Patients receiving immune checkpoint inhibitor (ICI) therapy should be carefully monitored for the occurrence of immune-related adverse events (irAEs). We report the case of an 87-year-old woman with primary mucosal malignant melanoma (T4aN0M0) of the nasal cavity who developed bullous pemphigoid (BP), a rare irAE, after treatment with pembrolizumab. The patient received definitive heavy ion therapy followed by monotherapy with pembrolizumab. After 6 courses of pembrolizumab, she developed grade 2 oral mucositis, and pembrolizumab was discontinued. Since the oral mucositis improved with a topical steroid spray, and computed tomography revealed shrinkage of the nasal tumor, we followed up the patient after discontinuation of pembrolizumab. However, on day 237 after the first dose of pembrolizumab, she developed bullous dermatitis on the skin of the hands and face. A dermatologist was consulted and biopsy led to the diagnosis of BP. Accordingly, we initiated the patient on oral steroid therapy, which led to resolution of the BP, and the condition has not relapsed since. Meanwhile, the nasal tumor continues to remain shrunk, and the patient has been followed up without treatment, including pembrolizumab administration. Prompt diagnosis and treatment are necessary in patients developing irAEs during/after ICI therapy, and collaboration with specialist physicians could be important.
