Optimizing MRI-guided radiotherapy for dose escalation in rectal cancer

Abstract

Dose escalation improves response rates in rectal cancer, potentially increasing eligibility for the watch-and-wait approach. Achieving high response rates requires high radiotherapy doses, which is challenging in conventional radiotherapy due to treatment related uncertainties, mainly anatomical changes during (intra) and between (inter) treatment fractions. Moreover, dose escalation requires accurate tumor definition, which is not possible when using CBCT as an imaging modality since the tumor is poorly visible on CBCT. These challenges can be addressed using MRI-integrated linear accelerators (MR-linac), which allow for on-line tumor visualization, daily online adaptation and motion management. The aim of this thesis was to optimize the MRI-guided workflow to allow for safe dose escalation to rectal tumors. By improving autosegmentation, applying second adaptations and personalizing margins based on tumor characteristics this thesis presents steps towards optimizing MRI-guided radiotherapy to possibly allow for safe dose escalation in rectal cancer.