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Multiple segment radiation therapy (MSRT) for breast cancer treatment after breast-conserving surgery
Proceedings of the 43rd Annual ASTRO Meeting
position. The patients were placed on a flat board with the involved breast hanging by gravity through an aperture. Treatment plans for WBRT, IMRT, tomotherapy, and in most patients, brachytherapy, were compared. The target volume was defined as the surgical cavity plus 2 cm surrounding tissue. DVH for target volume, breast, skin, lung, and heart were analyzed. Results: The motion of the breast as measured by displacement of skin marks relative to a fixed grid was within 1 mm during shallow breathing, enabling IMRT and tomotherapy. As compared with prone WBRT, IMRT and tomotherapy delivered much lower dose to the skin, but the lung and heart doses were similar (see figure below). Conclusion: IMRT and tomotherapy both provide adequate dose coverage of the target volume while sparing normal tissue. The prone position minimizes breathing motion of the breast, which is essential for non-gated external beam partial breast irradiation by either IMRT or tomotherapy. While our study demonstrates a difference for skin, the DVH curves of lung/heart are superimposable, because the tangential beam superior border could be placed at the chest wall as a result of the prone positioning. Left breast supine tangential WBRT would have included more lung and heart. Although conventional fixed-field IMRT produces a similar planned dose distribution, tomotherapy is potentially more time efficient in planning, patient setup and delivery. For future partial breast irradiation trials, IMRT, tomotherapy, and brachytherapy may be potential therapeutic arms for comparison.
2330
Investigation of IMRT Delivery for Lung Cancer
N. Papanikolaou, J. Penagaricano, V. Ratanatharathorn Radiation Oncology, University of Arkansas, Little Rock, AR Purpose: The goal of this work is to investigate the application of IMRT delivery to lung tumors in regards to the effect of lung heterogeneity in the relative dose distributions and absolute dose (MU) calculation. The study was done for both low (6MV) and high (18MV) photon beams. Materials and Methods: Intensity modulated radiation therapy techniques are slowly phased into clinical practice as more accelerator types are capable to deliver them and treatment planning computers to plan them. In our study we used a Varian 2100EX linear accelerator for the delivery and the Pinnacle treatment planning system for the IMRT field design and optimization. The delivery was implemented using the step-and-shoot technique. The CT dataset chosen was one for a patient with a spherically shaped mass approximately 4cm in diameter in his right lung. The fact that the tumor was almost entirely suspended into the healthy lung tissue provided an ideal scenario for the study of lateral scatter transport during the calculation. The optimization and final dose calculation was done for eight 6MV and 18 MV coplanar beams in three modes: homogeneous, 2.5D (convolution) and full 3D scatter transport (superposition) calculation. A Monte Carlo computation was also performed for verification. Relative and absolute dose distributions were generated for comparison as well as dose volume histograms for the target volume and the volumes at risk. Results: Unlike conventional 3D RTP that typically involves aperture modulated conformal beams, intensity modulated radiotherapy produces beams than have non-uniform intensity across a given field, often with severe fluence gradients between neighboring beamlets. It is the magnitude of these effects that we studied in this work as applied to the exclusion and inclusion of tissue inhomogeneity in the dose calculation. Although the energy selection did not seem to play a significant role for the case in study, the sophistication of the algorithm used for the optimization and dose computation had a significant impact. Differences as high as 15% were observed in absolute dose coverage and as high as 20% to adjacent tissues like spinal cord. The location of the hot spot also moved as the calculation algorithm was changed which could have significant impact on the clinical plan evaluation and approval. Conclusion: For lung tumors, should one choose to use IMRT techniques for treatment, an accurate 3D dose calculation algorithm should be employed for the optimization and final dose calculation, as the low density of the healthy lung can severely alter the spatial distribution of absolute dose. The choice of energy appears to be of lesser importance, hence a low energy beam (6MV) is recommended for practical reasons.
2331
Multiple Segment Radiation Therapy (MSRT) for Breast Cancer Treatment After Breast-Conserving Surgery
L. Xing1, T. Pawlicki1, L. Yuen1, C. Cotrutz1, J. Dogan2, C. Li2, F. Halberg2, A. Boyer1, G. Luxton1, D. Goffinet1 1 Radiation Oncology, Stanford University, Stanford, CA, 2Radiation Oncology, Marin Cancer Institute, Greenbrae, CA Purpose: Despite the well-known fact that intensity-modulation could significantly improve the dose distributions in breast irradiation, its clinical implementation has been hindered by deficiencies in the current inverse planning systems and by the lack
399
400
I. J. Radiation Oncology
● Biology ● Physics
Volume 51, Number 3, Supplement 1, 2001
of a comprehensive procedure. The aim of this work was to develop a general scheme of intensity-modulated breast treatment using MSRT and to demonstrate its superiority over IMRT as well as standard tangential-field (TF) technique using 20 clinical cases of various breast sizes. Methods: The patient setup and target definition were the same as that used in the TF treatment. Two planning methods were studied: (1)manual forward planning, and (2)computer MSRT optimization. A 3D planning system with a trial-and-error was used in the manual MSRT planning to stack MLC segments on top of the standard TFs to get a sensible MSRT plan. The underlying reason for the approach to be a viable choice is that the initial TFs have already brought the system to the vicinity of optimal solution. As a result, it is often sufficient to add 1⬃3 segments to greatly improve the dose distribution. MSRT optimization was also developed using a gradient method. The DVH-based score function depended on both segment weights and shapes. MLC constraints were included to prevent unphysical MLC configurations. The target volume, defined at the patient setup based on the palpable breast tissue, was used by the algorithm for calculation. The segmented fields were concatenated to form a step-and-shoot delivery. An algebraic method was devised to determine the segmented MU to optimally compensate the MLC transmission. The fluence map and MU were independently checked. 20 patients were planned and the results were compared with the standard TF plans, as well as the TF and multiple-field IMRT plans. Results: The MSRT significantly improved target dose uniformity. Figure 1 shows a manual plan. It was also possible to reduce the lung/heart dose with a slight deterioration of target dose. Optimization yielded consistent beam apertures and weights and became advantageous for complicated cases. Our results revealed that MSRT could easily reduce the dose uniformity from 105⬃120% (prescription was at 90%) to 100⬃112%. The results were comparable or even more favorable than the conventional TF IMRT plans. It was noticed that IMRT with 3⬃7 beams was inferior in that more normal tissues were irradiated. Conclusion: IMRT deviates from the conventional approach and requires additional steps in the treatment process. MSRT bridges the gap between conventional and IMRT treatments. For breast cancer, MSRT is a natural extension of standard procedure and improves the treatment without paying the excessive overhead associated with current IMRT.
2332
Dose-Rate Effects in Intensity Modulated Radiation Therapy 1
J.O. Deasy , J.F. Fowler2, J.L. Roti Roti1, D.A. Low1 1 Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University, St. Louis, MO, 2Human Oncology, University of Wisconsin-Madison, Madison, WI Purpose: Intensity modulated radiation therapy (IMRT) delivery methods use temporally nonuniform fluence distributions to shape high-dose treatment volumes. It is known from previous work, primarily that of Steel, Hall, and co-workers, that cell-kill is decreased significantly as dose-rates are reduced from 1 to 0.1 Gy/min. Current IMRT optimization and delivery algorithms do not distinguish between plans based on the effective dose-rate to dose-limiting structures or different regions of the target. We consider the implications of the dose-rate effect for IMRT delivery. Materials and Methods: We reviewed the literature on tumor and normal tissue in vivo and in vitro dose-rate effects in the range 0.1 to 1 Gy/min. From these, we extrapolated to 2 Gy fractions on the basis of linear-quadratic fitting. Results: Dose-rate effects vary markedly in vitro from cell-line to cell-line. Tumor dose-rate effects vary more than normal tissue dose-rate effects, perhaps due to intra-tumor heterogeneity. A change in dose-rate from 0.1 to 1 Gy/min could be expected to often increase the biologically effective dose to the tumor by 10% or more, although this figure is highly variable due to large cell-specific variations. Similar increases are expected for normal human cell lines, though the variations are smaller.
position. The patients were placed on a flat board with the involved breast hanging by gravity through an aperture. Treatment plans for WBRT, IMRT, tomotherapy, and in most patients, brachytherapy, were compared. The target volume was defined as the surgical cavity plus 2 cm surrounding tissue. DVH for target volume, breast, skin, lung, and heart were analyzed. Results: The motion of the breast as measured by displacement of skin marks relative to a fixed grid was within 1 mm during shallow breathing, enabling IMRT and tomotherapy. As compared with prone WBRT, IMRT and tomotherapy delivered much lower dose to the skin, but the lung and heart doses were similar (see figure below). Conclusion: IMRT and tomotherapy both provide adequate dose coverage of the target volume while sparing normal tissue. The prone position minimizes breathing motion of the breast, which is essential for non-gated external beam partial breast irradiation by either IMRT or tomotherapy. While our study demonstrates a difference for skin, the DVH curves of lung/heart are superimposable, because the tangential beam superior border could be placed at the chest wall as a result of the prone positioning. Left breast supine tangential WBRT would have included more lung and heart. Although conventional fixed-field IMRT produces a similar planned dose distribution, tomotherapy is potentially more time efficient in planning, patient setup and delivery. For future partial breast irradiation trials, IMRT, tomotherapy, and brachytherapy may be potential therapeutic arms for comparison.
2330
Investigation of IMRT Delivery for Lung Cancer
N. Papanikolaou, J. Penagaricano, V. Ratanatharathorn Radiation Oncology, University of Arkansas, Little Rock, AR Purpose: The goal of this work is to investigate the application of IMRT delivery to lung tumors in regards to the effect of lung heterogeneity in the relative dose distributions and absolute dose (MU) calculation. The study was done for both low (6MV) and high (18MV) photon beams. Materials and Methods: Intensity modulated radiation therapy techniques are slowly phased into clinical practice as more accelerator types are capable to deliver them and treatment planning computers to plan them. In our study we used a Varian 2100EX linear accelerator for the delivery and the Pinnacle treatment planning system for the IMRT field design and optimization. The delivery was implemented using the step-and-shoot technique. The CT dataset chosen was one for a patient with a spherically shaped mass approximately 4cm in diameter in his right lung. The fact that the tumor was almost entirely suspended into the healthy lung tissue provided an ideal scenario for the study of lateral scatter transport during the calculation. The optimization and final dose calculation was done for eight 6MV and 18 MV coplanar beams in three modes: homogeneous, 2.5D (convolution) and full 3D scatter transport (superposition) calculation. A Monte Carlo computation was also performed for verification. Relative and absolute dose distributions were generated for comparison as well as dose volume histograms for the target volume and the volumes at risk. Results: Unlike conventional 3D RTP that typically involves aperture modulated conformal beams, intensity modulated radiotherapy produces beams than have non-uniform intensity across a given field, often with severe fluence gradients between neighboring beamlets. It is the magnitude of these effects that we studied in this work as applied to the exclusion and inclusion of tissue inhomogeneity in the dose calculation. Although the energy selection did not seem to play a significant role for the case in study, the sophistication of the algorithm used for the optimization and dose computation had a significant impact. Differences as high as 15% were observed in absolute dose coverage and as high as 20% to adjacent tissues like spinal cord. The location of the hot spot also moved as the calculation algorithm was changed which could have significant impact on the clinical plan evaluation and approval. Conclusion: For lung tumors, should one choose to use IMRT techniques for treatment, an accurate 3D dose calculation algorithm should be employed for the optimization and final dose calculation, as the low density of the healthy lung can severely alter the spatial distribution of absolute dose. The choice of energy appears to be of lesser importance, hence a low energy beam (6MV) is recommended for practical reasons.
2331
Multiple Segment Radiation Therapy (MSRT) for Breast Cancer Treatment After Breast-Conserving Surgery
L. Xing1, T. Pawlicki1, L. Yuen1, C. Cotrutz1, J. Dogan2, C. Li2, F. Halberg2, A. Boyer1, G. Luxton1, D. Goffinet1 1 Radiation Oncology, Stanford University, Stanford, CA, 2Radiation Oncology, Marin Cancer Institute, Greenbrae, CA Purpose: Despite the well-known fact that intensity-modulation could significantly improve the dose distributions in breast irradiation, its clinical implementation has been hindered by deficiencies in the current inverse planning systems and by the lack
399
400
I. J. Radiation Oncology
● Biology ● Physics
Volume 51, Number 3, Supplement 1, 2001
of a comprehensive procedure. The aim of this work was to develop a general scheme of intensity-modulated breast treatment using MSRT and to demonstrate its superiority over IMRT as well as standard tangential-field (TF) technique using 20 clinical cases of various breast sizes. Methods: The patient setup and target definition were the same as that used in the TF treatment. Two planning methods were studied: (1)manual forward planning, and (2)computer MSRT optimization. A 3D planning system with a trial-and-error was used in the manual MSRT planning to stack MLC segments on top of the standard TFs to get a sensible MSRT plan. The underlying reason for the approach to be a viable choice is that the initial TFs have already brought the system to the vicinity of optimal solution. As a result, it is often sufficient to add 1⬃3 segments to greatly improve the dose distribution. MSRT optimization was also developed using a gradient method. The DVH-based score function depended on both segment weights and shapes. MLC constraints were included to prevent unphysical MLC configurations. The target volume, defined at the patient setup based on the palpable breast tissue, was used by the algorithm for calculation. The segmented fields were concatenated to form a step-and-shoot delivery. An algebraic method was devised to determine the segmented MU to optimally compensate the MLC transmission. The fluence map and MU were independently checked. 20 patients were planned and the results were compared with the standard TF plans, as well as the TF and multiple-field IMRT plans. Results: The MSRT significantly improved target dose uniformity. Figure 1 shows a manual plan. It was also possible to reduce the lung/heart dose with a slight deterioration of target dose. Optimization yielded consistent beam apertures and weights and became advantageous for complicated cases. Our results revealed that MSRT could easily reduce the dose uniformity from 105⬃120% (prescription was at 90%) to 100⬃112%. The results were comparable or even more favorable than the conventional TF IMRT plans. It was noticed that IMRT with 3⬃7 beams was inferior in that more normal tissues were irradiated. Conclusion: IMRT deviates from the conventional approach and requires additional steps in the treatment process. MSRT bridges the gap between conventional and IMRT treatments. For breast cancer, MSRT is a natural extension of standard procedure and improves the treatment without paying the excessive overhead associated with current IMRT.
2332
Dose-Rate Effects in Intensity Modulated Radiation Therapy 1
J.O. Deasy , J.F. Fowler2, J.L. Roti Roti1, D.A. Low1 1 Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University, St. Louis, MO, 2Human Oncology, University of Wisconsin-Madison, Madison, WI Purpose: Intensity modulated radiation therapy (IMRT) delivery methods use temporally nonuniform fluence distributions to shape high-dose treatment volumes. It is known from previous work, primarily that of Steel, Hall, and co-workers, that cell-kill is decreased significantly as dose-rates are reduced from 1 to 0.1 Gy/min. Current IMRT optimization and delivery algorithms do not distinguish between plans based on the effective dose-rate to dose-limiting structures or different regions of the target. We consider the implications of the dose-rate effect for IMRT delivery. Materials and Methods: We reviewed the literature on tumor and normal tissue in vivo and in vitro dose-rate effects in the range 0.1 to 1 Gy/min. From these, we extrapolated to 2 Gy fractions on the basis of linear-quadratic fitting. Results: Dose-rate effects vary markedly in vitro from cell-line to cell-line. Tumor dose-rate effects vary more than normal tissue dose-rate effects, perhaps due to intra-tumor heterogeneity. A change in dose-rate from 0.1 to 1 Gy/min could be expected to often increase the biologically effective dose to the tumor by 10% or more, although this figure is highly variable due to large cell-specific variations. Similar increases are expected for normal human cell lines, though the variations are smaller.