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Effects of multileaf collimator parameters on treatment planning of intensity-modulated radiotherapy
Medical Dosimetry, Vol. 32, No. 1, pp. 38-43, 2007 Copyright © 2007 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/07/$–see front matter
doi:10.1016/j.meddos.2006.11.005
EFFECTS OF MULTILEAF COLLIMATOR PARAMETERS ON TREATMENT PLANNING OF INTENSITY-MODULATED RADIOTHERAPY This paper was presented at the 15th Asian Conference of Radiological Technologists, organized by the Japan Association of Radiological Technogists November 22, 2005, Chiba, Japan.
VINCENT W. C. WU Hong Kong Polytechnic University, Hung Hom, Hong Kong (Received 13 November 2006; accepted 15 November 2006)
Abstract—In inverse planning of intensity-modulated radiotherapy (IMRT), the setting of multileaf collimator (MLC) parameters affects the optimization algorithms and dose distribution. We investigated the effects of varying the MLC leaf width, leaf insertion percentage, and leaf increment in treatment planning of IMRT in 3 cancer cases: nasopharynx, esophagus, and prostate. Inverse planning of the 3 cancer cases was performed using the XiO treatment planning system. MLCs with 0.5 and 1.0 cm were used to evaluate the leaf width effect, whereas leaf insertions of 20%, 50%, and 80% were used to demonstrate the effect of leaf insertion percentage, and leaf increments of 0.5, 1.0, and 2.0 cm were used to study the leaf increment effect. The treatment plans were evaluated by dose profiles, tumor control probability (TCP), and normal tissue complication probability (NTCP). The 0.5-cm MLC leaves showed better TCPs and NTCPs than the 1.0-cm leaves in the 3 cancer cases, although the differences were less than 2.5%. For the leaf insertion percentage, the dose profile differences among the 3 levels of increments were minimal, and their differences in TCP and NTCP were extremely small (< 1.5%). The effect of leaf increment was more prominent, dose profile, TCPs, and NTCPs were best for the smallest leaf increment and they deteriorated as the leaf increment increased. Narrower leaves gave slightly better sparing of organs at risk (OAR)s; changing the leaf insertion percentage brought about negligible changes, whereas increasing the leaf increment significantly degraded the treatment plans. © 2007 American Association of Medical Dosimetrists. Key Words: Intensity-modulated radiotherapy, Multileaf collimator, Inverse planning, Dose evaluation.
INTRODUCTION
workstation. Although inverse planning in IMRT treatment planning employs an automatic optimization process, it still requires human intervention. Apart from the input of dose constraints and associated importance factors, the adjustment of MLC parameters also affects the optimization algorithm and subsequently the dose distribution.7 The width of MLC leaves affects the smoothness of the field aperture, which in turn may affect the conformity of dose to the planning target volume (PTV). For most linear accelerators, the common MLC leaf width is 1.0 cm at isocenter and covers a field size of up to 40 ⫻ 40 cm2. Recently, more sophisticated MLC systems that contain 60 pairs of leaves, with the central leaves at a width of 0.5 cm, such as the Millennium-120 dynamic MLC has been available. Jin et al.,8 in a study of intensity-modulated radiosurgery using dynamic conformal arc techniques, demonstrated that MLCs with finer leaf widths produce better dose conformity than those with thicker leaves in the treatment of small targets. However, a study of leaf width effect by Burmeister et al.9 on standard IMRT using the Corvus treatment planning system revealed that there was no apparent clinically significant difference between 1.0 and 0.5 cm leaf-width plans except that better sparing of organs at risk (OARs) was observed in the narrower-leaf plans for very small or concave-shaped target volumes.
Intensity-modulated radiotherapy (IMRT) has been increasingly practiced in radiotherapy. Its goals are to shape the high dose zone tightly to the target volume and minimize the dose to the adjacent normal organs. The dosimetric advantages of IMRT over conventional techniques have been documented by many studies.1– 6 Apart from the development of a more advanced treatment planning system with an inverse planning algorithm, one important factor that has brought about the success of IMRT is the inclusion of the dynamic multileaf collimators (MLCs) in linear accelerators. The programmed movement of the MLC leaves according to the treatment plan during each beam segment governs the intensity of each beamlet and ultimately determines the overall dose distribution to the irradiated volume. The resultant plan is able to deliver a highly conformal dose distribution to the target volume. The determination of the MLC motions within beam segments is a complicated task that is beyond the capability of the human brain, and therefore is mainly achieved by inverse planning using a dedicated computer
Reprint requests to: Dr. Vincent W. C. Wu, Department of Health Technology and Informatics, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mail: htvinwu@ inet.polyu.edu.hk 38
MLC parameters on treatment planning of IMRT ● V. W. C. WU
Apart from the leaf width, the leaf insertion percentage (the degree of leaf intrusion into the PTV at the circumference) and leaf increment (the measure of minimum leaf movement between segments) are 2 additional factors that need to be taken into account during the optimization of IMRT plans. We investigated the effects of varying the leaf width, leaf insertion percentage, and leaf increment in the treatment planning of IMRT on 3 different cancer cases: nasopharyngeal, esophageal, and prostatic carcinoma. The study should help treatment planners acquire knowledge about the setting of the above-mentioned MLC parameters in the inverse planning of IMRT and to appreciate the effects of altering these parameters to establish optimum values. METHODS AND MATERIALS The treatment planning system used in this study was XiO (CMS Inc.). The system uses a conjugate gradient optimization algorithm10,11 for IMRT plans, which is a type of gradient descent optimization algorithm. The conjugate gradient method uses the negative gradient of a cost function, with respect to input parameters, to systematically find the minimum of a cost function. The 3 subjects selected for this study were patients with nasopharyngeal, esophageal, and prostatic carcinoma. These 3 patients were selected because they involved tumors from the head-and-neck down to the pelvis with good variation in tumor size, shape, and geometry in relation to the OARs. For the nasopharyngeal patient, the PTV was small (volume 73.9 cm3) and showed slight concavity at the posterior aspect where it was adjacent to the brain stem or spinal cord. The PTV of the esophageal tumor was relatively large (208.7 cm3), especially along the longitudinal axis of the thoracic region, and was roughly circular in the transverse section. The prostate tumor was situated in close proximity between the urinary bladder and rectum, with a PTV volume of 128.6 cm3. The computed tomography (CT) images of the 3 patients were transferred to the treatment planning system. Before starting the optimization, the PTVs and OARs were delineated on the corresponding CT slices of each patient. The dose constraints and importance factors were input for the PTVs and OARs, which was followed by the setting of planning parameters including those for the MLC. Five equispaced beams were used in the nasopharynx and eosphagus cases, and an equispaced 7-beam arrangement was used for the prostate case. Tumor doses of 66, 60, and 70 Gy were prescribed to 95% of the PTV for the nasopharynx, esophagus, and prostate cases, respectively. The calculation grid used for the calculation of the final dose distribution for all treatment plans was 0.25 ⫻ 0.25 cm. To evaluate the effect of leaf width, a 2300CD and 600C/D linear accelerators (Varian Medical Systems, Inc.) were used, which were equipped with Millennium
39
MLC-120 (60 pairs of leaves, with the central leaves having a width of 0.5 cm) and 40 pairs of 1.0-cm wide leaves, respectively. Both leaf widths were measured at the isocenter. The study of the effects of the leaf insertion percentage was performed using the 1.0-cm leaf machine. Leaf insertions of 20%, 50%, and 80% were applied to each case. To facilitate comparison, all other planning parameters were kept constant. The same machine was used for the evaluation of the leaf increment factor. Leaf increments of 0.5, 1.0, and 2.0 cm were applied to each case to generate the IMRT plans, while the other parameters were kept unchanged. A number of plan evaluation tools were used to evaluate the dose distribution of the treatment plans. First, dose profiles from 2 perpendicular planes in x (lateral) and y (antero-posterior) directions across the target were generated (Fig. 1a-c). These profiles provided information about the dose gradients at the interface between the PTVs and adjacent OARs, and were an indication of the target dose conformity. Second, the tumor control probabilities (TCPs) and normal tissue complication probabilities (NTCPs) were calculated for the PTVs and OARs, respectively. These biological indices were by no means absolute, but provided a relative ranking of the treatment plans under comparison. In addition, the total number of MLC segments and monitor units (MUs) per beam generated from each treatment plan were also recorded. These provided an indication of the complexity of the MLC configuration, which in turn affects the treatment time and whole body dose to the patient. For each case, these parameters were compared across the plans with different MLC settings. RESULTS Effect of leaf width The differences in dose profiles between the 0.5 and 1.0-cm leaf-width plans were small in all 3 clinical cases, although the plans with narrower leaves showed a slight advantage over those of the wider leaves in most cases. IMRT plans using 0.5-cm leaves produced a slightly higher target TCP and a lower NTCP for most OARs than those using 1.0-cm leaf widths (Table 1). In the nasopharynx case, the spinal cord and lens received the most benefit from the finer leaves, whereas in the eosphagus and prostate cases, the left lung and rectum, respectively, were more effectively spared by the narrower MLC leaves. There was no definite pattern in the differences in the number of segments or the total MU per beam between the 2 leaf-width plans (Table 1). Effect of leaf insertion percentage The dose difference was minimal between IMRT plans with different leaf insertion percentages. All 3 clinical cases demonstrated no difference in the dose profiles. The differences in the TCPs and NTCPs across
40
Medical Dosimetry
Volume 32, Number 1, 2007
Fig. 1. CT slices showing the 2 perpendicular planes (x and y) for the generation of dose profiles for (a) nasopharynx, (b) esophagus, and (c) prostate cancer patients.
the 3 levels of leaf insertion percentage were small in all cases. A maximum variation in TCP of 1.23% was observed in the esophagus case, while the greatest variation in NTCP (0.31%) was demonstrated by the spinal cord in the nasopharynx case.
With regard to the number of MLC segments and total MU per beam, no definite pattern was observed. Comparatively higher numbers of segments were seen in the plans with 50% leaf insertion in the nasopharynx and esophagus cases, whereas a slight increase in the total
Table 1. Comparison between IMRT plans with 0.5-cm (LW05) and 1.0-cm (LW10) MLC leaf widths for the 3 cancer cases Nasopharynx
TCP (%) Segments/beam Total MU/beam NTCP (%)
S. cord B. stem Op. Ch. Lens TMJ Parotid
Esophagus
Prostate
LW05
LW10
LW05
LW10
LW05
LW10
78.5 27.4 3562 3.73 0.04 0.00 0.07 0.12 0.01
77.5 29.2 3646 4.15 0.10 1.01 0.07 0.13 0.04
78.8 41.6 5247 1.35 0.72 0.24 0.09
76.8 36.3 4694 1.60 1.01 0.34 0.09
80.2 28.0 4243 1.22 2.14 0.00 0.00
79.0 35.1 4847 1.32 4.35 0.00 0.00
S. cord L lung R lung Heart
Bladder Rectum L femur R femur
S. cord ⫽ spinal cord; B. stem ⫽ brain stem; Op. Ch. ⫽ optic chiasm; TMJ ⫽ temporo-mandibular joint; L ⫽ left; R ⫽ right.
MLC parameters on treatment planning of IMRT ● V. W. C. WU
Fig. 2. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for nasopharyngeal case: (a) y-direction and (b) x-direction.
41
weighting factors for the targets and OARs is required in inverse planning for IMRT.7,12 In addition, the determination of MLC parameters prior to the optimization process also affects the dosimetric outcome of the treatment plan. With regard to the effect of MLC leaf width, a slight dosimetric advantage was achieved by reducing the leaf width from 1.0 cm to 0.5 cm. This was reflected by slightly better dose profiles within the PTV regions and a slight improvement in the TCPs and NTCPs in most cases. This was largely in line with the results from Burmeister et al.9 except that the use of 0.5-cm leaves in this study did not show significant dosimetric advantage in the smaller size nasopharynx tumor over the other larger size tumors. Usually, plans with finer leaves are more complex than plans with wider leaves, in terms of number of beam segments and total MUs. Such results were observed in the prostate and nasopharynx cases but not in the esophagus case. This could be explained by the more simple convex PTV geometry of the esophagus case, in which the 0.5-cm MLC did not require extra beam segments to achieve the optimum dose distribution as for the other 2 cases with concave targets. The dose conformity effect of the MLC leaf insertion percentage in IMRT plans was negligible, as the dose profiles in all cases were nearly identical. This illustrated that leaf insertion percentage was not an influential factor in the final dose distribution. It was be-
MU per beam was demonstrated by the 80% leaf insertion plans in the nasopharynx and prostate cases. Effect of leaf increment The dose profiles of the 0.5-cm leaf increment plans were the best, which was followed by the plans with 1.0 and 2.0-cm increments. The same pattern was present in all 3 cancer cases (Figs. 2– 4). Similar trends were also observed in the TCP and NTCP values. The plans with the smallest leaf increment demonstrated the highest TCP, with a maximum difference of 7.7% over the 2.0-cm increment plan in the nasopharynx case (Table 2). The NTCPs of most OARs were also lowest in plans with the smallest leaf increment, and they increased with the leaf increment increase. Greater variations in the number of MLC segments and total MUs per beam were observed for the leaf increment tests (Table 2). The plans with the smallest increment demonstrated the greatest number of MLC segments and total MU per beam, followed by the 1.0 and 2.0-cm leaf increment plans. DISCUSSION The change from forward to inverse planning involves a completely different treatment planning procedure, in which intervention by the treatment planner is important. Experience in the setting of dose constraints and
Fig. 3. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for esophagus case: (a) y-direction and (b) x-direction.
42
Medical Dosimetry
Fig. 4. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for prostate case: (a) y-direction and (b) x-direction.
lieved that the inverse planning approach was able to counteract the leaf insertion effect by adjusting other parameters such as re-allocating the weights to individual beamlets and altering the beam segments. For similar reasons, there was no definite relationship between the leaf insertion percentage and the number of segments or total MU values. Nevertheless, the 50% leaf insertion was recommended for treatment planning because it was the intermediate percentage between the 2 extremes, and was believed to render a more convenient optimization process in most clinical scenarios. The leaf increment was proven to be the most influential factors among the 3 MLC parameters examined
Volume 32, Number 1, 2007
here. An increase in the leaf increment factor resulted in a degradation of the dose distribution, regardless of the size and shape of the PTV. In terms of the dose conformity, both the x- and y-direction dose profiles were significantly affected, despite that the leaves were moving along the x-direction. Not only was there a loss of sharp dose gradient at the periphery of the PTV, there was also shifting of the high-dose regions outside the PTV in some cases. In addition, similar trends were observed in the TCPs and NTCPs of all plans. The smallest leaf increment plans (0.5 cm) provided the best dose coverage to the PTV and the lowest complication risks to the normal adjacent structures. It is understandable that a smaller leaf movement between beam segments allows much more flexible control of intensity for individual beamlets and achieves a dose distribution closer to the plan requirements. The leaf increment plans using larger increments were able to reduce the number of beam segments and total MU values per radiation beam, and therefore reduce the treatment time and whole body patient dose. However, because treatment outcomes, such as better tumor control and complication-free survival of patients, outweighs the time and extra whole body dose factors, a leaf increment of not greater than 0.5 cm for IMRT plans is recommended. A limitation of this study was that only one planning system, XiO (CMS, Inc.) was used in the comparison. In fact, to conduct this study on different planning systems would be very difficult because it was not possible to extract the effect of the same studied parameters from different models of planning systems. It would be expected that for different planning systems, the differences in their commissioning and planning parameters might lead to different outcomes and subsequent recommendations. CONCLUSIONS The MLC parameters in the inverse planning of IMRT affect the dosimetric outcome of treatment plans.
Table 2. Comparison between IMRT plans with 0.5-cm (In05), 1.0-cm (In10), and 2.0-cm (In20) MLC leaf increments for the 3 cancer cases Nasophaynx
TCP (%) Segments/beam Total MU/beam NTCP (%)
S. cord B. stem Op. Ch. Lens TMJ Parotid
Esophagus
Parotid
In05
In10
In20
In05
In10
In20
In05
In10
In20
78.5 27.4 3562 4.25 0.10 0.00 0.76 0.16 0.04
75.6 20.0 3248 3.75 0.20 0.00 0.75 0.26 0.08
70.9 14.8 3428 6.97 0.21 0.00 0.76 0.29 0.37
78.8 41.6 5247 1.60 1.01 0.34 0.09
77.8 25.8 2673 1.67 1.58 0.53 0.11
76.4 18.4 1855 1.68 3.63 1.29 0.46
80.5 35.2 5093 0.04 1.88 0.00 0.00
78.6 28.0 4142 0.04 1.94 0.00 0.00
77.0 19.1 2845 0.10 2.63 0.00 0.00
S. cord L lung R lung Heart
Bladder Rectum L. femur R. femur
S. cord ⫽ spinal cord; B. stem ⫽ brain stem; Op. Ch. ⫽ optic chaism; TMJ ⫽ temporo-mandibular joint; L ⫽ left; R ⫽ right.
MLC parameters on treatment planning of IMRT ● V. W. C. WU
The leaf width effect on the dose distribution is slight, with narrower leaves giving slightly better sparing of OARs, whereas the effect of the leaf insertion percentage is insignificant. Increasing the leaf increment significantly degrades the treatment plans in terms of the dose distributions to both PTV and OARs, although treatment time can be saved due to fewer number of beam segments. REFERENCES 1. Hunt, M.A.; Zelefsky, M.J.; Wolden, S.; et al. Treatment planning and delivery of intensity-modulated radiation therapy for primary nasopharynx cancer. Int. J. Radiat. Oncol. Biol. Phys. 49:623–32; 2001. 2. Sultanem, K.; Shu, H.K.; 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. 48:711–22; 2000. 3. Xia, P.; Fu, K.K.; Wong, G.W.; et al. Comparison of treatment plans involving intensity-modulated radiotherapy of nasopharyngeal carcinoma. Int. J. Radiat. Oncol. Biol. Phys. 48:329 –37; 2000. 4. Boyer, A.L.; Geis, P.; Grant, W.; et al. Modulated beam conformal therapy for head and neck tumors. Int. J. Radiat. Oncol. Biol. Phys. 39:227–36; 1997.
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5. Chao, K.S.; Low, D.A.; Perez, C.A.; et al. Intensity-modulated radiation therapy in head and neck cancers: The Mallinckrodt experience. Int. J. Cancer 90:92–103; 2000. 6. Zelefsky, M.J.; Fuks, Z.; Happersett, L.; et al. Clinical experience with intensity modulated radiation therapy (IMRT) in prostate cancer. Radiother. Oncol. 55:241–9; 2000. 7. Chauvet, I.; Petitfils, A.; Lehobey, C.; et al. The sliding slit test for dynamic IMRT: Useful tool for adjustment of MLC related parameters. Phys. Med. Biol. 50:563– 80; 2005. 8. 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. 32:405–11; 2005. 9. 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 planning system. Med. Phys. 31:3187– 93; 2004. 10. Nocedal, J.; Wright, S.J. Numerical Optimization. New York: Springer; 2000. 11. Press, W.H.; Teukosky, S.A.; Vetterling, W.I.; et al. Numerical Recipes in C⫹⫹. The Art of Scientific Computing. 2nd ed. Cambridge: Cambridge University Press; 2002. 12. Samuelsson, A.; Johansson, K.A. Intensity modulated radiotherapy treatment planning for dynamic multileaf collimator delivery: Influence of different parameters on dose distributions. Radiother. Oncol. 66:16 –28; 2003.
doi:10.1016/j.meddos.2006.11.005
EFFECTS OF MULTILEAF COLLIMATOR PARAMETERS ON TREATMENT PLANNING OF INTENSITY-MODULATED RADIOTHERAPY This paper was presented at the 15th Asian Conference of Radiological Technologists, organized by the Japan Association of Radiological Technogists November 22, 2005, Chiba, Japan.
VINCENT W. C. WU Hong Kong Polytechnic University, Hung Hom, Hong Kong (Received 13 November 2006; accepted 15 November 2006)
Abstract—In inverse planning of intensity-modulated radiotherapy (IMRT), the setting of multileaf collimator (MLC) parameters affects the optimization algorithms and dose distribution. We investigated the effects of varying the MLC leaf width, leaf insertion percentage, and leaf increment in treatment planning of IMRT in 3 cancer cases: nasopharynx, esophagus, and prostate. Inverse planning of the 3 cancer cases was performed using the XiO treatment planning system. MLCs with 0.5 and 1.0 cm were used to evaluate the leaf width effect, whereas leaf insertions of 20%, 50%, and 80% were used to demonstrate the effect of leaf insertion percentage, and leaf increments of 0.5, 1.0, and 2.0 cm were used to study the leaf increment effect. The treatment plans were evaluated by dose profiles, tumor control probability (TCP), and normal tissue complication probability (NTCP). The 0.5-cm MLC leaves showed better TCPs and NTCPs than the 1.0-cm leaves in the 3 cancer cases, although the differences were less than 2.5%. For the leaf insertion percentage, the dose profile differences among the 3 levels of increments were minimal, and their differences in TCP and NTCP were extremely small (< 1.5%). The effect of leaf increment was more prominent, dose profile, TCPs, and NTCPs were best for the smallest leaf increment and they deteriorated as the leaf increment increased. Narrower leaves gave slightly better sparing of organs at risk (OAR)s; changing the leaf insertion percentage brought about negligible changes, whereas increasing the leaf increment significantly degraded the treatment plans. © 2007 American Association of Medical Dosimetrists. Key Words: Intensity-modulated radiotherapy, Multileaf collimator, Inverse planning, Dose evaluation.
INTRODUCTION
workstation. Although inverse planning in IMRT treatment planning employs an automatic optimization process, it still requires human intervention. Apart from the input of dose constraints and associated importance factors, the adjustment of MLC parameters also affects the optimization algorithm and subsequently the dose distribution.7 The width of MLC leaves affects the smoothness of the field aperture, which in turn may affect the conformity of dose to the planning target volume (PTV). For most linear accelerators, the common MLC leaf width is 1.0 cm at isocenter and covers a field size of up to 40 ⫻ 40 cm2. Recently, more sophisticated MLC systems that contain 60 pairs of leaves, with the central leaves at a width of 0.5 cm, such as the Millennium-120 dynamic MLC has been available. Jin et al.,8 in a study of intensity-modulated radiosurgery using dynamic conformal arc techniques, demonstrated that MLCs with finer leaf widths produce better dose conformity than those with thicker leaves in the treatment of small targets. However, a study of leaf width effect by Burmeister et al.9 on standard IMRT using the Corvus treatment planning system revealed that there was no apparent clinically significant difference between 1.0 and 0.5 cm leaf-width plans except that better sparing of organs at risk (OARs) was observed in the narrower-leaf plans for very small or concave-shaped target volumes.
Intensity-modulated radiotherapy (IMRT) has been increasingly practiced in radiotherapy. Its goals are to shape the high dose zone tightly to the target volume and minimize the dose to the adjacent normal organs. The dosimetric advantages of IMRT over conventional techniques have been documented by many studies.1– 6 Apart from the development of a more advanced treatment planning system with an inverse planning algorithm, one important factor that has brought about the success of IMRT is the inclusion of the dynamic multileaf collimators (MLCs) in linear accelerators. The programmed movement of the MLC leaves according to the treatment plan during each beam segment governs the intensity of each beamlet and ultimately determines the overall dose distribution to the irradiated volume. The resultant plan is able to deliver a highly conformal dose distribution to the target volume. The determination of the MLC motions within beam segments is a complicated task that is beyond the capability of the human brain, and therefore is mainly achieved by inverse planning using a dedicated computer
Reprint requests to: Dr. Vincent W. C. Wu, Department of Health Technology and Informatics, Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong. E-mail: htvinwu@ inet.polyu.edu.hk 38
MLC parameters on treatment planning of IMRT ● V. W. C. WU
Apart from the leaf width, the leaf insertion percentage (the degree of leaf intrusion into the PTV at the circumference) and leaf increment (the measure of minimum leaf movement between segments) are 2 additional factors that need to be taken into account during the optimization of IMRT plans. We investigated the effects of varying the leaf width, leaf insertion percentage, and leaf increment in the treatment planning of IMRT on 3 different cancer cases: nasopharyngeal, esophageal, and prostatic carcinoma. The study should help treatment planners acquire knowledge about the setting of the above-mentioned MLC parameters in the inverse planning of IMRT and to appreciate the effects of altering these parameters to establish optimum values. METHODS AND MATERIALS The treatment planning system used in this study was XiO (CMS Inc.). The system uses a conjugate gradient optimization algorithm10,11 for IMRT plans, which is a type of gradient descent optimization algorithm. The conjugate gradient method uses the negative gradient of a cost function, with respect to input parameters, to systematically find the minimum of a cost function. The 3 subjects selected for this study were patients with nasopharyngeal, esophageal, and prostatic carcinoma. These 3 patients were selected because they involved tumors from the head-and-neck down to the pelvis with good variation in tumor size, shape, and geometry in relation to the OARs. For the nasopharyngeal patient, the PTV was small (volume 73.9 cm3) and showed slight concavity at the posterior aspect where it was adjacent to the brain stem or spinal cord. The PTV of the esophageal tumor was relatively large (208.7 cm3), especially along the longitudinal axis of the thoracic region, and was roughly circular in the transverse section. The prostate tumor was situated in close proximity between the urinary bladder and rectum, with a PTV volume of 128.6 cm3. The computed tomography (CT) images of the 3 patients were transferred to the treatment planning system. Before starting the optimization, the PTVs and OARs were delineated on the corresponding CT slices of each patient. The dose constraints and importance factors were input for the PTVs and OARs, which was followed by the setting of planning parameters including those for the MLC. Five equispaced beams were used in the nasopharynx and eosphagus cases, and an equispaced 7-beam arrangement was used for the prostate case. Tumor doses of 66, 60, and 70 Gy were prescribed to 95% of the PTV for the nasopharynx, esophagus, and prostate cases, respectively. The calculation grid used for the calculation of the final dose distribution for all treatment plans was 0.25 ⫻ 0.25 cm. To evaluate the effect of leaf width, a 2300CD and 600C/D linear accelerators (Varian Medical Systems, Inc.) were used, which were equipped with Millennium
39
MLC-120 (60 pairs of leaves, with the central leaves having a width of 0.5 cm) and 40 pairs of 1.0-cm wide leaves, respectively. Both leaf widths were measured at the isocenter. The study of the effects of the leaf insertion percentage was performed using the 1.0-cm leaf machine. Leaf insertions of 20%, 50%, and 80% were applied to each case. To facilitate comparison, all other planning parameters were kept constant. The same machine was used for the evaluation of the leaf increment factor. Leaf increments of 0.5, 1.0, and 2.0 cm were applied to each case to generate the IMRT plans, while the other parameters were kept unchanged. A number of plan evaluation tools were used to evaluate the dose distribution of the treatment plans. First, dose profiles from 2 perpendicular planes in x (lateral) and y (antero-posterior) directions across the target were generated (Fig. 1a-c). These profiles provided information about the dose gradients at the interface between the PTVs and adjacent OARs, and were an indication of the target dose conformity. Second, the tumor control probabilities (TCPs) and normal tissue complication probabilities (NTCPs) were calculated for the PTVs and OARs, respectively. These biological indices were by no means absolute, but provided a relative ranking of the treatment plans under comparison. In addition, the total number of MLC segments and monitor units (MUs) per beam generated from each treatment plan were also recorded. These provided an indication of the complexity of the MLC configuration, which in turn affects the treatment time and whole body dose to the patient. For each case, these parameters were compared across the plans with different MLC settings. RESULTS Effect of leaf width The differences in dose profiles between the 0.5 and 1.0-cm leaf-width plans were small in all 3 clinical cases, although the plans with narrower leaves showed a slight advantage over those of the wider leaves in most cases. IMRT plans using 0.5-cm leaves produced a slightly higher target TCP and a lower NTCP for most OARs than those using 1.0-cm leaf widths (Table 1). In the nasopharynx case, the spinal cord and lens received the most benefit from the finer leaves, whereas in the eosphagus and prostate cases, the left lung and rectum, respectively, were more effectively spared by the narrower MLC leaves. There was no definite pattern in the differences in the number of segments or the total MU per beam between the 2 leaf-width plans (Table 1). Effect of leaf insertion percentage The dose difference was minimal between IMRT plans with different leaf insertion percentages. All 3 clinical cases demonstrated no difference in the dose profiles. The differences in the TCPs and NTCPs across
40
Medical Dosimetry
Volume 32, Number 1, 2007
Fig. 1. CT slices showing the 2 perpendicular planes (x and y) for the generation of dose profiles for (a) nasopharynx, (b) esophagus, and (c) prostate cancer patients.
the 3 levels of leaf insertion percentage were small in all cases. A maximum variation in TCP of 1.23% was observed in the esophagus case, while the greatest variation in NTCP (0.31%) was demonstrated by the spinal cord in the nasopharynx case.
With regard to the number of MLC segments and total MU per beam, no definite pattern was observed. Comparatively higher numbers of segments were seen in the plans with 50% leaf insertion in the nasopharynx and esophagus cases, whereas a slight increase in the total
Table 1. Comparison between IMRT plans with 0.5-cm (LW05) and 1.0-cm (LW10) MLC leaf widths for the 3 cancer cases Nasopharynx
TCP (%) Segments/beam Total MU/beam NTCP (%)
S. cord B. stem Op. Ch. Lens TMJ Parotid
Esophagus
Prostate
LW05
LW10
LW05
LW10
LW05
LW10
78.5 27.4 3562 3.73 0.04 0.00 0.07 0.12 0.01
77.5 29.2 3646 4.15 0.10 1.01 0.07 0.13 0.04
78.8 41.6 5247 1.35 0.72 0.24 0.09
76.8 36.3 4694 1.60 1.01 0.34 0.09
80.2 28.0 4243 1.22 2.14 0.00 0.00
79.0 35.1 4847 1.32 4.35 0.00 0.00
S. cord L lung R lung Heart
Bladder Rectum L femur R femur
S. cord ⫽ spinal cord; B. stem ⫽ brain stem; Op. Ch. ⫽ optic chiasm; TMJ ⫽ temporo-mandibular joint; L ⫽ left; R ⫽ right.
MLC parameters on treatment planning of IMRT ● V. W. C. WU
Fig. 2. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for nasopharyngeal case: (a) y-direction and (b) x-direction.
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weighting factors for the targets and OARs is required in inverse planning for IMRT.7,12 In addition, the determination of MLC parameters prior to the optimization process also affects the dosimetric outcome of the treatment plan. With regard to the effect of MLC leaf width, a slight dosimetric advantage was achieved by reducing the leaf width from 1.0 cm to 0.5 cm. This was reflected by slightly better dose profiles within the PTV regions and a slight improvement in the TCPs and NTCPs in most cases. This was largely in line with the results from Burmeister et al.9 except that the use of 0.5-cm leaves in this study did not show significant dosimetric advantage in the smaller size nasopharynx tumor over the other larger size tumors. Usually, plans with finer leaves are more complex than plans with wider leaves, in terms of number of beam segments and total MUs. Such results were observed in the prostate and nasopharynx cases but not in the esophagus case. This could be explained by the more simple convex PTV geometry of the esophagus case, in which the 0.5-cm MLC did not require extra beam segments to achieve the optimum dose distribution as for the other 2 cases with concave targets. The dose conformity effect of the MLC leaf insertion percentage in IMRT plans was negligible, as the dose profiles in all cases were nearly identical. This illustrated that leaf insertion percentage was not an influential factor in the final dose distribution. It was be-
MU per beam was demonstrated by the 80% leaf insertion plans in the nasopharynx and prostate cases. Effect of leaf increment The dose profiles of the 0.5-cm leaf increment plans were the best, which was followed by the plans with 1.0 and 2.0-cm increments. The same pattern was present in all 3 cancer cases (Figs. 2– 4). Similar trends were also observed in the TCP and NTCP values. The plans with the smallest leaf increment demonstrated the highest TCP, with a maximum difference of 7.7% over the 2.0-cm increment plan in the nasopharynx case (Table 2). The NTCPs of most OARs were also lowest in plans with the smallest leaf increment, and they increased with the leaf increment increase. Greater variations in the number of MLC segments and total MUs per beam were observed for the leaf increment tests (Table 2). The plans with the smallest increment demonstrated the greatest number of MLC segments and total MU per beam, followed by the 1.0 and 2.0-cm leaf increment plans. DISCUSSION The change from forward to inverse planning involves a completely different treatment planning procedure, in which intervention by the treatment planner is important. Experience in the setting of dose constraints and
Fig. 3. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for esophagus case: (a) y-direction and (b) x-direction.
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Medical Dosimetry
Fig. 4. Comparison of dose profiles of plans with 1.0-cm (Inc-10), 2.0-cm (Inc-20), and 0.5-cm (Inc-05) MLC leaf increments for prostate case: (a) y-direction and (b) x-direction.
lieved that the inverse planning approach was able to counteract the leaf insertion effect by adjusting other parameters such as re-allocating the weights to individual beamlets and altering the beam segments. For similar reasons, there was no definite relationship between the leaf insertion percentage and the number of segments or total MU values. Nevertheless, the 50% leaf insertion was recommended for treatment planning because it was the intermediate percentage between the 2 extremes, and was believed to render a more convenient optimization process in most clinical scenarios. The leaf increment was proven to be the most influential factors among the 3 MLC parameters examined
Volume 32, Number 1, 2007
here. An increase in the leaf increment factor resulted in a degradation of the dose distribution, regardless of the size and shape of the PTV. In terms of the dose conformity, both the x- and y-direction dose profiles were significantly affected, despite that the leaves were moving along the x-direction. Not only was there a loss of sharp dose gradient at the periphery of the PTV, there was also shifting of the high-dose regions outside the PTV in some cases. In addition, similar trends were observed in the TCPs and NTCPs of all plans. The smallest leaf increment plans (0.5 cm) provided the best dose coverage to the PTV and the lowest complication risks to the normal adjacent structures. It is understandable that a smaller leaf movement between beam segments allows much more flexible control of intensity for individual beamlets and achieves a dose distribution closer to the plan requirements. The leaf increment plans using larger increments were able to reduce the number of beam segments and total MU values per radiation beam, and therefore reduce the treatment time and whole body patient dose. However, because treatment outcomes, such as better tumor control and complication-free survival of patients, outweighs the time and extra whole body dose factors, a leaf increment of not greater than 0.5 cm for IMRT plans is recommended. A limitation of this study was that only one planning system, XiO (CMS, Inc.) was used in the comparison. In fact, to conduct this study on different planning systems would be very difficult because it was not possible to extract the effect of the same studied parameters from different models of planning systems. It would be expected that for different planning systems, the differences in their commissioning and planning parameters might lead to different outcomes and subsequent recommendations. CONCLUSIONS The MLC parameters in the inverse planning of IMRT affect the dosimetric outcome of treatment plans.
Table 2. Comparison between IMRT plans with 0.5-cm (In05), 1.0-cm (In10), and 2.0-cm (In20) MLC leaf increments for the 3 cancer cases Nasophaynx
TCP (%) Segments/beam Total MU/beam NTCP (%)
S. cord B. stem Op. Ch. Lens TMJ Parotid
Esophagus
Parotid
In05
In10
In20
In05
In10
In20
In05
In10
In20
78.5 27.4 3562 4.25 0.10 0.00 0.76 0.16 0.04
75.6 20.0 3248 3.75 0.20 0.00 0.75 0.26 0.08
70.9 14.8 3428 6.97 0.21 0.00 0.76 0.29 0.37
78.8 41.6 5247 1.60 1.01 0.34 0.09
77.8 25.8 2673 1.67 1.58 0.53 0.11
76.4 18.4 1855 1.68 3.63 1.29 0.46
80.5 35.2 5093 0.04 1.88 0.00 0.00
78.6 28.0 4142 0.04 1.94 0.00 0.00
77.0 19.1 2845 0.10 2.63 0.00 0.00
S. cord L lung R lung Heart
Bladder Rectum L. femur R. femur
S. cord ⫽ spinal cord; B. stem ⫽ brain stem; Op. Ch. ⫽ optic chaism; TMJ ⫽ temporo-mandibular joint; L ⫽ left; R ⫽ right.
MLC parameters on treatment planning of IMRT ● V. W. C. WU
The leaf width effect on the dose distribution is slight, with narrower leaves giving slightly better sparing of OARs, whereas the effect of the leaf insertion percentage is insignificant. Increasing the leaf increment significantly degrades the treatment plans in terms of the dose distributions to both PTV and OARs, although treatment time can be saved due to fewer number of beam segments. REFERENCES 1. Hunt, M.A.; Zelefsky, M.J.; Wolden, S.; et al. Treatment planning and delivery of intensity-modulated radiation therapy for primary nasopharynx cancer. Int. J. Radiat. Oncol. Biol. Phys. 49:623–32; 2001. 2. Sultanem, K.; Shu, H.K.; 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. 48:711–22; 2000. 3. Xia, P.; Fu, K.K.; Wong, G.W.; et al. Comparison of treatment plans involving intensity-modulated radiotherapy of nasopharyngeal carcinoma. Int. J. Radiat. Oncol. Biol. Phys. 48:329 –37; 2000. 4. Boyer, A.L.; Geis, P.; Grant, W.; et al. Modulated beam conformal therapy for head and neck tumors. Int. J. Radiat. Oncol. Biol. Phys. 39:227–36; 1997.
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5. Chao, K.S.; Low, D.A.; Perez, C.A.; et al. Intensity-modulated radiation therapy in head and neck cancers: The Mallinckrodt experience. Int. J. Cancer 90:92–103; 2000. 6. Zelefsky, M.J.; Fuks, Z.; Happersett, L.; et al. Clinical experience with intensity modulated radiation therapy (IMRT) in prostate cancer. Radiother. Oncol. 55:241–9; 2000. 7. Chauvet, I.; Petitfils, A.; Lehobey, C.; et al. The sliding slit test for dynamic IMRT: Useful tool for adjustment of MLC related parameters. Phys. Med. Biol. 50:563– 80; 2005. 8. 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. 32:405–11; 2005. 9. 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 planning system. Med. Phys. 31:3187– 93; 2004. 10. Nocedal, J.; Wright, S.J. Numerical Optimization. New York: Springer; 2000. 11. Press, W.H.; Teukosky, S.A.; Vetterling, W.I.; et al. Numerical Recipes in C⫹⫹. The Art of Scientific Computing. 2nd ed. Cambridge: Cambridge University Press; 2002. 12. Samuelsson, A.; Johansson, K.A. Intensity modulated radiotherapy treatment planning for dynamic multileaf collimator delivery: Influence of different parameters on dose distributions. Radiother. Oncol. 66:16 –28; 2003.