DETECT: Target Volume for Rectal Endoluminal Radiation Boosting

The aim of the study is to provide prospective data regarding microscopic tumor spread in all directions from the macroscopic tumor in pathology specimens, as seen by eye, and on imaging to define the target volume for endoluminal radiation boosting in rectal cancer patients.

Inclusion Criteria: * ≥18 years of age and capable of giving informed consent. * ycT1-3N0(\*) residual(\*\*) histology confirmed rectal adenocarcinoma after neoadjuvant radiotherapy or long-course chemoradiotherapy for which patients will undergo TME surgery. * Minimal interval between end of neoadjuvant chemoradiotherapy or radiotherapy: 6 weeks. (\*)= as determined by clinical assessment (digital rectal examination, endoscopy with or without biopsy) and/or MRI. Biopsy/histology around the time of diagnosis is adequate; no biopsy/histology is needed after neoadjuvant therapy. (\*\*)= including tumor regrowths/local recurrence after an initial clinical complete response and a "watch and wait" approach. These patients will also be included after the local recurrence has been determined using endoscopy and/or MRI. Exclusion Criteria: * Patient has received brachytherapy as part of neoadjuvant treatment. * \