Follicular mycosis fungoides: a histopathologic, immunohistochemical, and genotypic review

Follicular mycosis fungoides (FMF) is a recognized variant of mycosis fungoides. In this review, the authors characterize the distinct histopathological and immunohistochemical patterns of FMF that have been reported in the literature. This article is an extensive review of the literature cited in Medline and own data of the authors. The major patterns of FMF histopathology, immunohistochemistry, and molecular genetics are summarized in this review. Histologically, the quintessential finding in FMF is small to medium atypical CD3+ CD4+ CD8- T lymphocytes around and within the epithelium of the hair follicles. This finding is requisite to the diagnosis. However, this finding may be obscured by a host of other patterns often identified in FMF. This includes basaloid folliculo-lymphoid hyperplasia, a granulomatous reaction, eosinophilic folliculitis, and follicular cystic changes with subtle atypical lymphocytes in the cyst wall. Follicular mucinosis (MF) and syringo-tropism are also variably present. Immunohistochemistry of all reported cases uniformly show a CD4+ T cell infiltrate. This review emphasizes and discusses the broad spectrum of histologic changes which may be seen in FMF, clues to the diagnosis, and some potential mimickers.