DVH, dose volume histogram; MLC, multileaf collimator; Pgtv, planning gross target volume; PTV, planning target volume; Dmax, Dmin, and Dmean, maximum, minimum, and average irradiation dose of the targets received.
alyzed applying “mean ⫾ SD.” Using Student’s t-test, a value of p ⬍ 0.05 (2-tailed) was considered statistically significant.
RESULTS Two IMRT treatment plans were generated for each of the patients in this study, based on the different Elekta MLC devices, respectively. In total, 20 IMRT plans were generated after the protocol and analyzed. Figure 1 shows the case-by-case comparison of the delivering MUs between these paired IMRT plans, indicating that the average MUs in plans with the MMLC (698.2 ⫾ 35.5) are significantly lower than those with the SMLC (745.7 ⫾ 33.6) (p ⬍ 0.01). The transverse sections of the representative IMRT plans of one patient are shown in Fig. 2, and it is obvious that the irradiation dose curves are similar. The evaluation of the DVH-based parameters of the targets is shown in Table 2. The maximum, minimum, and average doses of Pgtv and the average dose of PTV
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are similar between these 2 kinds of plans, respectively, with no statistical significance (p ⬎ 0.05). The comparisons of the dose conformality for the targets are summarized in Table 3. The average CI for Pgtv (0.790 ⫾ 0.036) and PTV (0.858 ⫾ 0.026) in IMRT plans with the MMLC are significantly better than those indexes in plans with the SMLC (for Pgtv: 0.754 ⫾ 0.038 and for PTV: 0.850 ⫾ 0.021) (p ⬍ 0.05). Also, the average HI for Pgtv (1.062 ⫾ 0.011) and PTV (1.185 ⫾ 0.011) in plans with MMLC are better than those in plans with the SMLC (for Pgtv: 1.070 ⫾ 0.010 and for PTV: 1.195 ⫾ 0.011) with the statistical significance of p ⬍ 0.05. Table 4 shows the comparisons of the DVH-based parameters of the spinal cord and the brainstem in the present study. Compared with the plans with the SMLC, the IMRT plans with the MMLC have no advantage in dose sparing of the spinal cord and brainstem. The differences of Dmax, D5, and gEUD of the spinal cord and the brainstem between the 2 groups are not statistically significant (p ⬎ 0.05). The comparisons of the DVH-based parameters of other OARs in the present study are shown in Table 5. Compared with the IMRT plans with the SMLC, the IMRT plans with the MMLC do not show advantage in dose sparing of the other evaluated OARs (D50, Dmax, gEUD, and NTCP for the parotid glands; Dmean, Dmax, gEUD, and NTCP for the temporomandibular joints, inner ears, and optic nerve/chiasm) (p ⬎ 0.05). DISCUSSION In the IMRT plan generation, there are so many factors affecting the dose distributions around the targets and OARs, such as the target delineation, inter/intrafractional tumor motion, patient’s set-up errors, beam orientation, and numbers. As with many studies mentioned here
Table 3. Comparisons of CI and HI for Pgtv and PTV in the present study (n ⫽ 10) Pgtv
PTV
CI Patient Number
10 mm
HI 4 mm
10 mm
CI 4 mm
10 mm
HI 4 mm
10 mm
4 mm
1 0.780 0.822 1.087 1.077 0.868 0.874 1.207 1.200 2 0.789 0.821 1.077 1.073 0.863 0.871 1.201 1.193 3 0.817 0.838 1.066 1.063 0.856 0.864 1.205 1.197 4 0.780 0.794 1.069 1.058 0.850 0.862 1.196 1.184 5 0.697 0.771 1.066 1.064 0.863 0.877 1.174 1.171 6 0.738 0.793 1.067 1.057 0.864 0.881 1.190 1.183 7 0.762 0.797 1.056 1.046 0.825 0.824 1.190 1.176 8 0.749 0.796 1.068 1.063 0.873 0.883 1.192 1.182 9 0.715 0.718 1.059 1.046 0.816 0.815 1.186 1.170 10 0.717 0.752 1.087 1.072 0.824 0.827 1.213 1.197 Average 0.754 ⫾ 0.038 0.790 ⫾ 0.036 1.070 ⫾ 0.010 1.062 ⫾ 0.011 0.850 ⫾ 0.021 0.858 ⫾ 0.026 1.195 ⫾ 0.011 1.185 ⫾ 0.011 (mean ⫾ SD) p-value 0.04 0.04 0.04 0.04 Abbreviations: CI, conformity index; HI, homogeneous index; Pgtv, planning gross target volume; PTV, planning target volume; 10 and 4 mm, multileaf collimator leaf widths as 10 and 4 mm, respectively.
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Table 4. Comparisons of the DVH-based parameters of spinal cord and brain stem in the present study (n ⫽ 10) Dmax
D5
gEUD
MLC Leaf Width
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
10 mm 4 mm 10 mm 4 mm
4454.5 ⫾ 173.8 4484.1 ⫾ 175.0 4998 ⫾ 546 4927.1 ⫾ 477.8
0.71
3921.4 ⫾ 195.2 3991.2 ⫾ 137.1 4868.2 ⫾ 187.5 4885.7 ⫾ 204.9
0.37
2930.5 ⫾ 616.5 3023.1 ⫾ 580.2 3989.0 ⫾ 234.3 4154.6 ⫾ 223.4
0.73
Spinal cord Brainstem
0.76
0.84
0.12
Abbreviations: DVH, dose volume histogram; MLC, multileaf collimator; Dmax, maximum irradiation dose; D5, irradiation dose received by the 5% of volume of OARs; gEUD, generalized equivalent uniform dose.
before, the leaf widths of the MLC may have an impact on the dose distribution in IMRT dose delivery accuracy. For the first time, we conducted a serial study focusing on the effect of the MLC leaf widths on the dose distribution in IMRT for NPC. Our results indicate that the IMRT plans with the MMLC are better than those plans with SMLC in the dose conformity and dose delivering efficiency. However, we find no significant difference on the dose sparing of the OARs between the 2 kinds of plans (p ⬎ 0.05). The IMRT technique has advantages not only on the 3D conformation to the tumor structure, but also on the dose protection for normal tissues around the targets. In theory, the less MLC leaf widths are, the better the dose optimization of an IMRT plan would be because it could achieve a more conformal shape around the tumor. In previously published studies, the conclusions have been made that the MMLC was better in the target’s conformity and uniformity of the dose distribution. Kubo et al. reported that the 1.7–3.0-mm MLCs could spare more bladder and rectum than the 10-mm MLC for prostate treatment planning.23 Monk et al. compared the 3- and 5-mm MLCs in intracranial radiosurgery planning and observed that the 3-mm MLC improves the conformity of PTV. However, the quantitative differences between
the 3- and 5-mm MLC collimation (5% for tissue sparing) may not be clinically significant for some cases.9 Jin et al. found that the 3-mm MLC has the better conformity parameters than those of the 5- and 10-mm MLCs; the advantage decreases when the target volume increases.11 Conversely, Burmeister et al. reported that there was no apparent clinically significant difference between the 5and 10-mm MLCs except for very small target volumes or those with concavities that are small, with respect to the MLC leaf widths.24 Similar to the studies mentioned before, our data indicate that in IMRT treatment for NPC, the plans with the MMLC show the more optimized dose coverage (CI: Pgtv 0.790 ⫾ 0.036 and PTV 0.858 ⫾ 0.026) and the better dose homogeneity for the targets (HI: Pgtv 1.062 ⫾ 0.011 and PTV 1.185 ⫾ 0.011) than those plans with the SMLC (CI: Pgtv 0.754 ⫾ 0.038 and PTV 0.850 ⫾ 0.021; HI: Pgtv 1.07 ⫾ 0.01 and PTV 1.195 ⫾ 0.011) (p ⬍ 0.05). In addition, we also find that to achieve the similar dose coverage of the targets, the delivering dose with the MMLC decreased significantly, compared with that of the SMLC (p ⬍ 0.01), indicating that the IMRT plans with MMLC are more efficient. Although these differences we observed were somewhat small and could be ignored in clinical practice, our results still indicate the
Table 5. Comparisons of the DVH-based parameters of other OARs in the present study (n ⫽ 10) Dmean (cGy)
Dmax (cGy)
gEUD (cGy)
OARs
MLC Leaf Width
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
Mean ⫾ SD
Right parotid gland
10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm
2574.8 ⫾ 78.9a 2615.0 ⫾ 47.8a 2607.2 ⫾ 41.8* 2644.4 ⫾ 40.6a 4270.2 ⫾ 411.3 4238.5 ⫾ 382.6 4426.1 ⫾ 528.4 4507.4 ⫾ 457.1 4495.2 ⫾ 166.2 4485.9 ⫾ 146.1 4358.6 ⫾ 254.8 4329.9 ⫾ 273.8 4413.2 ⫾ 164.6 4396.4 ⫾ 158.2
0.18
6266.9 ⫾ 286.9 6289.1 ⫾ 371.8 6317.0 ⫾ 538.4 6319.4 ⫾ 485.8 5844.1 ⫾ 435.9 5783.7 ⫾ 504.9 5984.5 ⫾ 355.1 6032.6 ⫾ 257.9 5326.9⫾216.2 5332.1 ⫾ 216.7 5400.0 ⫾ 281.8 5370.6 ⫾ 265.3 4890.6 ⫾ 139.6 4854.1 ⫾ 152.2
0.88
3317.2 ⫾ 111.4 3356.2 ⫾ 91.8 2820.9 ⫾ 113.8 2858.2⫾114.7 4346.6 ⫾ 553.5 4340.7 ⫾ 567.9 4395.4 ⫾ 518.5 4505.9 ⫾ 462.2 4494.9 ⫾ 162.7 4489.2 ⫾ 141.8 4492.3 ⫾ 163.1 4464.6 ⫾ 187.7 4590.9 ⫾ 164.8 4493.9 ⫾ 179.6
Left parotid gland Right joint Left joint Right inner ear Left inner ear Optic nerve/chiasm
0.06 0.86 0.72 0.89 0.81 0.82
0.99 0.78 0.73 0.96 0.81 0.58
NTCP (%)
p-Value Mean ⫾ SD p-Value 0.41 0.47 0.98 0.62 0.93 0.73 0.22
17.8 ⫾ 6 17.9 ⫾ 7 17.2 ⫾ 8 18.6 ⫾ 8 1.2 ⫾ 1 1.3 ⫾ 1 2.2 ⫾ 4.1 2.4 ⫾ 3.7 8.4 ⫾ 4.4 8.4 ⫾ 4.2 9.3 ⫾ 5.6 8.8 ⫾ 5.8 8.7 ⫾ 4.3 8.5 ⫾ 4.5
0.97 0.71 0.83 1 1 0.69 1
Abbreviations: DVH, dose volume histogram; OARs, organs at risk; MLC, multileaf collimator; Dmean and Dmax, average and maximum irradiation dose; gEUD, generalized equivalent uniform dose; NTCP, normal tissue complication probability. *Values represented as D50 (irradiation dose received by the 50% volume of parotid gland); Right and Left joint ⫽ right and left temporo-mandibular joint.
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Fig. 3. A beam’s-eye view of the MMLC (left) and SMLC (right) conforming to the targets of one patient abutting the brainstem, spinal cord, and optic nerve. The SMLC has a pair of movable backup jaws (gray arrows) that obviously decrease beam transmission in IMRT plans, whereas the MMLC only has a pair of the unmovable backup jaws (white arrows).
advantages of the MMLC and represent the ongoing progress of the precise radiotherapy techniques. Because the MMLC could achieve a more conformal dose distribution for the targets, it should achieve a more dose sparing of the OARs. In several reports, several researchers found that the small leaf width MLC improves surrounding tissue-sparing compared with the SMLC. Wang et al. reported that when used in a conventional fractionation scheme, IMRT with the MMLC for the prostate may reduce the toxicity of the critical organs.25 Wu et al. found that the radiosurgery for small lesions in cases involving complex target/OAR geometry will especially benefit from the use of a fine leaf width.10 Similar conclusions were reported by other investigators as well.11,12 Unlike these conclusions, in our study, the IMRT plans with the MMLC have no advantage on the dose sparing of the OARs compared with plans with the SMLC (p ⬎ 0.05). One possible explanation should be addressed here. The data in this study are based on 2 commercial Elekta MLC devices: the 10-mm leaf widths SMLC, which is equipped in the gantry head, has additional backup jaws that travel in the same direction as the MLC, and is full in the sense that they are not segmented; and the MMLC, which only has the unmovable backup jaws (Fig. 3). Consequently, the total transmission of the SMLC calculated in the TPS would be the fixed-jaw transmission factor (backup-jaw transmission factor 0.11) multiplied by the MLC transmission factor (0.003), whereas only a MLC transmission factor (0.007) would be applied in treatment planning of the MMLC.26 In our photon beam modeling and commissioning process, we also found that the total transmission of the MMLC out of the field was more noticeable than that of the SMLC. Galvin stated that the paired jaws of the SMLC could work together to bring the leakage radiation down to 0.5%,27 so the OARs
might receive less transmission dose in radiotherapy with the SMLC. To our knowledge, the different commercial MLC devices have respective technical characteristics. It should be emphasized that the data evaluated in this study only specifically depend on these 2 commercial Elekta MLC devices. To put it another way, the conclusions drawn from this study should not be applied directly to a different planning, delivery, and data translating system. Even for an identical system, the conclusions should be carefully verified by the treating physician and physicist. Nevertheless, the methodology presented here could be adopted by clinics with similar devices and software to optimize IMRT plan quality and delivery efficiency for NPC. In addition, our study indicates a potential effect of the movable backup jaws on the protection for OAR in radiotherapy. CONCLUSIONS In this analysis of the plans with these 2 commercial Elekta MLC devices, compared with the SMLC, the MMLC showed more optimal target coverage and more efficient dose delivery, but failed to improve the dose sparing of the OARs in IMRT for NPC. REFERENCES 1. Jason, C.H.; Cheng, K.S.; Chao, C.; et al. Comparison of intensity modulated radiation therapy (IMRT) treatment techniques for nasopharyngeal carcinoma. Int. J. Cancer. (Radiat. Oncol. Invest.) 96:126 –31; 2001. 2. Marcial, V.A.; Hanley, J.A.; Chang, C.; et al. Split-course radiation therapy of carcinoma of the nasopharynx: Results of a national collaborative clinical trial of the Radiation Therapy Oncology Group. Int. J. Radiat. Oncol. Biol. Phys. 6:409 –14; 1980. 3. Al-Sarraf, M.; Pajak, T.F.; Cooper, J.S.; et al. Chemoradiotherapy in patients with locally advanced nasopharyngeal carcinoma: A
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Radiation Therapy Oncology Group study. J. Clin. Oncol. 8:1342–51; 1990. Al-Sarraf, M.; LeBlanc, M.; Giri, P.G.; et al. Chemoradiotherapy versus radiotherapy in patients with advanced nasopharyngeal cancer: Phase III randomized Intergroup study 0099. J. Clin. Oncol. 16:1310 –7; 1998. Han, C.H.; Chen, Y.J.; Lin, A.; et al. Actual dose variation of parotid glands and spinal cord for nasopharyngeal cancer patients during radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 70:1256 – 62; 2008. Longobardia, B.; Martina, E.D.; Fiorinoa, C.; et al. Comparing 3DCRT and inversely optimized IMRT planning for head and neck cancer: Equivalence between step-and-shoo and sliding window techniques. Radiother. Oncol. 77:148 –56; 2005. Xia, P.; Fu, K.K.; Wong, G.W.; et al. Comparison of treatment plans involving intensity-modulated radiotherapy for nasopharyngeal carcinoma. Int. J. Radiat. Oncol. Biol. Phys. 48:329 –37; 2000. Zhu, X.R.; Prado, K.; Liu, H.H.; et al. Intensity-modulated radiation therapy for mesothelioma: Impact of multileaf collimator leaf width and pencil beam size on planning quality and delivery. Int. J. Radiat. Oncol. Biol. Phys. 62:1525–34; 2005. Monk, J.E.; Perks, J.R.; Doughty, D.; et al. Comparison of a micro-multileaf collimator with a 5-leaf-width collimator for intracranial stereotactic radiotherapy. Int. J. Radiat. Oncol. Biol. Phys. 57:1443–9; 2003. Wu, Q.J.; Wang, Z.H.; Kirkpatrick, J.P.; et al. Impact of collimator leaf width and treatment technique on stereotactic radiosurgery and radiotherapy plans for intra- and extracranial lesions. Radiat. Oncol. 4:3; 2009. Jin, J.Y.; Yin, F.F.; Ryu, S.; et al. Dosimetric study using different leaf-width MLCs for treatment planning of dynamic conformal arcs and intensity-modulated radiosurgery. Med. Phys. 32:405–7; 2005. Ding, M.S.; Newman, F.; Chen, C.H.; et al. Dosimetric comparison between 3DCRT and IMRT using different multileaf collimators in the treatment of brain tumors. Med. Dosim. 34:1– 8; 2009. Cheung, K.Y.; Choi, P.H.K.; Chau, R.M.C.; et al. The roles of multileaf collimators and micro-multileaf collimators in conformal and conventional nasopharyngeal carcinoma radiotherapy treatments. Med. Phys. 26:2077– 85; 1999.
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14. Integrated MLC (MLCi) for Precise Treatment SystemTM Functional Description. Elekta Ltd, Document no. 45133710188; 2005. 15. Patel, I.; Glendinning, A.G.; Kirby, M.C. Dosimetric characteristics of the Elekta Beam ModulatorTM. Phys. Med. Biol. 50:5479 – 92; 2005. 16. Gong, Y.L.; Wang, J.; Bai, S.; et al. Conventionally-fractionated image-guided intensity modulated radiotherapy (IG-IMRT): A safe and effective treatment for cancer spinal metastasis. Radiat. Oncol. 3:11; 2008. 17. Beam ModulatorTM Functional Description. Elekta Ltd, Document no. 45133710486; 2005. 18. American Joint Committee on Cancer. Manual for Staging of Cancer. 5th ed. Philadelphia: JB Lippincott; 1997. 19. International Commission on Radiation Units and Measurements. Prescribing, Recording, and Reporting photon beam therapy. Bethesda, MD: Author; 1993. 20. Wambersie, A.; Landberg, T. ICRU Report 62: Prescribing, recording and reporting photon beam therapy (Supplement to ICRU Report 50). 1999. 21. Nowak, P.J.; Wijers, O.B.; Lagerwaard, F.J.; et al. A three-dimensional CT-based target definition for elective irradiation of the neck. Int. J. Radiat. Oncol. Biol. Phys. 45:33–9; 1999. 22. Bratengeier, K.; Meyer, J.; Flentje, M. Research Pre-segmented 2-Step IMRT with subsequent direct machine parameter optimization—a planning study. Radiat. Oncol. 3:38; 2008. 23. Kubo, H.D.; Wilder, R.B.; Pappas. C.T.E. Impact of collimator leaf width on stereotactic radiosurgery and 3D conformal radiotherapy treatment plans. Int. J. Radiat. Oncol. Biol. Phys. 44:937– 45; 1999. 24. Burmeister, J.; McDermott, P.N.; Bossenberger, T.; et al. Effect of MLC leaf width on the planning and delivery of SMLC IMRT using the CORVUS inverse treatment planning system. Med. Phys. 31:3187–93; 2004. 25. Wang, L.; Hoban, P.; Paskalev, K.; et al. Dosimetric advantage and clinical implication of a micro-multileaf in the treatment prostate with intensity-modulated radiotherapy. Med. Dosim. 30:97–103; 2005. 26. Clinical User Manual for Physics of Pinnacle3 8.0. Philips Medical System 6.10:129; 2006. 27. Galvin, J.M. The Multileaf Collimator—A Complete Guide. Available at: https://www.aapm.org/meetings/99AM/pdf/2787-9625.pdf. Accessed 2009.
doi:10.1016/j.meddos.2010.02.007
IMPACTS OF MULTILEAF COLLIMATORS LEAF WIDTH ON INTENSITY-MODULATED RADIOTHERAPY PLANNING FOR NASOPHARYNGEAL CARCINOMA: ANALYSIS OF TWO COMMERCIAL ELEKTA DEVICES SHICHAO WANG, B.S., YOULING GONG, M.D., PH.D., QINGFENG XU, M.S., SEN BAI, PH.D., YOU LU, M.D., QINGFENG JIANG, and NIANYONG CHEN, M.D., PH.D. Radiation and Physics Center; Department of Thoracic Oncology and Radiation Oncology; State Key Laboratory of Biotherapy; and Department of Head & Neck Oncology and Radiation Oncology, West China Hospital, Sichuan University, Chengdu, Sichuan Province, PR China (Received 7 July 2009; accepted 18 February 2010)
Abstract—We compared the impacts of multileaf collimator (MLC) widths (standard MLC width of 10 mm [SMLC] and micro-MLC width of 4 mm [MMLC]) on intensity-modulated radiotherapy (IMRT) planning for nasopharyngeal carcinoma (NPC). Ten patients with NPC were recruited in this study. In each patient’s case, plans were generated with the same machine setup parameter and optimizing methods in a treatment planning system according to 2 commercial Elekta MLC devices. All of the parameters were collected from dose-volume histograms of paired plans and evaluated. The average conformity index (CI) and homogeneous index (HI) for the planning gross target volume in IMRT plans with MMLC were 0.790 ⴞ 0.036 and 1.062 ⴞ 0.011, respectively. Data in plans with SMLC were 0.754 ⴞ 0.038 and 1.070 ⴞ 0.010, respectively. The differences were statistically significant (p < 0.05). Compared with CI and HI for planning target volume in paired plans, data with MMLC obviously were better than those with SMLC (CI: 0.858 ⴞ 0.026 vs. 0.850 ⴞ 0.021, p < 0.05; and HI: 1.185 ⴞ 0.011 vs. 1.195 ⴞ 0.011, p < 0.05). However, there was no statistical significance between evaluated parameters (Dmean, Dmax, D5, gEUD, or NTCP) for organs at risk (OARs) in the 2 paired IMRT plans. According to these two kinds of Elekta MLC devices, IMRT plans with the MMLC have significant advantages in dose coverage for the targets, with more efficiency in treatment for NPC but fail to improve dose sparing of the OARs. © 2011 American Association of Medical Dosimetrists. Key Words: Multileaf collimators, Leaf width, Intensity-modulated radiotherapy, Nasopharyngeal carcinoma.
INTRODUCTION
powerful tool to improve the quality of the delivered dose distribution in head and neck radiotherapy both for its radiation of planning target volume (PTV) and the sparing of OARs.6,7 The introduction of the multileaf collimator (MLC) has streamlined the radiotherapy process, although the target conformity is limited by the discrete step size of the leaves. For some kinds of tumors, the anatomy location between targets and OARs is very close and complicated; MLC leaf widths may affect the dose distribution of targets and OARs because of the differences of every single MLC to conform to the outline of targets, which have a close relationship with the leaf width of the MLC. Recently, a few researchers evaluated impacts of MLC leaf width on treatment planning for several kinds of tumors, and the results are somewhat controversial.8 –12 So far, few studies investigated the impact of MLC leaf width on IMRT planning for NPC, although 1 case and 3 cases have been studied by Monk et al. and Jin et al., respectively.9,11 Although Cheung et al. studied the treatment planning for NPC and concluded that a 3-mm MLC is more suitable for tumor volumes very close to the critical normal structures than a 10-mm MLC, the technique they evaluated was the 3D-CRT.13
Nasopharyngeal carcinoma (NPC) is one of the most common head and neck cancers in Asia and Africa, but the incidences are rare in Western countries.1 Patients with NPC are typically treated with radiation therapy rather than surgery because of the anatomically challenging and difficult location and a demonstrated favorable response to radiotherapy and chemotherapy.2– 4 It has been considered that definitive radiotherapy with or without chemotherapy is the standard treatment of NPC.5 For patients with NPC, it is crucial for treatment success to minimize the dose to the surrounding organs at risk (OARs) while maintaining adequate radiation dose coverage to the target volume. In the recent 2 decades, IMRT has been stated to have advantages not only in physical dosimetry, but also in clinical practice compared with 3D conformal radiotherapy (3D-CRT). As Longobardia et al. and Xia et al. summarized, intensitymodulated radiotherapy (IMRT) was recognized as a The first two authors contributed equally to this work. Reprint requests to: Youling Gong, M.D., Ph.D., Department of Thoracic Oncology and Radiation Oncology, Tumor Center, West China Hospital, Sichuan University, Chengdu, Sichuan Province, PR China. E-mail: [email protected] 153
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Here, for the first time, we conducted a study to compare the impacts of MLC leaf width in IMRT planning for NPC. All plans were generated with our therapy planning system (TPS, Philips Pinnacle3 version 8.0 m, Milpitas, CA, USA) commissioned for the MLC device on the Elekta Precise Treatment System (Elekta Oncology System, Stockholm, Sweden) and the MLC device on the Elekta Synergy Treatment System.
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whole group was 43 years (range, 35–56). All patients were staged according to the 1997 UICC/AJCC staging system.18 The number of patients in Stage II was 2 (20%), Stage III 7 (70%), and Stage IVa 1 (10%). The T and N stage distribution in the present study is shown in Table 1. Permission to conduct the study was granted by the Research Ethics Board of the University Health Network. Clinically, 6 patients accepted treatment with the SMLC device and 4 patients accepted treatment with the MMLC device.
METHODS AND MATERIALS Collimators The standard MLC (SMLC) we analyzed is the Elekta MLCi, which is equipped in the Elekta Precise Treatment System. The SMLC is fully integrated in the linear accelerator both physically within the standard collimator housing and electrically within the control system. The maximum clearance between the isocenter and collimator is 45 cm. The diameter and the height of the SMLC are 62 and 42 cm, respectively. The leaf width of the SMLC is 10 mm at the isocenter. It has 80 leaves (40 pairs), with a travel range of 32.5 cm in the y-direction (12.5 cm beyond the central axis) covering a full 40 ⫻ 40-cm field. Preparation and treatment of non-MLC and MLC prescriptions is controlled via a single control console. There are no separate dedicated displays for the SMLC. The conventional collimators are in the x-direction. The backup diaphragms in the y-direction ensure a minimum X-ray leakage specification.14 There is a minimum leaf gap across banks. The micro-MLC (MMLC) we analyzed is another commercial system newly installed in our center, the Elekta Beam Modulator (Elekta Oncology Systems, Crawley, UK), which is equipped on the Elekta Synergy Treatment System.15,16 It has 80 individually controlled leaves that have a travel range of ⬎21 cm for each leaf. Each of the leaves is capable of interdigitation and projects a width of 0.4 cm at the isocenter. The true rectangular field size is 16 ⫻ 21 cm. The leaf bank is positioned approximately 39 cm from the target. There are no movable backup diaphragms and therefore the field is defined only by the open leaves.15 Leaf movements are achieved by individual motors using a rack and pinion drive to the leaf. The position readout is controlled by a high-resolution optical system. The speed of leaf movement is 0.2–3.0 cm per second.17 As a result, there would be a variation in penumbra as the leaves move to different positions with respect to the central axis. To counter this effect, the leaf ends are rounded. It is possible to create islands of radiation because it is not necessary to have all open leaves defined in a contiguous block.15
Target delineation and dose prescription All of the patients were immobilized with head and neck thermoplastic masks. Each of the patients in this study underwent a spiral computer tomography (CT, Siemens Sensation 4) with 3-mm-slice thickness in the supine position. Three radio-opaque ball markers made of lead and fixed on a tiny self-sticker paper were placed coplanarly on the surface of the mask to mark the CT reference center; 2 were on the lateral sides and 1 was on the anterior. All of the CT images of the patients acquired were transferred to and registered in the TPS with the same method and a standard of couch removal and laser center localization. All target delineations in this study were in accordance with ICRU 50 and 62 reports.19,20 The gross tumor volume (GTV) included all known gross disease (primary tumor ⫹ grossly enlarged lymph nodes) as determined by the imaging, clinical, and endoscopic findings. A planning gross target volume (Pgtv) was created based on the GTV with an additional 3-mm margin (GTV ⫹ 3 mm), allowing for any setup variability to prevent the gross tumor from being missed by the high-dose zone. The clinical target volume (CTV) was defined as the GTV ⫹ 1-cm margin (0.5-cm margin posteriorly) to encompass any microscopic extension, together with the regional lymphatics. Regional lymphatics were outlined according to the recommendation by Nowak et al.21 Surgical neck levels Ib (submandibular nodes) to V were electively irradiated. Submental region (level Ia) was included if the submandibular nodes or oral cavity were grossly involved. The PTV was the CTV ⫹ 3-mm margin, based on the observation of the average setup variability of our institution. Critical normal structures were contoured as OARs, including brainstem, spinal cord, parotid glands, optic nerves/chiasm, eyeballs, temporomandibular joints, and inner ears. A 3-mm planning organ-at-risk volume
Table 1. T- and N-stage distribution of the patients in this study (n ⫽ 10) T-Stage
Patient data This study was conducted between March 2007 and October 2008. Ten patients with pathologically confirmed NPC were recruited in the present study. There were 8 males and 2 females, and the median age for the
N-Stage
1
2
3
4
0 1 2 3
0 0 0 0
0 2 1 0
2 3 1 0
0 1 0 0
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Fig. 1. Case-by-case comparison of MUs between IMRT plans with MMLC and SMLC (10 and 4 mm ⫽ MLC leaf widths of 10 and 4 mm, respectively).
and evaluated using the TPS mentioned previously. The plans were generated using nine coplanar beams whose angels were 200°, 240°, 280°, 320°, 0°, 40°, 80°, 120°, and 160°, respectively. In plan generation, the direct machine parameter optimization (DMPO, Raysearch Laboratories, Stockholm, Sweden) method was applied. Such algorithms are clearly superior to those with sequencing after a complete fluence optimization process.22 The plan optimizing based on the 2 MLC devices was generated with the same dose constraints and optimizing parameters. The maximum iterations of plan optimizing were 40, and the maximum number of all segments in 1 plan was restricted to 100. There is no limitation to the minimum monitor units (MUs) per segment.
(PRV)-margin was added to the spinal cord and brainstem during optimization to allow for possible positional errors. The planned treatment for each patient consisted of 68.8 Gy to be delivered to Pgtv in 32 fractions, 60.8 Gy for PTV in 32 fractions, and the prescription dose covered at least 95% of the volume of Pgtv and PTV, respectively. The maximum tolerance doses to the critical normal structures were as follows: brain stem 54 Gy, spinal cord 45 Gy, optic nerve and chiasm 54 Gy, inner ears 54 Gy, and parotid gland (V26 ⬍50%). Because identical lower neck portals were generated in the IMRT plans with the different MLC devices, the dose-volume histograms (DVHs) of the tumor targets and normal structures in the lower neck and supraclavicular regions were not compared.
Evaluation of DVH-based parameters The conformity index (CI) for Pgtv was calculated as per the formula: CI ⫽ Pgtvref/Pgtv ⫻ Pgtvref/Vref.8 The Pgtvref is the overlap volume between the Pgtv and volume of prescription isodose surface. The Vref is tissue volume that is enclosed by the prescription isodose surface also outside of Pgtv. The prescription isodose was 100% isodose to Pgtv. The same method was applied in analysis of PTV. The higher the CI is, the more conformal the plan is. The homogeneous index (HI) for the targets was defined as follows: HI ⫽ D5/D95, where D5 and D95 are the doses received by the 5% and 95% volumes of Pgtv and PTV. The more D5 and D95 approach each other, the steeper the target’s curve in DVHs. The generalized equivalent uniform dose (gEUD) and normal tissue complication probability (NTCP) for OARs were calculated by the TPS automatically.
Treatment planning and optimizing In the process of treatment planning setup and optimizing, all of the inverse IMRT plans were generated
Statistical analysis The statistical analysis was performed using SPSS software (version 13.0, Chicago, IL). All data were an-
Fig. 2. Transverse sections of the representative IMRT plans of 1 patient with the irradiation isodose curves (10 and 4 mm ⫽ MLC leaf widths of 10 and 4 mm, respectively).
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Table 2. Comparisons of the DVH-based parameters for targets in the present study (n ⫽ 10) MLC Leaf Width Targets Pgtv Dmax Dmin Dmean PTV Dmean
10 mm
4 mm
p-Value
7570.0 ⫾ 90.1 5976.8 ⫾ 327.4 7173.7 ⫾ 28.4
7531.8 ⫾ 83.9 6132.2 ⫾ 247.0 7166.7 ⫾ 25.3
0.34 0.25 0.57
6600.9 ⫾ 65.8
6592.7 ⫾ 60.9
0.78
Abbreviations: DVH, dose volume histogram; MLC, multileaf collimator; Pgtv, planning gross target volume; PTV, planning target volume; Dmax, Dmin, and Dmean, maximum, minimum, and average irradiation dose of the targets received.
alyzed applying “mean ⫾ SD.” Using Student’s t-test, a value of p ⬍ 0.05 (2-tailed) was considered statistically significant.
RESULTS Two IMRT treatment plans were generated for each of the patients in this study, based on the different Elekta MLC devices, respectively. In total, 20 IMRT plans were generated after the protocol and analyzed. Figure 1 shows the case-by-case comparison of the delivering MUs between these paired IMRT plans, indicating that the average MUs in plans with the MMLC (698.2 ⫾ 35.5) are significantly lower than those with the SMLC (745.7 ⫾ 33.6) (p ⬍ 0.01). The transverse sections of the representative IMRT plans of one patient are shown in Fig. 2, and it is obvious that the irradiation dose curves are similar. The evaluation of the DVH-based parameters of the targets is shown in Table 2. The maximum, minimum, and average doses of Pgtv and the average dose of PTV
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are similar between these 2 kinds of plans, respectively, with no statistical significance (p ⬎ 0.05). The comparisons of the dose conformality for the targets are summarized in Table 3. The average CI for Pgtv (0.790 ⫾ 0.036) and PTV (0.858 ⫾ 0.026) in IMRT plans with the MMLC are significantly better than those indexes in plans with the SMLC (for Pgtv: 0.754 ⫾ 0.038 and for PTV: 0.850 ⫾ 0.021) (p ⬍ 0.05). Also, the average HI for Pgtv (1.062 ⫾ 0.011) and PTV (1.185 ⫾ 0.011) in plans with MMLC are better than those in plans with the SMLC (for Pgtv: 1.070 ⫾ 0.010 and for PTV: 1.195 ⫾ 0.011) with the statistical significance of p ⬍ 0.05. Table 4 shows the comparisons of the DVH-based parameters of the spinal cord and the brainstem in the present study. Compared with the plans with the SMLC, the IMRT plans with the MMLC have no advantage in dose sparing of the spinal cord and brainstem. The differences of Dmax, D5, and gEUD of the spinal cord and the brainstem between the 2 groups are not statistically significant (p ⬎ 0.05). The comparisons of the DVH-based parameters of other OARs in the present study are shown in Table 5. Compared with the IMRT plans with the SMLC, the IMRT plans with the MMLC do not show advantage in dose sparing of the other evaluated OARs (D50, Dmax, gEUD, and NTCP for the parotid glands; Dmean, Dmax, gEUD, and NTCP for the temporomandibular joints, inner ears, and optic nerve/chiasm) (p ⬎ 0.05). DISCUSSION In the IMRT plan generation, there are so many factors affecting the dose distributions around the targets and OARs, such as the target delineation, inter/intrafractional tumor motion, patient’s set-up errors, beam orientation, and numbers. As with many studies mentioned here
Table 3. Comparisons of CI and HI for Pgtv and PTV in the present study (n ⫽ 10) Pgtv
PTV
CI Patient Number
10 mm
HI 4 mm
10 mm
CI 4 mm
10 mm
HI 4 mm
10 mm
4 mm
1 0.780 0.822 1.087 1.077 0.868 0.874 1.207 1.200 2 0.789 0.821 1.077 1.073 0.863 0.871 1.201 1.193 3 0.817 0.838 1.066 1.063 0.856 0.864 1.205 1.197 4 0.780 0.794 1.069 1.058 0.850 0.862 1.196 1.184 5 0.697 0.771 1.066 1.064 0.863 0.877 1.174 1.171 6 0.738 0.793 1.067 1.057 0.864 0.881 1.190 1.183 7 0.762 0.797 1.056 1.046 0.825 0.824 1.190 1.176 8 0.749 0.796 1.068 1.063 0.873 0.883 1.192 1.182 9 0.715 0.718 1.059 1.046 0.816 0.815 1.186 1.170 10 0.717 0.752 1.087 1.072 0.824 0.827 1.213 1.197 Average 0.754 ⫾ 0.038 0.790 ⫾ 0.036 1.070 ⫾ 0.010 1.062 ⫾ 0.011 0.850 ⫾ 0.021 0.858 ⫾ 0.026 1.195 ⫾ 0.011 1.185 ⫾ 0.011 (mean ⫾ SD) p-value 0.04 0.04 0.04 0.04 Abbreviations: CI, conformity index; HI, homogeneous index; Pgtv, planning gross target volume; PTV, planning target volume; 10 and 4 mm, multileaf collimator leaf widths as 10 and 4 mm, respectively.
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Table 4. Comparisons of the DVH-based parameters of spinal cord and brain stem in the present study (n ⫽ 10) Dmax
D5
gEUD
MLC Leaf Width
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
10 mm 4 mm 10 mm 4 mm
4454.5 ⫾ 173.8 4484.1 ⫾ 175.0 4998 ⫾ 546 4927.1 ⫾ 477.8
0.71
3921.4 ⫾ 195.2 3991.2 ⫾ 137.1 4868.2 ⫾ 187.5 4885.7 ⫾ 204.9
0.37
2930.5 ⫾ 616.5 3023.1 ⫾ 580.2 3989.0 ⫾ 234.3 4154.6 ⫾ 223.4
0.73
Spinal cord Brainstem
0.76
0.84
0.12
Abbreviations: DVH, dose volume histogram; MLC, multileaf collimator; Dmax, maximum irradiation dose; D5, irradiation dose received by the 5% of volume of OARs; gEUD, generalized equivalent uniform dose.
before, the leaf widths of the MLC may have an impact on the dose distribution in IMRT dose delivery accuracy. For the first time, we conducted a serial study focusing on the effect of the MLC leaf widths on the dose distribution in IMRT for NPC. Our results indicate that the IMRT plans with the MMLC are better than those plans with SMLC in the dose conformity and dose delivering efficiency. However, we find no significant difference on the dose sparing of the OARs between the 2 kinds of plans (p ⬎ 0.05). The IMRT technique has advantages not only on the 3D conformation to the tumor structure, but also on the dose protection for normal tissues around the targets. In theory, the less MLC leaf widths are, the better the dose optimization of an IMRT plan would be because it could achieve a more conformal shape around the tumor. In previously published studies, the conclusions have been made that the MMLC was better in the target’s conformity and uniformity of the dose distribution. Kubo et al. reported that the 1.7–3.0-mm MLCs could spare more bladder and rectum than the 10-mm MLC for prostate treatment planning.23 Monk et al. compared the 3- and 5-mm MLCs in intracranial radiosurgery planning and observed that the 3-mm MLC improves the conformity of PTV. However, the quantitative differences between
the 3- and 5-mm MLC collimation (5% for tissue sparing) may not be clinically significant for some cases.9 Jin et al. found that the 3-mm MLC has the better conformity parameters than those of the 5- and 10-mm MLCs; the advantage decreases when the target volume increases.11 Conversely, Burmeister et al. reported that there was no apparent clinically significant difference between the 5and 10-mm MLCs except for very small target volumes or those with concavities that are small, with respect to the MLC leaf widths.24 Similar to the studies mentioned before, our data indicate that in IMRT treatment for NPC, the plans with the MMLC show the more optimized dose coverage (CI: Pgtv 0.790 ⫾ 0.036 and PTV 0.858 ⫾ 0.026) and the better dose homogeneity for the targets (HI: Pgtv 1.062 ⫾ 0.011 and PTV 1.185 ⫾ 0.011) than those plans with the SMLC (CI: Pgtv 0.754 ⫾ 0.038 and PTV 0.850 ⫾ 0.021; HI: Pgtv 1.07 ⫾ 0.01 and PTV 1.195 ⫾ 0.011) (p ⬍ 0.05). In addition, we also find that to achieve the similar dose coverage of the targets, the delivering dose with the MMLC decreased significantly, compared with that of the SMLC (p ⬍ 0.01), indicating that the IMRT plans with MMLC are more efficient. Although these differences we observed were somewhat small and could be ignored in clinical practice, our results still indicate the
Table 5. Comparisons of the DVH-based parameters of other OARs in the present study (n ⫽ 10) Dmean (cGy)
Dmax (cGy)
gEUD (cGy)
OARs
MLC Leaf Width
Mean ⫾ SD
p-Value
Mean ⫾ SD
p-Value
Mean ⫾ SD
Right parotid gland
10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm 10 mm 4 mm
2574.8 ⫾ 78.9a 2615.0 ⫾ 47.8a 2607.2 ⫾ 41.8* 2644.4 ⫾ 40.6a 4270.2 ⫾ 411.3 4238.5 ⫾ 382.6 4426.1 ⫾ 528.4 4507.4 ⫾ 457.1 4495.2 ⫾ 166.2 4485.9 ⫾ 146.1 4358.6 ⫾ 254.8 4329.9 ⫾ 273.8 4413.2 ⫾ 164.6 4396.4 ⫾ 158.2
0.18
6266.9 ⫾ 286.9 6289.1 ⫾ 371.8 6317.0 ⫾ 538.4 6319.4 ⫾ 485.8 5844.1 ⫾ 435.9 5783.7 ⫾ 504.9 5984.5 ⫾ 355.1 6032.6 ⫾ 257.9 5326.9⫾216.2 5332.1 ⫾ 216.7 5400.0 ⫾ 281.8 5370.6 ⫾ 265.3 4890.6 ⫾ 139.6 4854.1 ⫾ 152.2
0.88
3317.2 ⫾ 111.4 3356.2 ⫾ 91.8 2820.9 ⫾ 113.8 2858.2⫾114.7 4346.6 ⫾ 553.5 4340.7 ⫾ 567.9 4395.4 ⫾ 518.5 4505.9 ⫾ 462.2 4494.9 ⫾ 162.7 4489.2 ⫾ 141.8 4492.3 ⫾ 163.1 4464.6 ⫾ 187.7 4590.9 ⫾ 164.8 4493.9 ⫾ 179.6
Left parotid gland Right joint Left joint Right inner ear Left inner ear Optic nerve/chiasm
0.06 0.86 0.72 0.89 0.81 0.82
0.99 0.78 0.73 0.96 0.81 0.58
NTCP (%)
p-Value Mean ⫾ SD p-Value 0.41 0.47 0.98 0.62 0.93 0.73 0.22
17.8 ⫾ 6 17.9 ⫾ 7 17.2 ⫾ 8 18.6 ⫾ 8 1.2 ⫾ 1 1.3 ⫾ 1 2.2 ⫾ 4.1 2.4 ⫾ 3.7 8.4 ⫾ 4.4 8.4 ⫾ 4.2 9.3 ⫾ 5.6 8.8 ⫾ 5.8 8.7 ⫾ 4.3 8.5 ⫾ 4.5
0.97 0.71 0.83 1 1 0.69 1
Abbreviations: DVH, dose volume histogram; OARs, organs at risk; MLC, multileaf collimator; Dmean and Dmax, average and maximum irradiation dose; gEUD, generalized equivalent uniform dose; NTCP, normal tissue complication probability. *Values represented as D50 (irradiation dose received by the 50% volume of parotid gland); Right and Left joint ⫽ right and left temporo-mandibular joint.
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Volume 36, Number 2, 2011
Fig. 3. A beam’s-eye view of the MMLC (left) and SMLC (right) conforming to the targets of one patient abutting the brainstem, spinal cord, and optic nerve. The SMLC has a pair of movable backup jaws (gray arrows) that obviously decrease beam transmission in IMRT plans, whereas the MMLC only has a pair of the unmovable backup jaws (white arrows).
advantages of the MMLC and represent the ongoing progress of the precise radiotherapy techniques. Because the MMLC could achieve a more conformal dose distribution for the targets, it should achieve a more dose sparing of the OARs. In several reports, several researchers found that the small leaf width MLC improves surrounding tissue-sparing compared with the SMLC. Wang et al. reported that when used in a conventional fractionation scheme, IMRT with the MMLC for the prostate may reduce the toxicity of the critical organs.25 Wu et al. found that the radiosurgery for small lesions in cases involving complex target/OAR geometry will especially benefit from the use of a fine leaf width.10 Similar conclusions were reported by other investigators as well.11,12 Unlike these conclusions, in our study, the IMRT plans with the MMLC have no advantage on the dose sparing of the OARs compared with plans with the SMLC (p ⬎ 0.05). One possible explanation should be addressed here. The data in this study are based on 2 commercial Elekta MLC devices: the 10-mm leaf widths SMLC, which is equipped in the gantry head, has additional backup jaws that travel in the same direction as the MLC, and is full in the sense that they are not segmented; and the MMLC, which only has the unmovable backup jaws (Fig. 3). Consequently, the total transmission of the SMLC calculated in the TPS would be the fixed-jaw transmission factor (backup-jaw transmission factor 0.11) multiplied by the MLC transmission factor (0.003), whereas only a MLC transmission factor (0.007) would be applied in treatment planning of the MMLC.26 In our photon beam modeling and commissioning process, we also found that the total transmission of the MMLC out of the field was more noticeable than that of the SMLC. Galvin stated that the paired jaws of the SMLC could work together to bring the leakage radiation down to 0.5%,27 so the OARs
might receive less transmission dose in radiotherapy with the SMLC. To our knowledge, the different commercial MLC devices have respective technical characteristics. It should be emphasized that the data evaluated in this study only specifically depend on these 2 commercial Elekta MLC devices. To put it another way, the conclusions drawn from this study should not be applied directly to a different planning, delivery, and data translating system. Even for an identical system, the conclusions should be carefully verified by the treating physician and physicist. Nevertheless, the methodology presented here could be adopted by clinics with similar devices and software to optimize IMRT plan quality and delivery efficiency for NPC. In addition, our study indicates a potential effect of the movable backup jaws on the protection for OAR in radiotherapy. CONCLUSIONS In this analysis of the plans with these 2 commercial Elekta MLC devices, compared with the SMLC, the MMLC showed more optimal target coverage and more efficient dose delivery, but failed to improve the dose sparing of the OARs in IMRT for NPC. REFERENCES 1. Jason, C.H.; Cheng, K.S.; Chao, C.; et al. Comparison of intensity modulated radiation therapy (IMRT) treatment techniques for nasopharyngeal carcinoma. Int. J. Cancer. (Radiat. Oncol. Invest.) 96:126 –31; 2001. 2. Marcial, V.A.; Hanley, J.A.; Chang, C.; et al. Split-course radiation therapy of carcinoma of the nasopharynx: Results of a national collaborative clinical trial of the Radiation Therapy Oncology Group. Int. J. Radiat. Oncol. Biol. Phys. 6:409 –14; 1980. 3. Al-Sarraf, M.; Pajak, T.F.; Cooper, J.S.; et al. Chemoradiotherapy in patients with locally advanced nasopharyngeal carcinoma: A
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