Novel and recurrent histopathologic patterns of mogamulizumab-associated rash: diagnostic implications and insights for accurate diagnosis.

Background: Mogamulizumab-associated rash (MAR) is a frequent side-effect of a treatment with Mogamulizumab that poses diagnostic challenges.

Patients and methods: We reviewed our database for novel histological patterns in MAR.

Results: Four new histological patterns were identified: (1) A pagetoid pattern, which could be mistaken for CD8⁺ aggressive epidermotropic cytotoxic T-cell lymphoma. (2) A lupus erythematosus-like pattern, characterized vacuolar interface dermatitis, deep perivascular lymphocytic infiltrate, interstitial mucin deposits, and clusters of CD123⁺ plasmacytoid dendritic cells. (3) Pustular folliculitis/acneiform pattern, which may clinically and histologically resemble acne. (4) A combined pattern with MAR coexisting with residual or new infiltrates of the original/underlying lymphoma. Previously described main patterns were reproducible. The folliculotropic and MF-like epidermotropic infiltrate should be also mentioned as separate patterns. Immunohistochemistry and the comparison with the original lymphoma remain crucial for diagnosis, especially a shift towards CD8⁺ cells, CD7 expression by infiltrating lymphocytes, and an increased number of histiocytes across all MAR patterns.

Conclusions: MAR presents a range of histological patterns that mimic other conditions, underscoring the importance of careful histopathological and immunohistochemical analysis and clinicopathological correlation for accurate diagnosis and management.