Magnetic resonance image guided adaptive radiotherapy for gynaecological and rectal cancer

Multiple challenges exist in radiotherapy for gynaecological and rectal cancer. These lead to uncertainty and error in dose delivered to the tumour target and organs at risk (OARs). Adequate planning target volume (PTV) safety margins are required to avoid a geographical miss. In this thesis the potential to reduce geometric uncertainty and improve outcomes with Magnetic Resonance Image (MRI)-guided adaptive radiotherapy (ART) is investigated. New MRI sequences have been developed and validated for target volume delineation in radiotherapy treatment planning for gynaecological and rectal cancer. Uncertainties in target volume delineation are investigated and the potential to move away from traditional clinical target volume (CTV) definition, to individualised CTVs based on tumour extent, as seen on MRI, is explored. Tumour targets are highly mobile deformable structures influenced by adjacent bladder and rectal filling. Target motion is evaluated and new population-based PTV margins are calculated that account for geometric uncertainty from target motion, target volume delineation and patient set-up. The alternative to large population-based margins is ART. Image-guided radiotherapy with cone beam computerised tomography is limited by its ability to visualise the target and OARs. The dosimetric benefit of different ART strategies, including full plan re-optimisation based on anatomy at the time of treatment, is investigated. The work presented in this thesis demonstrates dosimetric benefit and reduction in error and uncertainty with MRI-guided ART. This work was necessary before implementation of large prospective studies to evaluate clinical outcome.