ABSTRACT
UNLABELLED: A 76-years-old woman underwent a partial mastectomy and a low-grade malignant homologous phyllodes tumor measuring 45 mm in maximum diameter was diagnosed. Beyond its typical dual composition the tumor displayed extensive intraductal squamous metaplasia. Approximately in one third of the lesion the original mesenchymal component cytologically and structurally changed which ultimately led to seeming stromal overgrowth. The loose storiform background contained isolated larger atypical elements with ample eosinophilic cytoplasm and obvious mitotic activity. This final fibromatosis-like arrangement was completed either by multiple dispersed abrupt squamous morules or just by pearl-like abortive form of squamous differentiation. Conventional in situ or invasive ductal carcinoma was not present. A combined expression of both low and high molecular weight cytoketatins, S100 protein, p63, CD10 and GFAP confirmed the incomplete basal/myoepithelial phenotype and ultimately led to the diagnosis of a spindle cell metaplastic carcinoma arising in a phyllodes tumor - a neoplasm unpublished so far. A review of the literature concerning epithelial malignancies originating from a milieu of phyllodes tumor guides discussion/speculation over the possible histopathogenesis of this vanishingly rare lesion. KEYWORDS: breast - phylloid tumor - phyllodes tumor - spindle cell sarcomatoid/metaplastic carcinoma - squamous metaplasia.
