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Histiocytic sarcoma transdifferentiated from follicular lymphoma presenting as a cutaneous tumor

Histiocytic sarcoma represents a rare and poorly understood tumor of histiocytic/dendritic cell lineage that can rarely present in the skin. Previously reported cases of histiocytic sarcoma after follicular lymphoma suggested that follicular lymphoma can transdifferentiate into histiocytic sarcoma. We describe another case involving a 40‐year old male who developed histiocytic sarcoma in his right thigh 4 years after the diagnosis of grade 1 follicular lymphoma in the left neck. The two neoplasms were morphologically and immunophenotypically different but had identical immunoglobulin heavy chain gene and bcl‐2 gene rearrangements, as demonstrated by polymerase chain gene reaction analysis, and the presence of t(14;18)(q32;q21) translocation was confirmed via fluorescence in situ hybridization (FISH) analysis. Because of spindle cell morphology and focal S‐100 positivity, malignant peripheral nerve sheath tumor and melanoma diagnoses were made initially and extensive workup was required to discover the correct diagnosis. Lineage transdifferentiation can occur in mature lymphoid neoplasms and awareness of this phenomenon and appropriate workup is crucial for correct diagnosis, as different treatment protocols and prognosis may vary.