Pembrolizumab-induced lichen planus in patients with metastatic melanoma: a report of two cases and prognostic implications of cutaneous immune-related adverse events.

Patients receiving immune checkpoint inhibitors (ICIs) commonly experience cutaneous immune-related adverse events (irAEs). We present two cases, a 51-year-old female and a 70-year-old male, that were undergoing treatment with pembrolizumab for metastatic melanoma and developed scaly, erythematous papules on their skin. Following skin biopsies, histological analysis confirmed the diagnosis of lichen planus. In the first patient, acitretin at a dosage of 25 mg/day was administered for 6 months, resulting in complete resolution of lichen lesions. Imaging scans showed no signs of melanoma. The second patient was treated with topical betamethasone dipropionate ointment for several weeks, which led to a favorable therapeutic response. During follow-up, a thoracic CT scan showed several micronodular lesions in the right lung, whereas brain and abdomen CT scans showed no signs of the disease. Lichen planus is not a commonly reported irAE in patients treated with ICIs. This report underscores the importance of conducting skin biopsies in patients receiving ICI therapy and highlights the potential prognostic importance of skin irAEs in patients with melanoma receiving such treatment.