This study is a prospective multicentre cohort trial in ≥50 patients with a residual ycT1-3N0 tumor after neoadjuvant chemoradiotherapy or radiotherapy for rectal adenocarcinoma at least 6 weeks after the neoadjuvant treatment.~In addition to standard workup and treatment (e.g. a flexible endoscopy and an MRI scan at 6-8 weeks post-neoadjuvant therapy), patients will undergo pre-operatively, after induction of general anaesthesia, an endorectal ultrasound and rigid rectoscopy as study procedures if these procedures are not already part of standard workup. Furthermore, the pathological specimens of some patients will be scanned using MR imaging during certain parts of the pathological process.~Objectives include determining the maximum distance of microscopic tumor spread per patient in all directions, creating a tissue deformation model to account for changes due to e.g. fixation and pathological processing, using this tissue deformation model to translate the microscopic tumor spread back to the in vivo situation (e.g. back to in vivo MRI scans, 3D endo-ultrasounds), and evaluating/determining risk factors for the presence and/or extent of microscopic tumor spread.~This data will be used for target volume definition in 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.