Pembrolizumab-Induced Lichen Planus on the Scalp of a Patient with Non-Small-Cell Lung Carcinoma

A 67-year-old man with non-small-cell lung carcinoma was referred to our department because of a pruritic rash on his head and upper extremities. Prior to the development of the rash, he had received 4 cycles of combination therapy with pemetrexed, carboplatin, and pembrolizumab, followed by 2 cycles of pembrolizumab monotherapy. On physical examination, violaceous scaly erythema grouped on his scalp and upper extremities. Histologically, the scalp lesions demonstrated irregular acanthosis that formed a characteristic saw-tooth appearance with hypergranulosis and typical lichenoid tissue reaction. These findings suggested that the scalp lesions were lichen planus. Two-week administration of topical corticosteroid dramatically improved the rash. Immunotherapy with pembrolizumab, an anti-PD-1 antibody, can induce T-cell activation that results in various immune-related adverse effects such as lichenoid tissue reaction. However, lichen planus is generally found on the extremities and/or oral mucosa, and unlike in this case, the scalp is rarely affected. Although the exact mechanism underlying predominant scalp involvement is unknown, the present case indicates that anti-PD-1 therapy-induced lichen planus can develop not only on the extremities and oral mucosa but also on the scalp. Interestingly, the lesions were not induced by the combination of chemotherapy and pembrolizumab; rather, they occurred soon after initiation of pembrolizumab monotherapy. In the present case, pembrolizumab-induced T-cell activation which triggered lichenoid tissue reaction may have been suppressed by chemotherapy-induced immunosuppression. Dermatologists should have a thorough knowledge of the cutaneous lesions that manifest as irAEs of anti-PD-1 therapy.