Sebaceous differentiation in poroid neoplasms: report of 11 cases, including a case of metaplastic carcinoma associated with apocrine poroma (sarcomatoid apocrine porocarcinoma).

We describe 11 poroid neoplasms with sebaceous differentiation, including a metaplastic (sarcomatoid) carcinoma arising in association with an apocrine poroma. Six lesions had the silhouette of a classical poroma, 3 of poroid hidradenoma and 1 of dermal duct tumor. In all cases, sebaceous differentiation was identified as clustered or solitary, mature sebocytes occurring mainly at the periphery of intradermal cellular aggregations, accompanied by sebaceous ducts. In one poroma, clusters of sebocytes were seen within intradermal aggregates and intraepidermally. In 1 of the 3 poroid hidradenomas, the eosinophilic cuticle lining the cyst was crenulated in foci associated with sebocytes. In none of the cases were there signs of follicular differentiation. One poroma, in addition to sebaceous differentiation, showed decapitation secretion in some ductular structures. The single carcinoma was an ulcerated oval to spindle cell neoplasm surrounded laterally by the residuum of a poroma containing groups of sebocytes. The epithelial islands of the poroma were prominently keratinized and blended gradually with the pleomorphic cells of the metaplastic carcinoma that immunohistochemically stained focally for cytokeratins and simultaneously showed strong vimentin expression. Our study supports previous findings that sebaceous differentiation can be identified not only in classical poroma but also in the related lesions known as dermal duct tumor and poroid hidradenoma. Occurrence of metaplastic carcinoma in association with apocrine poroma is a rare event which indicates the existence of a malignant counterpart of the latter entity, which can be descriptively referred to as "sarcomatoid apocrine porocarcinoma."