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Optimal Design and Evaluation of Electron Beam Energy Degrader for Breast Boost Irradiation
Poster Viewing Abstracts S931
Volume 90 Number 1S Supplement 2014 being introduced into the clinics and their advantages are being demonstrated particularly for hypofractionation and SBRT, where online replanning is important. In this study, we determine the applicability of the SAM-SWO algorithm for the replanning with FFF beams. Materials/Methods: The previously developed replanning tool based on the SAM-SWO algorithm for FF beams was used to evaluate the effectiveness of the SAM-SWO algorithm for FFF beams. Daily CTs collected during IGRT using a CT-on-Rails for representative cases were used. Three types of plans, repositioning, full-blown reoptimization and SAM-SWO plans were generated on daily CTs for representative prostate cancer cases. In addition, the SAM-SWO plans to correct for translational errors were generated on daily CTs for representative head and neck cancer cases with the shifts of leaves in the range of -15 to 15 mm with 3 mm increments in the direction of the MLC motion. The qualities of these plans generated based on daily CTs were compared with their corresponding original plans. Results: Various dose volume parameters for the three type of plans to account for deformations for two representative prostate cancer cases with small (Dice Coefficient (DC) Z 0.87) and moderate-large (DC Z 0.61) deformations are tabulated in the top portion of the table. The SAM-SWO plans with FFF beams are generally better than the repositioning plans and are equivalent to the reoptimization plans for small deformations, but they were worse than the reoptimization plans for large deformations. The selected DV parameters of the SAM-SWO plans generated for representative head and neck cancer cases, are included in the bottom portion of the table. The SAM-SWO plans preserve the most of the DV parameters, within 3% and 5%, as in the original plans for shifts up to 5mm and 1cm respectively. Conclusions: The aperture-morphing based replanning algorithm (e.g., SAM-SWO), is effective for online replanning with FFF beams, provided that the translational errors are not too big (< 1 cm) and/or organ deformations are not too large (e.g., DC>0.8). More complex replanning algorithms would be needed for larger shifts and/or deformations. Author Disclosure: E.E. Ahunbay: None. A.X. Li: None.
3826 Optimal Design and Evaluation of Electron Beam Energy Degrader for Breast Boost Irradiation J. Park,1 J. Kim,2 L. HyunSeok,1 J. Lee,1 K. Hyuckjun,3 H. Sung Whan,2,3 and S. Ye1,2; 1Program in Biomedical Radiation Sciences, Department of Transdisciplinary Studies, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, Korea, Republic of Korea, 2 Department of Radiation Oncology ,Seoul National University Hospital, Seoul, Korea, Republic of Korea, 3Interdisciplinary Program in Radiation Applied Life Science, Seoul National University, Seoul, Korea, Republic of Korea Purpose/Objective(s): Electron boost irradiation takes advantage of minimizing lung and heart doses because of the rapid fall-off in depthdoses. However, an electron mode of linear accelerator lacks sufficient energy spacing to treat optimal therapy for patients. Therefore, for patients who are treated in therapeutic ranges between 6 and 9 MeV electron beams, energy degraders (EDs) were specially designed to increase the skin dose and reduce penetration depths of 9 MeV electron beam. Materials/Methods: The EGS4/BEAMnrc and DOSXYZnrc codes were used to benchmark and assess the impact of the ED on a 9 MeV electron beam. The 9 MeV electron beam with ED placed on the lowermost scraper of the electron applicator is referred to as 7 MeV. In order to satisfy the depth dose characteristics and uniform dose profile at dmax, Monte Carlo simulations (MC) were performed to optimize the shape and thickness of the EDs. Percent depth doses (PDDs), profiles, and output factors were measured with a cylindrical ionization chamber to validate the MC results of EDs for cones of 10 10 cm2 and 15 15 cm2. In-vivo measurements were performed for three breast boost patients. Optically stimulated luminescent dosimeters (OSLDs) were used to measure in-vivo skin doses for breast boost patients. The chest wall thicknesses of post-mastectomy patients in our institution were measured by using patient’s CT-images. Results: Simulated results were in agreement with measured data within 2% for all of the energies. The skin doses with EDs were increased by 8% 9%, compared to those of 9 MeV electron beam. The shape of optimized
EDs was truncated a cone attached on the plane plate and the optimal thickness of EDs was 1.1 cm. Then the skin dose of nearly 90% was achieved by using the EDs for both the cone sizes. Compared to the output of 9 MeV 10 10 cm2 cone, the outputs of 7 MeV were slightly reduced into 0.882 and 0.972 for 10 10 cm2 and 15 15 cm2 cone sizes, respectively. Photon contamination was not increased. Uniformity of 7 MeV skin dose was 0.93 0.03, compared to 9 MeV skin dose. In our institution, about 22.5% of post-mastectomy patients had a chest wall of 0.9 to 2.7 cm thickness that could be adequate for 7 MeV treatments. Conclusions: The newly designed 7 MeV electron beam provides breast conserving and mastectomy patients with a new energy option for breast boost and chest wall irradiation. It effectively spares the lung tissue, while providing a sufficient skin dose. Author Disclosure: J. Park: None. J. Kim: None. L. HyunSeok: None. J. Lee: None. K. Hyuckjun: None. H. Sung Whan: None. S. Ye: None.
3827 Simultaneously Integrated Boost (SIB) Improves Sparing of Normal Tissues in Locally Advanced Cervical Cancer While Reducing Overall Treatment Time H. Feng,1 Y. Hasan,2 M. Kopec,2 and H.A. Al-Hallaq2; 1The University of Chicago Pritzker School of Medicine, Chicago, IL, 2Department of Radiation and Cellular Oncology, The University of Chicago Medical Center, Chicago, IL Purpose/Objective(s): IMRT for locally advanced cervical cancer has emerged as an advantageous technique for radiation therapy. Previous data from our institution has shown that extended treatment time decreases pelvic disease control in this patient population. We performed a dosimetric comparison of sequential IMRT (sIMRT) boost of lymph nodes (LNs) in locally advanced cervical cancer and simultaneously integrated boost (SIB) plans to assess dose to target volumes and organs-at-risk (OAR). Materials/Methods: We retrospectively identified 6 patients with intact PET avid LN positive cervical cancer. Delineating PTVs (planning target volume, extended field n Z 4, pelvis n Z 2) and LN boost volumes from pretreatment CT simulation and PET scans, SIB and sequential IMRT plans were created and compared for each patient. The PTV prescription (45 Gy in 25 fractions) goal was 95% coverage of target volumes while maximally sparing small bowel, bladder, rectum, and bone marrow. Accounting for brachytherapy contribution, the prescribed LN dose was a median 7.5 Gy (5.4-10.8 Gy) for both sIMRT and SIB. Dosimetric parameters for PTV coverage and homogeneity, and OAR were compared between the two methods. Paired two-tailed t-tests with Bonferroni correction of 5% significance level (p < 0.01) were used for all statistical comparisons. Results: The median PTV volume was 1843 cc (1088-2225 cc) and the median boost volume was 43 cc (15-129 cc). Comparable target volume coverage (V90% and V95%) was achieved with sequential and SIB plans, while hot spots (V110% and V115%) were significantly reduced using SIB. SIB plans improved sparing for all OAR, though only rectum and small bowel doses were statistically significant. Comparing sequential and SIB plans averaged over all patients, rectal doses were V45: 69.0% vs 65.7% and 0.1 cc: 50.2 Gy vs 48.7 Gy. For small bowel, sequential and SIB plans yielded mean V45: 10.9% vs 8.7% and 250 cc: 38.2 Gy vs 36.7 Gy, respectively. V30 and doses to 2 cc, 1 cc, and 0.1 cc for rectum and small bowel were also significantly reduced in SIB plans. The doses to femoral heads and bladder trended towards significance in favor of SIB plans while bone marrow doses were comparable between plans. The mean treatment time was 25 vs 29 days for SIB and sIMRT plans, respectively. Conclusions: SIB, when compared to sequential IMRT, provides comparable target coverage with improved homogeneity to the PTV. Furthermore, SIB delivers a lower dose to OAR, especially rectum and small bowel, despite boosting a larger pre-treatment nodal volume. Further investigation is warranted for the potential integration of SIB into clinical management of locally advanced cervical cancer. Author Disclosure: H. Feng: None. Y. Hasan: None. M. Kopec: None. H.A. Al-Hallaq: None.
Volume 90 Number 1S Supplement 2014 being introduced into the clinics and their advantages are being demonstrated particularly for hypofractionation and SBRT, where online replanning is important. In this study, we determine the applicability of the SAM-SWO algorithm for the replanning with FFF beams. Materials/Methods: The previously developed replanning tool based on the SAM-SWO algorithm for FF beams was used to evaluate the effectiveness of the SAM-SWO algorithm for FFF beams. Daily CTs collected during IGRT using a CT-on-Rails for representative cases were used. Three types of plans, repositioning, full-blown reoptimization and SAM-SWO plans were generated on daily CTs for representative prostate cancer cases. In addition, the SAM-SWO plans to correct for translational errors were generated on daily CTs for representative head and neck cancer cases with the shifts of leaves in the range of -15 to 15 mm with 3 mm increments in the direction of the MLC motion. The qualities of these plans generated based on daily CTs were compared with their corresponding original plans. Results: Various dose volume parameters for the three type of plans to account for deformations for two representative prostate cancer cases with small (Dice Coefficient (DC) Z 0.87) and moderate-large (DC Z 0.61) deformations are tabulated in the top portion of the table. The SAM-SWO plans with FFF beams are generally better than the repositioning plans and are equivalent to the reoptimization plans for small deformations, but they were worse than the reoptimization plans for large deformations. The selected DV parameters of the SAM-SWO plans generated for representative head and neck cancer cases, are included in the bottom portion of the table. The SAM-SWO plans preserve the most of the DV parameters, within 3% and 5%, as in the original plans for shifts up to 5mm and 1cm respectively. Conclusions: The aperture-morphing based replanning algorithm (e.g., SAM-SWO), is effective for online replanning with FFF beams, provided that the translational errors are not too big (< 1 cm) and/or organ deformations are not too large (e.g., DC>0.8). More complex replanning algorithms would be needed for larger shifts and/or deformations. Author Disclosure: E.E. Ahunbay: None. A.X. Li: None.
3826 Optimal Design and Evaluation of Electron Beam Energy Degrader for Breast Boost Irradiation J. Park,1 J. Kim,2 L. HyunSeok,1 J. Lee,1 K. Hyuckjun,3 H. Sung Whan,2,3 and S. Ye1,2; 1Program in Biomedical Radiation Sciences, Department of Transdisciplinary Studies, Graduate School of Convergence Science and Technology, Seoul National University, Seoul, Korea, Republic of Korea, 2 Department of Radiation Oncology ,Seoul National University Hospital, Seoul, Korea, Republic of Korea, 3Interdisciplinary Program in Radiation Applied Life Science, Seoul National University, Seoul, Korea, Republic of Korea Purpose/Objective(s): Electron boost irradiation takes advantage of minimizing lung and heart doses because of the rapid fall-off in depthdoses. However, an electron mode of linear accelerator lacks sufficient energy spacing to treat optimal therapy for patients. Therefore, for patients who are treated in therapeutic ranges between 6 and 9 MeV electron beams, energy degraders (EDs) were specially designed to increase the skin dose and reduce penetration depths of 9 MeV electron beam. Materials/Methods: The EGS4/BEAMnrc and DOSXYZnrc codes were used to benchmark and assess the impact of the ED on a 9 MeV electron beam. The 9 MeV electron beam with ED placed on the lowermost scraper of the electron applicator is referred to as 7 MeV. In order to satisfy the depth dose characteristics and uniform dose profile at dmax, Monte Carlo simulations (MC) were performed to optimize the shape and thickness of the EDs. Percent depth doses (PDDs), profiles, and output factors were measured with a cylindrical ionization chamber to validate the MC results of EDs for cones of 10 10 cm2 and 15 15 cm2. In-vivo measurements were performed for three breast boost patients. Optically stimulated luminescent dosimeters (OSLDs) were used to measure in-vivo skin doses for breast boost patients. The chest wall thicknesses of post-mastectomy patients in our institution were measured by using patient’s CT-images. Results: Simulated results were in agreement with measured data within 2% for all of the energies. The skin doses with EDs were increased by 8% 9%, compared to those of 9 MeV electron beam. The shape of optimized
EDs was truncated a cone attached on the plane plate and the optimal thickness of EDs was 1.1 cm. Then the skin dose of nearly 90% was achieved by using the EDs for both the cone sizes. Compared to the output of 9 MeV 10 10 cm2 cone, the outputs of 7 MeV were slightly reduced into 0.882 and 0.972 for 10 10 cm2 and 15 15 cm2 cone sizes, respectively. Photon contamination was not increased. Uniformity of 7 MeV skin dose was 0.93 0.03, compared to 9 MeV skin dose. In our institution, about 22.5% of post-mastectomy patients had a chest wall of 0.9 to 2.7 cm thickness that could be adequate for 7 MeV treatments. Conclusions: The newly designed 7 MeV electron beam provides breast conserving and mastectomy patients with a new energy option for breast boost and chest wall irradiation. It effectively spares the lung tissue, while providing a sufficient skin dose. Author Disclosure: J. Park: None. J. Kim: None. L. HyunSeok: None. J. Lee: None. K. Hyuckjun: None. H. Sung Whan: None. S. Ye: None.
3827 Simultaneously Integrated Boost (SIB) Improves Sparing of Normal Tissues in Locally Advanced Cervical Cancer While Reducing Overall Treatment Time H. Feng,1 Y. Hasan,2 M. Kopec,2 and H.A. Al-Hallaq2; 1The University of Chicago Pritzker School of Medicine, Chicago, IL, 2Department of Radiation and Cellular Oncology, The University of Chicago Medical Center, Chicago, IL Purpose/Objective(s): IMRT for locally advanced cervical cancer has emerged as an advantageous technique for radiation therapy. Previous data from our institution has shown that extended treatment time decreases pelvic disease control in this patient population. We performed a dosimetric comparison of sequential IMRT (sIMRT) boost of lymph nodes (LNs) in locally advanced cervical cancer and simultaneously integrated boost (SIB) plans to assess dose to target volumes and organs-at-risk (OAR). Materials/Methods: We retrospectively identified 6 patients with intact PET avid LN positive cervical cancer. Delineating PTVs (planning target volume, extended field n Z 4, pelvis n Z 2) and LN boost volumes from pretreatment CT simulation and PET scans, SIB and sequential IMRT plans were created and compared for each patient. The PTV prescription (45 Gy in 25 fractions) goal was 95% coverage of target volumes while maximally sparing small bowel, bladder, rectum, and bone marrow. Accounting for brachytherapy contribution, the prescribed LN dose was a median 7.5 Gy (5.4-10.8 Gy) for both sIMRT and SIB. Dosimetric parameters for PTV coverage and homogeneity, and OAR were compared between the two methods. Paired two-tailed t-tests with Bonferroni correction of 5% significance level (p < 0.01) were used for all statistical comparisons. Results: The median PTV volume was 1843 cc (1088-2225 cc) and the median boost volume was 43 cc (15-129 cc). Comparable target volume coverage (V90% and V95%) was achieved with sequential and SIB plans, while hot spots (V110% and V115%) were significantly reduced using SIB. SIB plans improved sparing for all OAR, though only rectum and small bowel doses were statistically significant. Comparing sequential and SIB plans averaged over all patients, rectal doses were V45: 69.0% vs 65.7% and 0.1 cc: 50.2 Gy vs 48.7 Gy. For small bowel, sequential and SIB plans yielded mean V45: 10.9% vs 8.7% and 250 cc: 38.2 Gy vs 36.7 Gy, respectively. V30 and doses to 2 cc, 1 cc, and 0.1 cc for rectum and small bowel were also significantly reduced in SIB plans. The doses to femoral heads and bladder trended towards significance in favor of SIB plans while bone marrow doses were comparable between plans. The mean treatment time was 25 vs 29 days for SIB and sIMRT plans, respectively. Conclusions: SIB, when compared to sequential IMRT, provides comparable target coverage with improved homogeneity to the PTV. Furthermore, SIB delivers a lower dose to OAR, especially rectum and small bowel, despite boosting a larger pre-treatment nodal volume. Further investigation is warranted for the potential integration of SIB into clinical management of locally advanced cervical cancer. Author Disclosure: H. Feng: None. Y. Hasan: None. M. Kopec: None. H.A. Al-Hallaq: None.