Optimising head and neck radiotherapy treatment using adaptive radiotherapy

Abstract

Radiotherapy delivery in head and neck cancer (HNC) has dramatically improved recently with the introduction of advanced techniques such as intensity-modulated radiation therapy (IMRT) and image-guided radiation therapy (IGRT). The aim of IGRT is to correct for setup errors such as positional shifts without modifying the original treatment plan. Intra-treatment structural and spatial changes are not corrected using IGRT. In contrast, adaptive radiotherapy (ART) is an exciting concept that aims to modify the radiotherapy treatment plan in response to temporal changes to the anatomy and tumour. In an anatomy-adapted ART, repeat planning is based on intra-treatment structural changes to reduce organs at risk (OARs) planned dose deviations and improve dose homogeneity to preserve target coverage. Treatment can be individualised further by adapting to a changing target volume in a response-adapted ART. Improved soft tissue contrast with magnetic resonance (MR) guidance has made the latter a real possibility. This thesis lays the groundwork for the introduction of ART at our institution. To assist with the increased clinician workload, I first demonstrate the utility of using deformable image registration (DIT) for contour propagation during repeat planning. This improves delineation efficiency by significantly reducing clinician delineation time. Whilst intra-treatment changes are evident in CT-based studies, there are few magnetic resonance imaging (MRI) studies reporting this. This thesis demonstrates that these intra-treatment changes in target volume and OARs are evident on MR imaging and that MR-guided response-adapted ART leads to AOR dose reduction and improves target volume coverage. The clinical introduction of a hybrid MRI and linear accelerator (MR-Linac) has made daily MR-guided ART possible but this has important practical differences to a conventional linear accelerator. Patient selection is crucial due to a restricted carniocaudal treatment field length and a longer treatment time. A hyo-sternal neck length <14.6 cm in a neutral neck position can be optimised using an oral cavity constraint, allaying concerns about increased oral cavity doses. Finally, this thesis reports the first HNC patient to be treated on the MR-Linac using a simple online adapted worflow. Whilst certain findings in this thesis such as the utility of DIR for contour propagation can be applied to current clinical practice, the response-adapted ART findings remain investigative and lay the groundwork for future randomised controlled studies.