3235 - MR-Guided Stereotactic Radiotherapy for Oligometastatic Prostate Cancer: Toward Optimal Patient Selection
Presenter(s)
L. Burns1,2, I. Mansour1,2, J. Winter1,2, A. Moreira1,2, A. Berlin1,2, C. Catton1,2, A. J. Bayley1,2, A. Mesci1,3, B. Id Said1,2, P. Chung1,2, R. Glicksman1,2, and A. McPartlin1,2; 1Department of Radiation Oncology, University of Toronto, Toronto, ON, Canada, 2Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, ON, Canada, 3Princess Margaret Hospital, UNH, University of Toronto, Toronto, ON, Canada
Purpose/Objective(s): MR-guided SBRT (MR-SBRT) allows for superb target visualization and online adaptation to daily anatomical changes. The high resource cost compared to conventional SBRT necessitates careful utilization. We assess outcomes and daily treatment plans for prostate cancer oligometastasis (OM) patients treated with MR-SBRT to inform future patient selection.
Materials/Methods:
Prostate cancer patients receiving OM-directed MR-SBRT on a single-center prospective registry study from March 2021 to July 2024 were included. Local failure (LF) was defined as any amount of radiographic progression. Serial patient-reported toxicity data were analyzed with a linear mixed effects model. All reference and daily adapted treatment plans were collected. PTVs are 5mm isotropic expansions of CTV. An adapt-to-shape workflow was used. Each treated OM was classified by whether the PTV was in proximity to a dose-limiting organ-at-risk (OAR) on reference plan, defined as direct overlap or within 5mm. Dosimetric analysis included mean dose, D99%, V100%, and difference between reference and adapted V100% (?V100%), compared using nonparametric Wilcoxon signed-rank tests.Results:
38 patients received 44 courses of MR-SBRT, with median 2 PTVs treated per MR-SBRT course (range 1-6; 71 nodes, 13 bones, 1 visceral). Median follow-up was 29.6 months (range 9.9-57.3) from first MR-SBRT course. Excluding 8 patients who started androgen deprivation with MR-SBRT, probability of no detected progression at 1 year was 63.3% (95% CI: 48.2-83.2%). 7 patients had no further treatment and remain without disease progression (median 25.5 months, range 14.2-40.5). Fatigue was the only toxicity with significant change from baseline. Of 56 PTVs with follow-up imaging, LF occurred in 3 bones and 1 node. There was significant variation in coverage for PTVs in proximity vs not in proximity to an OAR, both among the 85 reference PTVs (median V100% = 0.77, IQR 0.63-0.92 vs 0.84, IQR 0.64-0.97; p=0.011) and the 349 daily adapted PTVs (median V100% = 0.77, IQR 0.64-0.92 vs 0.84, IQR 0.63-0.95, p=0.023). This difference was not observed among CTVs in any group (median V100% = 1.00 for each). 132, 64, or 30 daily adapted PTVs have ?V100% of at least 0.05, 0.10, or 0.15, respectively. Of the ?V100% > 0.15 group, 8/30 had reference V100% > 95%, and another 8/30 had better coverage on adaptation than reference. Across the cohort, PTV ?V100% did not show significant variation for in proximity vs not (median 0.005, range -0.238 to +0.322, vs median 0.003, range -0.192 to +0.357, p=0.58).Conclusion: Online adaptive MR-SBRT was well-tolerated for this cohort with excellent local control. We demonstrate that PTV coverage is affected by OAR in proximity, but CTV coverage is not, with several cases identified where adapted PTV coverage is degraded to preserve OAR constraints. Without adaptation, these compromised OARs may receive higher doses than planned.