An Alternating Image Registration Approach for Large Scale Bladder Deformations in Radiation Therapy

In adaptive radiotherapy, deformable image registration (DIR) is a crucial component for tasks such as contour propagation and dose summation. In the pelvic region, DIR of planning CTs to daily CBCTs is especially challenging. Organs such as the bladder deform in a way such that common DIR approaches struggle compensating for their strong variations, caused by e.g. varying bladder fillings. In this work, we introduce a novel alternating approach which aims to capture huge differences in bladder filling while maintaining accuracy in adjacent regions.