Two cases of smooth muscle tumor of the breast with high tendency to local relapses are reported. The common cytohistologic feature of these infrequent neoplasms is interfacing bundles with spindle-shaped cells showing eosinophilic cytoplasm and blunt-ended nuclei. In estimating the prognosis of these lesions, the mitotic index must be evaluated together with evidence of necrosis and hypercellularity. The present cases suggest that, in the absence of the ancillary features, a mitotic rate of 1-3 × 10 HPF might define a “grey zone” in the spectrum of lesions between leiomyomas and leiomyosarcomas: the smooth muscle tumors of indeterminate prognosis (low risk lesions).
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