Microscopic intramural spread in patients with rectal cancer after neoadjuvant chemoradiation☆

Objective: This study investigates microscopic intramural spread (MIS) after neoadjuvant (chemo)radiotherapy on Total Mesorectal Excision (TME) specimens of rectal cancer patients and explores the necessity of an additional treatment margin for endorectal radiation boosts (for example through contact brachytherapy (CXB)) or local excisions. Methods: A cohort of patients from Maastricht University Medical Center (MUMC + ) treated between 2016 and 2022 was analyzed. Patients underwent MRI, CT scans, and sigmoidoscopy six weeks after radiotherapy, followed by surgery. Pathological analysis of TME specimens, including whole mount macro-cassettes, was performed to measure residual macroscopic tumor and MIS. Fragmented and continuous MIS were recorded parallel and perpendicular to the bowel wall. Results: Out of 54 patients, 37 (69%) exhibited no MIS. MIS was observed in 4/18 (22%) of patients with ycT1-2 tumors and 13/36 (36%) of patients with ycT3-4 tumors. 4 patients (7%) showed continuous MIS and 15 (28%) showed fragmented MIS. No patients with ypT1-2 had MIS. Conclusions: 69% of patients do not retain MIS post-neoadjuvant therapy. Knowledge of tumor thickness seems crucial for patient selection for CXB.