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

Abstract

<h3>Purpose/Objective(s)</h3> 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. <h3>Materials/Methods</h3> 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 (P_A) was then created and optimized based on these volumes. Adaptive plan metrics for target/OARs were then compared to the initial plan (P_I) for each Fx. The PTV_5500 V₁₀₀ (%), V₉₅ (%), D_97% (Gy), and D_95% (Gy); rectum D_47Gy (cc); and non-sigmoid bowel OARs were compared between the P_I and P_A plans using paired samples t-test (Table). <h3>Results</h3> 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 D_95% (<i>p</i><0.01 for all). The P_I plan did not meet PTV V₁₀₀ ≥95% in 39 of 42 Fxs (92.9%) vs. 4 of 42 Fx (9.5%) with the P_A plans. P_A 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]. <h3>Conclusion</h3> 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.