Trichoblastic sarcoma: a high-grade stromal tumor arising in trichoblastoma.

We report a case of primary sarcoma of the skin with a biphasic histological pattern, being composed of areas of mixed mesenchymal-epithelial cell proliferation and areas of purely sarcomatous growth. The tumor occurred in the posterior cervical region of a 93-year-old man, and its history was marked by sudden, rapid enlargement after many years of stable duration. The excised lesion was about 4 cm in diameter, had a firm consistency and was covered by intact skin. Histological examination showed a multifocal proliferation of follicular germinative cells arranged in corymbiform and petaloid shapes with an overall retiform growth pattern. Epithelial cords and strands were composed of cytologically uniform cells with bland nuclear features and were surrounded by a prominent, fibroblast-rich stroma reminiscent of a perifollicular sheath. In many areas of the tumor the stroma showed abrupt transition into a pleomorphic proliferation of large sarcomatous cells with frequent and often atypical mitoses. Multinucleated neoplastic cells infiltrated the epithelial structures to cause their partial or total obliteration in many fields of the lesion. Immunohistochemically, the epithelial cells displayed expression of various keratins, with a particularly intense staining for 34betaE12, and were partly positive for the CD10 antigen. A strong immunostaining for this antigen was also observed in malignant-appearing stromal areas, where no expression of cytokeratins was detected. Moreover, nuclear positivity for p53 protein was seen in sarcomatous cells, whereas it resulted in total lack of epithelial elements. Our case emphasizes that high-grade sarcoma may occur in the spectrum of trichoblastic tumors and that it may share some features of other noncutaneous biphasic neoplasms, such as mammary cystosarcoma phyllodes.