Follicular mycosis fungoides mimicking a cutaneous B-cell lymphoproliferative disorder

Follicular mycosis fungoides (MF) is an uncommon histological variant of MF characterized by infiltrates of atypical lymphocytes around and within the epithelium of the hair follicles (folliculotropism). Here we report a patient with rapidly progressive follicular MF on the face, associated with concurrent typical MF lesions elsewhere. The histology was unusual, as apart from dense lymphoid infiltrates showing folliculotropism and epidermotropism, there was a prominent B-cell component with germinal centres, leading to an initial diagnosis of cutaneous B-cell lymphoma. The final diagnosis of follicular MF was established on demonstration of clonal T-cell receptor gene arrangements and lack of clonality for heavy chain gene rearrangements. This case illustrates a variant of MF that has a more rapid progression than the otherwise indolent course of classical MF over many years, and the diagnostic pitfalls, whereby the histology can mimic a B-cell proliferative disorder.