A rare case of cytomegalovirus mucositis resembling an immune-related adverse event during treatment with nivolumab and ipilimumab.

Immune checkpoint inhibitor (ICI) therapy may be accompanied by immune-related adverse events (irAEs). Severe irAEs frequently require immunosuppressive treatment with high-dose corticosteroids and, in refractory cases, additional agents such as infliximab. Under these circumstances, patients may become susceptible to opportunistic infections, including cytomegalovirus (CMV), which can present with clinical features closely resembling irAEs. We report the case of a 79-year-old man with malignant melanoma of the sphenoid sinus who was treated using nivolumab and ipilimumab. He developed immune-related colitis, requiring corticosteroid and infliximab treatment. Subsequently, he presented with severe pharyngeal pain and extensive oral and pharyngeal mucosal erosions, which were initially suspected to represent irAE-related mucositis. However, the lesions did not improve despite corticosteroid re-escalation. CMV antigenemia increased over time, and biopsy of the buccal mucosa confirmed CMV mucositis by immunohistochemistry. Intravenous ganciclovir led to marked clinical and virological improvement, allowing resumption of oral intake. Therefore, CMV mucositis can mimic an irAE during ICI therapy, and the risk of opportunistic infection in this setting arises primarily from immunosuppressive treatment for irAEs rather than from ICIs themselves. When severe mucosal lesions are persistent or refractory to corticosteroid escalation, clinicians should consider opportunistic infections and pursue prompt tissue evaluation for definitive diagnosis.