CT-Guided Adaptive Radiotherapy (CT-ART) Improves Target Coverage in Hypofractionated Radiation Therapy for Bladder Cancer

Purpose/Objective(s) We assess the feasibility and dosimetric characteristics of CT-guided daily adaptive radiotherapy (CT-ART) for patients with muscle-invasive bladder cancer (MIBC). Compared to the scheduled treatment, we hypothesized that CT-ART would allow us to optimize target coverage and dose to organs at risk (OARs) by accounting for inter-fraction motion of the bladder and OARs. Materials/Methods Four patients with cT2a-T4aN0M0 MIBC treated with hypofractionated RT or chemoRT at 2.75 Gy/Fx (55Gy/20) with empty bladder using CT-ART were retrospectively reviewed. There were 42 total fractions delivered with CT-ART. For each Fx, cone beam CT (CBCT) scans were acquired with artificial-intelligent segmented target and OARs. Contours were manually edited and a patient-specific internal target volume (ITV) was applied for bladder filling during the adaptive process. An adaptive plan (PA) was then created and optimized based on these volumes. Adaptive plan metrics for target/OARs were then compared to the initial plan (PI) for each Fx. The PTV_5500 V100 (%), V95 (%), D97% (Gy), and D95% (Gy); rectum D47Gy (cc); and non-sigmoid bowel OARs were compared between the PI and PA plans using paired samples t-test (Table). Results CT-ART improved PTV coverage, with mean per Fx improvement of 12.4% and 28.3% for V95 and V100 and 0.4 Gy/Fx for both D 97% and D95% (p<0.01 for all). The PI plan did not meet PTV V100 ≥95% in 39 of 42 Fxs (92.9%) vs. 4 of 42 Fx (9.5%) with the PA plans. PA plans did not result in increased dose to OARs (Table). Median CT-ART treatment duration improved over time [median 33 min (range 30-47) for the last patient treated]. Conclusion CT-based online adaptive radiation therapy was feasible and significantly improved PTV coverage without a corresponding increase in dose to OARs. This study demonstrates the potential value of CT-ART for MIBC. This approach will be tested in an upcoming phase II trial.