Cytomegalovirus-associated ulceration complicating immune checkpoint inhibitor-related colitis.

We present a case of cytomegalovirus (CMV)-associated rectal ulceration that developed as a secondary complication of immune checkpoint inhibitor-induced colitis. A 74-year-old man who received pembrolizumab for metastatic squamous cell carcinoma of the lung developed severe immune-related colitis and required high-dose corticosteroid therapy. Although his condition initially improved, his symptoms recurred approximately four weeks after the initiation of steroid tapering. Colonoscopy revealed a circumferential, band-shaped ulcer located just above the dentate line, and a histopathological examination demonstrated nuclear inclusion bodies suggestive of CMV infection. The diagnosis was confirmed by immunohistochemical staining. Treatment with valganciclovir resulted in complete clinical and virological remission within four weeks. This case highlights the need for monitoring opportunistic infections during immunosuppressive therapy for immune-related adverse events (irAEs). It also underscores the importance of early recognition and prompt intervention for clinical deterioration.