Abstract
BACKGROUND: Immunotherapy is used routinely for treating deficient mismatch repair (dMMR) colon cancer (CC). This case series highlights an emerging safety issue, where patients develop bowel obstruction associated with immunotherapy response. PATIENTS AND METHODS: Patients with dMMR CC who developed bowel obstruction while responding to immunotherapy were retrospectively identified. Data on patient, disease, treatment, and response-specific factors were explored for potential risk factors. Overall treatment numbers were used to estimate incidence. RESULTS: Nine patients from eight European centres were included. Common features were hepatic flexure location (5/9), T4 radiological staging (6/9), annular shape (8/9), radiological stricturing (5/9), and endoscopic obstruction (6/9). All received pembrolizumab and obstructed between 45 and 652 days after starting treatment. Seven patients underwent surgical resection; one was managed with a defunctioning stoma; and one was managed conservatively. One patient died from obstruction. Radiological response was seen in eight patients, including two complete responses. Pathological response was seen in all seven who underwent resection, including four complete responses. The overall incidence of immunotherapy response-related obstruction in these centres was 1.51%. CONCLUSIONS: Bowel obstruction associated with immunotherapy response may represent a rare treatment-related complication in dMMR CC. Clinicians must recognise this safety signal and share experience to maintain patient safety.