Background and purpose Adaptive radiotherapy (ART) using plan selection is being introduced clinically for bladder cancer, but the challenge of how to compensate for intra-fractional motion remains. The purpose of this study was to assess target coverage with respect to intra-fractional motion and the potential for normal tissue sparing in MRI-guided ART (MRIGART) using isotropic (MRIGARTiso), an-isotropic (MRIGARTanIso) and population-based margins (MRIGARTpop). Materials and methods Nine bladder cancer patients treated in a phase II trial of plan selection underwent 6-7 weekly repeat MRI series, each with volumetric scans acquired over a 10 min period. Adaptive re-planning on the 0 min MRI scans was performed using density override, simulating a hypo-fractionated schedule. Target coverage was evaluated on the 10 min scan to quantify the impact of intra-fractional motion. Results MRIGARTanIso reduced the course-averaged PTV by median 304 cc compared to plan selection. Bladder shifts affected target coverage in individual fractions for all strategies. Two patients had a v95% of the bladder below 98% for MRIGARTiso. MRIGARTiso decreased the bowel V25 with 15-46 cc compared to MRIGARTpop. Conclusion Online re-optimised ART has a considerable normal tissue sparing potential. MRIGART with online corrections for target shift during a treatment fraction should be considered in ART for bladder cancer.
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