Abstract

Folliculotropic mycosis fungoides (FMF) represents a variant of MF histologically characterized by folliculotropic T-cell infiltrates, with or without mucinous degeneration of the hair follicles. We present an unusual case of squamous cell carcinoma (SCC) occurring in a man with undiagnosed FMF, where background syringoid metaplasia was mistaken for residual SCC. An 87-year-old man with a biopsy-proven 3x2 cm moderately differentiated squamous cell carcinoma of the forehead was referred for Mohs micrographic surgery (MMS). During the procedure, he had no appreciable clinical tumor after the first stage but was noted histopathologically to have extensive lobules of atypical keratinocytes to all margins after an additional six stages, resulting in a 6.5 cm x 6.0 cm defect without clearance. He was subsequently referred to medical oncology for consideration of systemic treatment for extensive/residual SCC, and on evaluation was noted to have extensive background erythema and scaling of the entire face and scalp that the patient reported present for over a decade. Punch biopsy of a representative area revealed a prominent folliculotropic CD3-positive infiltrate with marked increase in CD4 to CD8 ratio and loss of CD7. T-cell receptor gene rearrangement PCR studies demonstrated a monoclonal T-cell population. Involved follicles were also noted to have marked mucinous degeneration and syringoid metaplasia of the follicular epithelium. Specimens taken from prior MMS stages were reviewed retrospectively, revealing that areas concerning for SCC consisted of en face follicles with mucinous degeneration and syringoid metaplasia. In conclusion, FMF may present with diffuse erythema and scaling, resembling extensive actinic damage. Syringoid metaplasia can occur in the setting of FMF and mimic SCC. Awareness of this phenomenon may help clinicians and dermatopathologists avoid this diagnostic pitfall.

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