Joan Guitart

Background Interstitial mycosis fungoides (IMF) is a rare subtype of mycosis fungoides (MF) characterized by atypical lymphocytes infiltrating the reticular dermis between collagen bundles with limited epidermotropism and variable granulomatous features. Methods Retrospective single institution review of 31 cases of IMF including clinical characteristics, disease course and pathological features. Results Our cohort was predominately male (19; 61%, M:F 1.6:1) with a mean age at diagnosis of 43 years (range 11–85), mean signs/symptoms duration of 7 years prior to diagnosis, and 6 years mean follow‐up duration. Clinically, patients often exhibited symmetric ill‐defined patches/plaques involving intertriginous regions with tan‐yellow hyperpigmentation and follicular‐based papules, wrinkling, and alopecia. Lymphadenopathy was noted in seven patients. Fifteen (52%) patients were in near or complete clinical remission at the latest follow‐up. T‐cell receptor gene rearrangement was positive in 23/24 (96%) cases. Histopathologically, atypical cells were small–medium, CD4+ (29; 94%) or rarely CD4+/CD8+ (1; 3%) lymphocytes infiltrating the reticular dermis with thickened collagen bundles (27; 87%), multinucleated giant cells (12; 39%), and often tracing along adnexa with subtle folliculotropism (12/20; 60%). Conclusions Our study demonstrates IMF is an indolent subtype of MF with distinct features, including frequent granulomatous and subtle follicular involvement resulting in alopecia.