Rare Concomitant Langerhans Cell Histiocytosis in a Patient With Folliculotropic Mycosis Fungoides

ABSTRACT The relationship between lymphomas such as mycosis fungoides and Langerhans cell histiocytosis (LCH) is exceedingly rare. Cases reported in the literature describe lymphomas preceding a diagnosis of LCH, as well as LCH followed by a diagnosis of lymphoma. Here, we present a 76‐year‐old patient with a years‐long history of recalcitrant folliculotropic mycosis fungoides (FMF) who presented to the outpatient dermatology clinic with newly developed painful and itchy cutaneous nodules. Subsequent flow cytometry showed no evidence of Sezary syndrome. NM PET/CT FDG showed no systemic involvement but revealed areas of intense FDG uptake in multiple sites. Biopsy from one of these sites revealed dense, nodular atypical lymphohistiocytic infiltrate and several large intrafollicular Langerhans cell collections, consistent with LCH. Immunohistochemical staining was strongly and diffusely positive for S100, Langerin, and CD1a, confirming the diagnosis of LCH. This is a very rare and unusual case of a patient with FMF who subsequently developed LCH. We emphasize the importance of distinguishing mycosis fungoides from LCH using histopathological, immunohistochemical, and clinical findings. Additionally, we encourage the differentiation between reactive Langerhans cell hyperplasia and neoplastic, malignant LCH and to consider repeat biopsies as appropriate.