Improvements in the treatment of rectal cancer patients have led to increased survival. Therefore, long-term outcome has become an increasingly important factor. Both neoadjuvant (chemo... Show moreImprovements in the treatment of rectal cancer patients have led to increased survival. Therefore, long-term outcome has become an increasingly important factor. Both neoadjuvant (chemo)radiotherapy and total mesorectal excision surgery are associated with toxicity. As a result, research for rectal cancer treatment has focused on the reduction of radiation dose to healthy tissue and less extensive surgery or omission of surgery in selected patients. The work described in this thesis can be used to reduce uncertainties related to image-guided external beam radiotherapy (EBRT) and high-dose-rate endorectal brachytherapy (HDREBT) of rectal cancer. By decreasing treatment related uncertainties, dose to surrounding healthy tissue can be reduced and/or a higher dose to the target volume can be delivered with an isotoxic effect. The MRI visibility of four types of fiducial markers as a surrogate for gross tumor volume (GTV) location was evaluated. In addition, it was found that the use of fiducial markers as a surrogate for GTV location in an EBRT GTV boost setting allows smaller treatment margins. For HDREBT, it was found that daily CT imaging should be the minimal standard to verify a correct applicator setup. In addition, we propose the use of an ultrashort echo time MRI sequence to visualize the applicator on MRI. Lastly, we determined the added value of a national study group meeting on the quality and variability of EBRT treatment plans for the introduction of a novel target volume that includes only the mesorectum. Show less
