Intrafractional Anatomic and Dosimetric Variations of MR-Guided SBRT Treatment of Prostate Bed After Radical Prostatectomy
Y Gao1*, S Yoon1, T Ma1, Y Yang1, K Sheng1, D Low1, M Steinberg1, L Ballas2, A Kishan1, M Cao1, (1) University of California, Los Angeles, Los Angeles, CA, (2) University Of Southern California, Los Angeles, CA
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PO-GePV-M-265 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: To investigate intrafractional anatomic variation and its dosimetric impact for patients receiving MR-guided SBRT treatment to the prostate bed after radical prostatectomy.
Methods: Nineteen patients received SBRT treatment(30-34Gy in 5 fractions) on a 0.35T MR-guided Linac as part of a phase II clinical trial. Pre- and post-treatment MRIs were acquired for each fraction. Prostate bed CTV, PTV (generated with an isotropic 3mm margin), bladder, rectum, and rectal wall were contoured on pre- and post-treatment MRIs obtained for each fraction (n=75 fractions analyzed). Percent volume change and Dice similarity coefficient (DSC) of each structure per fraction were calculated to quantify intrafractional anatomical variations. To evaluate dosimetric impact, the planning dose was transferred to pre- and post-treatment MRI with updated contours, and changes in target coverage and OAR dose were compared.
Results: The median treatment time was 29.5min (IQR 23.6-33.5min). Median CTV and PTV volume changes were less than 2%, and DSC was greater than 0.95. Bladder volume increased by around 50%, and the shape change was moderate (median DSC 0.8). Volume changes were minimal for the rectum and rectal wall (median 1.05% and -0.09%), but there were moderate and large shape changes with median DSC of 0.84 and 0.5, respectively. No statistically significant dosimetric changes were observed except for an increase in bladder hotspot of V35.7Gy from 0cc to 0.06cc. Increased bladder volume also led to favorable dosimetry with bladder V32.5Gy decreasing from 18.66% to 13.34% and mean dose reducing from 17.48Gy to 12.55Gy.
Conclusion: With an isotropic 3mm PTV margin, minimal CTV coverage change was observed despite moderate-large intrafractional shape and size changes in the bladder, rectum, and rectal wall. No significant intrafractional OAR dose changes were observed except for a slight increase in the bladder hotspot.
Funding Support, Disclosures, and Conflict of Interest: Dr. Yang and Dr. Ma received consulting fees from ViewRay Inc. Dr. Cao received personal fee from ViewRay Inc outside the submitted work. Dr. Kishan discloses research support related to this study from ViewRay, Inc., consulting fees from ViewRay, Inc., and low-value stock held in ViewRay Inc.
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