Undifferentiated/unclassified sarcomas are defined in the 2013 World Health Organization (WHO) monograph as a heterogeneous group of sarcomas without evidence of a particular line of differentiation. Most undifferentiated sarcomas with round cell morphology arise in young patients, while those of pleomorphic morphology (previously pleo-morphic malignant fibrous histiocytoma; MFH) usually occur in elderly adults. Morphologically, they can be either spindle cell, epithelioid, round cell, or pleomorphic (Fig. 1) (previous MFH). MFH was considered to be a distinct entity for many years. Retrospective examinations of a series of cases diagnosed as pleomorphic MFH have identified lipogenic, myogenic, and Schwann cell differentiation, as well as nonmesenchymal histogenesis in the majority of re-examined cases [2]. A diagnosis of undifferentiated/unclassified sarcoma diagnosis should only be rendered from fine-needle aspiration smears if there is access to immunocyto-chemical and molecular studies that exclude any specific subtype of sarcoma.
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