A Case of Folliculotropic Mycosis Fungoides Initially Misleading as a Poikilodermatous Rash: The Importance of Gene Rearrangement Testing in Early-Stage Disease (Folliculotropic T-Cell Lymphocytosis)

Folliculotropic mycosis fungoides (FMF) is a distinct and often aggressive variant of cutaneous T-cell lymphoma (CTCL) that can mimic benign dermatoses, particularly in its early stages. We present the case of a 42-year-old man with a chronic, progressively worsening, intensely pruritic, scaly eruption involving the lower back, buttocks, and proximal extremities. Despite initial management with topical corticosteroids, his condition persisted and evolved into poikilodermatous patches with follicular accentuation and hyperpigmented plaques, prompting further evaluation. Serial skin biopsies from the left anterior distal thigh, right posterior shoulder, and right inferior medial lower back revealed superficial and deep perivascular and perifollicular atypical lymphoid infiltrates. Molecular studies demonstrated clonal T-cell receptor (TCR) gene rearrangement, raising concern for early-stage cutaneous T-cell lymphoma with folliculotropic involvement. Given the clinical presentation and histopathologic findings, a diagnosis of early-stage folliculotropic mycosis fungoides was made. The patient was initiated on narrowband ultraviolet B (nbUVB) phototherapy three times per week in addition to topical corticosteroids, with close clinical follow-up.