Bowel Stenosis Following Immunotherapy in dMMR Colorectal Cancer: A Case Series

Background The use of immune checkpoint inhibitors in the treatment of deficient mismatch repair (dMMR) colorectal cancers (CRC) has become standard in the metastatic setting and is promising in the perioperative setting of locally advanced CRCs. While immune-related adverse events are well recognized, bowel stenosis following ICI therapy is an emerging and rare but serious complication of immunotherapy. Case presentation We report a case series of five patients with metastatic or locally advanced dMMR colon adenocarcinoma treated with ICIs who developed bowel stenosis despite complete radiologic and pathologic tumor response. All tumors were circumferential and initially impassable by endoscope. Two patients presented with acute obstructive symptoms requiring urgent surgery, both with complete pathologic response at resection. Three patients were asymptomatic and diagnosed through routine imaging or endoscopy; one underwent a hemicolectomy and two remain stenotic but clinically well under surveillance. Conclusion Bowel stenosis following ICI in dMMR CRC appears to represent a fibrosis-driven response and may be more frequent than clinically apparent. A low threshold for investigation of new occlusive or subocclusive symptoms is warranted in this patient population, even in responders. Optimal surveillance and risk stratification strategies remain undefined and require prospective evaluation. Mini-abstract Immune checkpoint inhibitors (ICI) improve outcomes in metastatic and locally advanced deficient mismatch repair colorectal cancer. We report a case series of five patients with bowel stenosis following ICI therapy despite radiologic and pathologic tumor response. Clinicians should have a low threshold for investigation of new obstructive symptoms and consider surveillance strategies for this rare but emerging complication of immunotherapy.