Lichenoid dermatitis from immune checkpoint inhibitor therapy: An immune‐related adverse event with mycosis‐fungoides‐like morphologic and molecular features

Cutaneous immune‐related adverse events (irAEs) are a known consequence of immune checkpoint inhibitor (ICI) therapy and may exhibit a spectrum of morphologic features both clinically and histologically. Lichenoid dermatitis associated with ICI therapy (LD‐ICI) is the most frequently encountered histopathologic type of irAE biopsied by dermatologists. There is frequent clinical and histologic overlap between irAEs and several reactive and neoplastic dermatologic disorders; thus, clinical information is essential. LD‐ICI with histologic, immunohistochemical, and molecular features typical of mycosis fungoides (MF) are unique. Here, we report a patient who developed LD‐ICI with MF‐like morphologic features with monoclonal T‐cell receptor gene rearrangement on consecutive biopsies during ICI therapy. The development of monoclonal LD‐ICI is important for clinicians and pathologists to recognize in patients receiving ICI therapy.