Adaptive radiotherapy for muscle-invasive bladder cancer: optimisation of plan sizes.

Introduction: Plan-of-the-day adaptive radiotherapy (ART) that has not been optimally designed may result in inefficient plan sizes. This can lead to unused plans, which may potentially reduce overall conformality. We compared two methods of individualising ART plan sizes for muscle-invasive bladder cancer to determine which provides a more balanced distribution of plan selections.

Methods: Twenty-seven previously treated patients had small, medium and large ART plans generated from CTV contours on the simulation CT and initial cone beam CTs (CBCT). In the original clinical method, the smallest plan was based on the smallest CTV, while the experimental method used the Boolean summation of the two smallest CTVs. The large plan was identical in both methods. The medium plans were created midway between small and large CTVs. Credentialed treatment staff performed plan selection clinically for the original plans and retrospectively for the experimental plans.

Results: A total of 646 CBCTs from 26 patients were included. The small, medium and large adaptive CTVs, and the conventional CTV, were used 29.7%, 45.4%, 22.0% and 2.9% of the time, respectively, compared to the previous 9.8%, 49.2%, 39.5% and 1.5%. The differences were significant between previous and new CTV (small), and CTV (large).

Conclusions: The new design method resulted in the three adaptive CTV choices being selected more evenly, however, a reduction in a surrogate for normal tissue irradiation was not observed.