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. K E Y W O R D S dermatologic toxicity, immune checkpoint inhibitor, immune-related adverse events, lichenoid dermatitis, mycosis fungoides

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