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How many Arc is Enough? A Dosimetric Comparison of RapidArc and Static-Field IMRT for High-Risk Prostate Cancer
I. J. Radiation Oncology d Biology d Physics
S820
3377
Volume 78, Number 3, Supplement, 2010
How many Arc is Enough? A Dosimetric Comparison of RapidArc and Static-Field IMRT for High-Risk Prostate Cancer
M. Yeginer1, B. Aydogan2 1
University of Illinois at Chicago, Chicago, IL, 2University of Chicago, Chicago, IL
Purpose/Objective(s): To compare single (sRA), double (dRA), and triple (tRA) -arc RapidArc (RA) plans with seven (7IMRT) and nine (9IMRT) field IMRT plans prepared for complex-shaped high-risk prostate cancers. Materials/Methods: The RA and IMRT plans were prepared in Eclipse v8.6 treatment planning system for 7 prostate cancer patients. The primary PTV included the prostate, seminal vesicles and lymph nodes with margins while OAR included the bladder, rectum and small bowel. The sRA was obtained with a 360 counter-clockwise gantry rotation between 179.9 and 180.1 . For the dRA, the gantry continued its next 360 rotation clockwise in the second arc-field. The tRA had three sequential 330 arcs because of the total arc limitation of 1000 . In order to compare the OAR sparing performance of the treatment plans more accurately, plans are optimized using the RTOG 0521 protocol. For instance, PTV volume covered by the prescribed dose was set to 95%. In addition, the optimization process was repeated by fine-tuning the dose volume constraints to limit the PTV volume receiving more than 107% of prescribed dose to 1.5%-3%. The PTV DVHs were, therefore, shaped such that the PTV coverage and PTV homogeneity of each plan was similar and comparable. Results: On average, percent bladder median doses were 60.9% (sRA), 53.5% (dRA), 48.5% (tRA), 60.1% (7IMRT), and 54.3% (9IMRT). For the rectum, percent median doses were found to be very similar for the 7IMRT and 9IMRT (?66%) while it was 69.2%, 64.4% and%60.1 for the sRA, dRA, and tRA, respectively. The D20 for small bowel was around 6% for all the IMRT and RA plans. SDs were found to be less than 3% in this study. The monitor units were 1295 MU and 1565 MU for the 7IMRT and 9IMRT, respectively. As expected, the RA monitor units were substantially lower than that for IMRT even for the tRA (715 MU). Conclusions: Nine treatment fields are commonly preferred in the IMRT treatment of localized high risk prostate cancers. While the one arc is the most efficient RA technique, a minimum of two arcs is required to achieve a comparable OAR sparing with the 9IMRT. It is worthwhile to note that the OAR sparing obtained with the sRA and dRA is almost equivalent to that obtained with the 7IMRT and 9IMRT, respectively, suggesting that the sRA can only perform as good as the 7IMRT. This study concludes that the tRA provides the lowest OAR doses with only a modest increase in monitor units (80 MU). Author Disclosure: M. Yeginer, Varian Medical Systems, Inc. (Palo Alto, CA), B. Research Grant; B. Aydogan, Varian Medical Systems, Inc. (Palo Alto, CA), B. Research Grant.
3378
Comparison of Dosimetry of a New Linear Accelerator under Gated Operation vs. Non-gated Operation
S. Chen, S. Zhou, L. Schubert, S. Li, C. Lin, A. Wahl, C. Enke University of Nebraska Medical Center, Omaha, NE Purpose/Objective(s): Beam characteristics may be influenced when a linear accelerator is operated with gated operation. In this study, we evaluated the dosimetry of a Siemens Artiste linear accelerator under gated operation and investigated if there are dosimetric limitations to use the treatment planning system based on non-gated beam data for delivering respiratory gated treatment. Materials/Methods: The Siemens Artiste linear accelerator was installed at our institution with ANZAI-733V respiratory gating system (ANZAI MEDICAL CO, LTD.). The system utilizes a pressure sensor to track the respiratory motion. The signal from the sensor triggers the linac to turn radiation beam on or off. In this study, a motion phantom simulated the respiratory signal. Dosimetric measurements were performed using an IMRT MatriXX system (IBA Dosimetry, Inc.) at 10 cm depth with 100 cm sourceto-surface distance setup. The MatriXX system is a 2D ion chamber array with 7.6 mm spatial resolution; therefore it can be used to measure both the absolute dose and the 2D dose distribution. 6MV and 15MV photon beam data under gated operation were collected for two radiation fields (10*10 cm2 and 20*20 cm2) and 8 duty cycles (50, 25, 10, 5, 4, 3, 2, and 1 monitor unit (MU) delivered per 4 second gate cycle). Measurements were compared with corresponding non-gated operation. The absolute readings of the central ion chamber for gated deliveries were compared to those from the non-gated delivery. 2D dose distribution comparison was performed with gamma analysis (1% and 1 mm criterion). Results: For the 6MV photon beam, all dose deviations were within 1%, even for an extremely low duty cycle (1MU per gate). Similarly, the 2D dose distributions were similar between the gated and non-gated readings. The gamma values of all points in the radiation fields were\1 with exception of 1 MU delivery per respiratory cycle, which had 95% points with gamma value 5 MU per gate. For the condition of\5 MU per gate, the maximum dose difference between gated and non-gated delivery was 3%. The points with gamma values \1 were 90% for 1 MU per gate. However, all points had gamma less than 1 if 2% and 2mm criterion were used. The radiation field size (10*10 cm2 vs. 20*20 cm2) did not significantly influence dosimetric differences. Conclusions: The dosimetric performance (absolute dose and 2D dose distribution) of the Siemens Artiste linear accelerator was not found to be significantly affected by the gating operation. A treatment planning system based on non-gated beam data can be used for the gated treatment planning. Cautious use of planning with the 15 MV photon beam using a small number of MU (\5) per gate may be considered. Author Disclosure: S. Chen, None; S. Zhou, None; L. Schubert, None; S. Li, None; C. Lin, None; A. Wahl, None; C. Enke, None.
3379
Comparison of Full Arcs, Avoidance Sectors, and Partial Arcs for RapidArc Planning
V. A. Dumane, J. Kao, S. Green, V. Gupta, Y. Lo The Mount Sinai Medical Center, New York, NY Purpose/Objective(s): To compare RapidArc plans using full arcs, avoidance sectors and partial arcs for lateral PTvs. and full arcs vs. avoidance sectors for PTvs. that are centrally located. Materials/Methods: Ten cases with laterally located PTvs. (consisting of 3 abdomen, 3 head and neck and 4 pelvis cases) and eight cases with centrally located PTvs. (4 pelvis and 4 head and neck) were selected for the study. The choice of avoidance sectors and
S820
3377
Volume 78, Number 3, Supplement, 2010
How many Arc is Enough? A Dosimetric Comparison of RapidArc and Static-Field IMRT for High-Risk Prostate Cancer
M. Yeginer1, B. Aydogan2 1
University of Illinois at Chicago, Chicago, IL, 2University of Chicago, Chicago, IL
Purpose/Objective(s): To compare single (sRA), double (dRA), and triple (tRA) -arc RapidArc (RA) plans with seven (7IMRT) and nine (9IMRT) field IMRT plans prepared for complex-shaped high-risk prostate cancers. Materials/Methods: The RA and IMRT plans were prepared in Eclipse v8.6 treatment planning system for 7 prostate cancer patients. The primary PTV included the prostate, seminal vesicles and lymph nodes with margins while OAR included the bladder, rectum and small bowel. The sRA was obtained with a 360 counter-clockwise gantry rotation between 179.9 and 180.1 . For the dRA, the gantry continued its next 360 rotation clockwise in the second arc-field. The tRA had three sequential 330 arcs because of the total arc limitation of 1000 . In order to compare the OAR sparing performance of the treatment plans more accurately, plans are optimized using the RTOG 0521 protocol. For instance, PTV volume covered by the prescribed dose was set to 95%. In addition, the optimization process was repeated by fine-tuning the dose volume constraints to limit the PTV volume receiving more than 107% of prescribed dose to 1.5%-3%. The PTV DVHs were, therefore, shaped such that the PTV coverage and PTV homogeneity of each plan was similar and comparable. Results: On average, percent bladder median doses were 60.9% (sRA), 53.5% (dRA), 48.5% (tRA), 60.1% (7IMRT), and 54.3% (9IMRT). For the rectum, percent median doses were found to be very similar for the 7IMRT and 9IMRT (?66%) while it was 69.2%, 64.4% and%60.1 for the sRA, dRA, and tRA, respectively. The D20 for small bowel was around 6% for all the IMRT and RA plans. SDs were found to be less than 3% in this study. The monitor units were 1295 MU and 1565 MU for the 7IMRT and 9IMRT, respectively. As expected, the RA monitor units were substantially lower than that for IMRT even for the tRA (715 MU). Conclusions: Nine treatment fields are commonly preferred in the IMRT treatment of localized high risk prostate cancers. While the one arc is the most efficient RA technique, a minimum of two arcs is required to achieve a comparable OAR sparing with the 9IMRT. It is worthwhile to note that the OAR sparing obtained with the sRA and dRA is almost equivalent to that obtained with the 7IMRT and 9IMRT, respectively, suggesting that the sRA can only perform as good as the 7IMRT. This study concludes that the tRA provides the lowest OAR doses with only a modest increase in monitor units (80 MU). Author Disclosure: M. Yeginer, Varian Medical Systems, Inc. (Palo Alto, CA), B. Research Grant; B. Aydogan, Varian Medical Systems, Inc. (Palo Alto, CA), B. Research Grant.
3378
Comparison of Dosimetry of a New Linear Accelerator under Gated Operation vs. Non-gated Operation
S. Chen, S. Zhou, L. Schubert, S. Li, C. Lin, A. Wahl, C. Enke University of Nebraska Medical Center, Omaha, NE Purpose/Objective(s): Beam characteristics may be influenced when a linear accelerator is operated with gated operation. In this study, we evaluated the dosimetry of a Siemens Artiste linear accelerator under gated operation and investigated if there are dosimetric limitations to use the treatment planning system based on non-gated beam data for delivering respiratory gated treatment. Materials/Methods: The Siemens Artiste linear accelerator was installed at our institution with ANZAI-733V respiratory gating system (ANZAI MEDICAL CO, LTD.). The system utilizes a pressure sensor to track the respiratory motion. The signal from the sensor triggers the linac to turn radiation beam on or off. In this study, a motion phantom simulated the respiratory signal. Dosimetric measurements were performed using an IMRT MatriXX system (IBA Dosimetry, Inc.) at 10 cm depth with 100 cm sourceto-surface distance setup. The MatriXX system is a 2D ion chamber array with 7.6 mm spatial resolution; therefore it can be used to measure both the absolute dose and the 2D dose distribution. 6MV and 15MV photon beam data under gated operation were collected for two radiation fields (10*10 cm2 and 20*20 cm2) and 8 duty cycles (50, 25, 10, 5, 4, 3, 2, and 1 monitor unit (MU) delivered per 4 second gate cycle). Measurements were compared with corresponding non-gated operation. The absolute readings of the central ion chamber for gated deliveries were compared to those from the non-gated delivery. 2D dose distribution comparison was performed with gamma analysis (1% and 1 mm criterion). Results: For the 6MV photon beam, all dose deviations were within 1%, even for an extremely low duty cycle (1MU per gate). Similarly, the 2D dose distributions were similar between the gated and non-gated readings. The gamma values of all points in the radiation fields were\1 with exception of 1 MU delivery per respiratory cycle, which had 95% points with gamma value 5 MU per gate. For the condition of\5 MU per gate, the maximum dose difference between gated and non-gated delivery was 3%. The points with gamma values \1 were 90% for 1 MU per gate. However, all points had gamma less than 1 if 2% and 2mm criterion were used. The radiation field size (10*10 cm2 vs. 20*20 cm2) did not significantly influence dosimetric differences. Conclusions: The dosimetric performance (absolute dose and 2D dose distribution) of the Siemens Artiste linear accelerator was not found to be significantly affected by the gating operation. A treatment planning system based on non-gated beam data can be used for the gated treatment planning. Cautious use of planning with the 15 MV photon beam using a small number of MU (\5) per gate may be considered. Author Disclosure: S. Chen, None; S. Zhou, None; L. Schubert, None; S. Li, None; C. Lin, None; A. Wahl, None; C. Enke, None.
3379
Comparison of Full Arcs, Avoidance Sectors, and Partial Arcs for RapidArc Planning
V. A. Dumane, J. Kao, S. Green, V. Gupta, Y. Lo The Mount Sinai Medical Center, New York, NY Purpose/Objective(s): To compare RapidArc plans using full arcs, avoidance sectors and partial arcs for lateral PTvs. and full arcs vs. avoidance sectors for PTvs. that are centrally located. Materials/Methods: Ten cases with laterally located PTvs. (consisting of 3 abdomen, 3 head and neck and 4 pelvis cases) and eight cases with centrally located PTvs. (4 pelvis and 4 head and neck) were selected for the study. The choice of avoidance sectors and