Main Session
Sep
29
PQA 06 - Genitourinary Cancer, Gynecological Cancer, and Health Care Access and Engagement
3782 - Dosimetric Impact of MRI-Based Planning Plus Multichannel Cylinder for Vaginal Cuff Brachytherapy
Presenter(s)
Colton Ladbury, MD - City of Hope National Medical Center, Duarte, CA
A. L. Schwer1, C. Shi1, R. Etemadpour1, S. N. Lim2, B. Maraghechi1, H. Ai1, J. Zhu2, S. Hendley2, W. T. Watkins2, K. Qing2, B. Nakamura3, J. Kim3, L. G. Brunette3, J. G. Cohen3, H. Kim2, S. M. Glaser2, and C. J. Ladbury2; 1Department of Radiation Oncology, Orange County Lennar Foundation Cancer Hospital, Irvine, CA, 2Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, 3Department of Gynecologic Oncology, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s):
Vaginal cuff brachytherapy (VCB) is a high-value component of curative treatment regimens for select uterine cancers. A common approach to planning VCB is to prescribe to 5 mm depth, with the corresponding target volume defined as a 5 mm expansion off the upper vaginal cylinder. Due to variability in anatomy, a 5 mm expansion will not always conform to vaginal wall and may extend into organs at risk (OARs), leading to unnecessarily high OAR doses. MRI can aid in defining vaginal wall thickness along the target volume, but the resulting structure can be asymmetric, which leads to a tradeoff between coverage and OAR dose when using a standard single-channel cylinder. This study hypothesized that by using a combination of MRI-based planning and multi-channel cylinder, stepwise improvements in OAR dosimetry could be achieved while maintaining suitable target coverage.Materials/Methods:
Ten cases where patients underwent VCB with treatment planning MRI were identified. All patients were prescribed a BT dose of 2100 cGy/3 fx. Each patient was planned in three approaches: with a single channel cylinder with HR-CTV defined as a 5 mm expansion off the upper 3 cm of the vaginal cylinder (SC 5 mm); with a single channel cylinder with HR-CTV defined as the vaginal wall thickness based on MRI along the upper 3 cm of the vaginal cylinder (SC MRI); with a multi-channel cylinder (with 7 total channels) with HR-CTV defined using MRI as before (MC MRI). All plans kept target coverage D90 at 700 ± 50 cGy. Cumulative dosimetry was then calculated assuming dosimetry would be identical across all three fractions with a/ß=3 for OARs. Comparisons between plans were performed using a one-sided Wilcoxon signed-rank test.Results:
A total of 30 plans were generated from 10 patients. Cylinder diameter distribution included two 2.5 cm, six 3.0 cm, and two 3.5 cm. Relative to the SC 5 mm plans, the SC MRI plans resulted in an average per fraction decrease in bladder D2cc of 64 ± 18 cGy (range 33 to 88; p<0.001), rectum D2cc of 71 ± 20 cGy (range 49 to 118; p<0.001), and vaginal wall D0.1cc of 193 ± 51 cGy (range 96 to 264; p<0.001). Over a three-fraction course, that corresponded to a decrease of EQD2 492 cGy, 604 cGy, and 3769 cGy for bladder D2cc, rectum D2cc, and vaginal D0.1cc. Relative to the SC MRI plans, the MC MRI plans resulted in an average per fraction decrease in bladder D2cc of 9 ± 13 cGy (range 33 to -10; p=0.053), rectum D2cc of 25 ± 24 cGy (range 82 to 1; p<0.001), and vaginal wall D0.1cc of 174 ± 146 cGy (range 385 to -10 p=0.005). Over a three-fraction course, that corresponded to a decrease of EQD2 67 cGy, 216 cGy, and 3120 cGy for bladder D2cc, rectum D2cc, and vaginal D0.1cc.Conclusion:
MRI-based planning leads to improvements in VCB OAR doses. Use of a multi-channel cylinder could decrease OAR dose while maintaining coverage of the true target, though the magnitude of benefit was generally small and varied by patient. Dosimetric benefit could be modeled on an individual basis to determine if it warrants increased planning complexity and treatment time.