2041 - Hypofractionated Online Adaptive Radiotherapy for a Personalized Approach to Muscle Invasive Bladder Cancer

Purpose/Objective(s): Neoadjuvant chemotherapy with surgical extirpation or trimodal therapy (TMT) in select patients are effective options for muscle invasive bladder cancer (MIBC). Non-adaptive bladder radiation (RT) methods have risk of target miss and increased organ-at-risk (OAR) toxicity due to daily variation. Online-adaptive RT (oART) is a novel approach to optimize target coverage and reduce OAR dose. We hypothesize that oART improves target coverage and OAR sparing.

Materials/Methods: Patients (n=12) with MIBC (cT2-4a N0) treated by TMT using hypofractionated oART (3/2024-1/2025) were retrospectively analyzed; 92% received concurrent chemotherapy. CT simulation was performed with an empty bladder/rectum with contrast followed by a scan 15 minutes later to evaluate patient-specific bladder fill for personalized Planning target volume (PTV) margins. Targets included the whole empty bladder and pelvic lymph nodes. Bladder PTVs were treated to 51-55 Gy (guided by OAR constraints) with nodes to 40 Gy in 20 fractions. We employed an oART hybrid coverage scheme with weekly physician and daily physicists (trained on target and OAR delineation) presence (daily offline physician review) to recontour targets/OARs and adapt dose daily.

Results: Customized bladder PTV margins had a mean expansion of 1.0 cm (range: 0.8-1.2 cm). Mean adaptive session time was 29 ± 7 minutes. oART fraction coverage: direct physician (30%) and physics (70%). The bladder fill and proximity of bowel to high-dose region (i.e., overlap volume with PTV) varied by 23% ± 17% and 10.7 ± 28.6 cc, respectively. Adaptive was selected over scheduled plan in 99.6% of fractions for improved target coverage and/or OAR sparing. Percentage of scheduled/adaptive fractions that met clinical goals: Bladder PTV V_100%: 30%/80%, Bowel V_55Gy: 60%/80%, Bowel D_0.03cc: 90%/100%, and Rectum D_0.03cc: 50%/100%, respectively (see Table 1). At median follow-up of 3 months (range 1-9 months), no grade 3 or higher toxicities were observed and grade 2 GI toxicity was 8%.

Conclusion: