Pembrolizumab-Induced Lichenoid Dermatitis in a Patient With Metastatic Cancer of Unknown Primary.

Pembrolizumab is an immune checkpoint inhibitor approved for use in many cancer types such as non-small cell lung cancer (NSCLC), metastatic melanoma, head and neck cancers, hepatocellular carcinoma, and renal cell carcinoma. There are many reported cases of patients on immunotherapy who have discontinued treatment due to the development of immune-related adverse effects (irAE). Recognition of the histopathologic patterns of dermatologic toxicities due to immunotherapy will become increasingly important for ensuring appropriate management and optimal patient care. Here, we present a case of a 72-year-old man with metastatic carcinoma of unknown primary origin treated with pembrolizumab who developed an immune-related cutaneous adverse event (ircAE) in the form of lichenoid dermatitis.