Rare variants of mycosis fungoides: a practical approach with emphasis on differential diagnosis

Mycosis fungoides (MF) is the most common form of primary cutaneous T-cell lymphoma, representing around 50% of all primary cutaneous lymphomas. Its incidence in the United States is estimated at 0.52 per 100,000 cases annually, with a male-to-female ratio of 2:1. MF predominantly affects adults, with a median age of onset between 55 and 60 years. The classic form is characterized by scaly erythematous patches and plaques, which may evolve into tumor stage, signifying more advanced disease with poorer overall survival. Large cell transformation (LCT) can occur at any stage, resulting in more aggressive disease with a median overall survival of 19–36 months. MF can mimic various inflammatory dermatoses, such as eczema and psoriasis, complicating early diagnosis. Uncommon variants like folliculotropic MF (FMF) and syringotropic MF (STMF) exhibit unique clinical and histopathological features, further broadening the differential diagnosis. FMF, characterized by follicular involvement, has a poorer prognosis compared to conventional MF. STMF involves lymphocytic infiltration of eccrine structures and typically presents on the extremities. Pagetoid reticulosis (PR) and granulomatous slack skin (GSS) are rare variants of MF, each with distinct clinical and histological presentations. Accurate diagnosis often requires a multidisciplinary approach due to the overlapping features with other neoplastic and inflammatory conditions. This review highlights the key histopathological features of these rare MF variants and discusses their differential diagnosis.