Mycosis Fungoides with a Follicular Helper T-Cell Immunophenotype: A Diagnostically Challenging Case Report

Summary: Mycosis fungoides (MF), a primary cutaneous T-cell lymphoma (CTCL), can rarely express follicular helper T-cell (TFH) markers. This atypical presentation complicates diagnosis, often mimicking TFH T-cell lymphomas with cutaneous involvement. A 65-year-old woman, initially misdiagnosed with angioimmunoblastic T-cell lymphoma in 2016, later diagnosed with leukemic CTCL (with clonal TCR gamma gene rearrangement studies and negative gene sequencing studies), and treated with multiple therapies, presented with disseminated acute, pruritic, weeping skin lesions and lymphadenopathy. A skin biopsy demonstrated an atypical lymphoid infiltrate of small, medium-sized, and large atypical T cells (≥25% medium-to-large cells) in the superficial dermis with focal epidermotropism. Immunohistochemistry showed that the T-cells are positive for CD2, CD3, CD4, CD5, CD10, CD30(partial, 30–50%, mainly large forms), BCL6(partial, 5-10%), PD1(partial, 20-25%), and CXCL13 (partial, 5-10%) and negative for CD7, TdT, and EBER(ISH). TCR gamma gene rearrangement studies revealed the same clonal rearrangements seen on peripheral blood. These findings led to a diagnosis of MF with large cell transformation and a TFH immunophenotype. This case highlights the diagnostic challenge that MF with a TFH immunophenotype may represent as this entity can easily be mistaken for cutaneous involvement by a nodal-based TFH lymphoma.