Pembrolizumab‐Exacerbated Widespread Pigmented Purpuric Dermatosis in an Elderly Patient, a Potential Diagnostic Pitfall Mimicking Pigmented Purpuric Dermatosis‐Like Mycosis Fungoides

Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1), programmed cell death ligand 1 (PD-L1), and cytotoxic lymphocyte-associated antigen-4 (CTLA-4) have revolutionized cancer treatment but are associated with immune-related adverse events, particularly cutaneous toxicities. We report a rare case of pembrolizumab-exacerbated pigmented purpuric dermatosis (PPD) in a 77-year-old male with a history of metastatic non-small cell lung cancer. His rash, initially confined to the lower extremities, worsened and became widespread to involve the trunk after pembrolizumab initiation. Histopathology showed perivascular lymphocytic infiltrate with extravasated erythrocytes without vasculitis, compatible with PPD; however, due to some lymphocyte atypia and exocytosis, together with the clinically widespread lesions, there was initial concern for PPD-like mycosis fungoides (MF). Subsequent T-cell receptor gene rearrangement studies revealed no monoclonal lymphoid population, and the later resolution of the rash with treatments typical for PPD did not support MF. This represents only the second reported case of ICI-associated PPD and highlights a potential diagnostic pitfall with histopathology and clinical presentation mimicking PPD-like MF. Our case contributes to the expanding spectrum of ICI-related cutaneous reactions and underscores the importance of recognizing inflammatory dermatoses with atypical histopathologic features for dermatologists and dermatopathologists.