Langerhans cell sarcoma is a rare neoplastic proliferation of Langerhans cells with a poor prognosis. We report FDG PET/CT findings of Langerhans cell sarcoma in the left forearm of a 77-year-old woman who presented with progressive enlargement of the left forearm mass for over 2 months. 18F-FDG PET/CT demonstrated a high FDG uptake by the mass along with several FDG-avid enlarged lymph nodes in the left arm and axilla. Pathology examination of the biopsied specimen from the left forearm mass led to a diagnosis of Langerhans cell sarcoma.

Langerhans cell sarcoma (LCS) is rare and aggressive; patients have an overall survival rate of less than 50%. We present a 62-year-old man with a history of superficial spreading melanoma of the upper back with sentinel lymph node metastasis, Langerhans cell histiocytosis, and LCS. The patient presented with erythematous papules and scaly areas on his face, neck, arms, chest, abdomen, and legs. A skin biopsy revealed a proliferation of large neoplastic cells involving the dermis and with epidermotropism. These cells had atypical bean-shaped nuclei, with ample cytoplasm and abundant mitotic figures including atypical forms. Immunohistochemical studies showed the tumor to be diffusely positive for CD1a, S100 protein, and langerin (CD207) and negative for melanocytic markers. Some tumor cells were positive for cyclin D1. A diagnosis of LCS involving the skin was established. The present study is a very unusual case of LCS showing epidermotropism. The patient's history of metastatic melanoma posed additional challenges for diagnosis, underlying the need of immunophenotyping in these cases. Consensus for optimal standard therapy has not been established in LCS, and thus, early recognition is important since these neoplasms tend to recur and metastasize. LCS in skin is discussed and published cases are comprehensively reviewed.