Abstract

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. We present the largest case series to date of this condition with comprehensive clinical and pathological characterization. We reviewed the clinical history and biopsy specimens of 29 patients with IMF. Our cohort was predominately male (18; 62%, M:F 1.6:1) with a mean age of diagnosis at 43 years (range 11-85), mean duration of signs/symptoms prior to diagnosis of 7 years, and mean follow-up duration of 5.9 years. Clinically, patients often exhibited symmetric ill-defined patches/plaques often involving intertriginous regions with tan-yellow hyperpigmentation and follicular-based papules, wrinkling, or alopecia. Lymphadenopathy with a single lymph node involvement was noted in five (18%) patients at presentation. Twelve (44.5%) 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. Histologically, atypical cells were small-medium, CD4+ (27; 93%) or rarely CD4+/CD8+ (1; 3.5%) lymphocytes infiltrating the reticular dermis with thickened collagen bundles (26; 90%), multinucleated giant cells (11; 38%), and occasionally tracing along adnexa with subtle folliculotropism (10/18; 56%). Our study reports that IMF is an indolent subtype of MF with distinct clinico-histopathological features.