Abstract

Solitary reticulohistiocytoma is an uncommon entity within the C-group of histiocytoses that has been described as having RAF1 and TSC2 mutations.  The presence of BRAFV600E typically raises the differential of L-group histiocytoses, such as Langerhans Cell Histiocytosis (LCH) or Erdheim-Chester Disease (ECD), or is associated with multiple histiocytic lesions. Herein we describe a 48 year old patient without symptoms or signs of L-group histiocytoses or other systemic disease who demonstrated a BRAFV600E positive histiocytic neoplasm with features of solitary reticulohistiocytoma. The patient presented for full body skin exam given a history of a mildly dysplastic nevus and was noted to have a 0.5 cm firm pink papule of the left thigh. Histologic sections of a shave biopsy revealed a well-circumscribed dermal-based proliferation comprised of eosinophilic glassy-appearing mononuclear and multinucleated cells with a rare Touton giant cell. Immunostaining with CD68 highlighted the lesional cells, confirming histiocytic origin, and immunohistochemistry for S100, Langerin, and CD1a was negative, ruling out a diagnosis of LCH. Additional immunohistochemistry with BRAFV600E demonstrated strong and diffuse cytoplasmic staining of the histiocytes, prompting additional work-up to exclude systemic disease. Skeletal survey revealed no radiographic findings to suggest a diagnosis of ECD, and no additional foci suggestive of multicentric reticulohistiocytosis were identified on physical exam. Overall, the morphological appearance of the lesion supports a diagnosis of solitary reticulohistiocytoma; however, the presence of BRAFV600E mutation has not been described in the literature. Additional studies to further characterize the molecular profile of solitary reticulohistiocytoma are needed to better understand its pathogenesis and relationship to other histiocytoses.