Epithelioid glomus tumor.

Reported are five cases of an unusual histological variant of glomus tumor that we have designated epithelioid glomus tumor. Unlike conventional glomus tumors, which consist of small polygonal cells with dark round nuclei and scanty cytoplasm, the epithelioid lesions were composed of large polygonal to spindle-shaped cells with abundant eosinophilic cytoplasm and large, irregularly shaped nuclei: The cells had both epithelioid and myoid qualities. Two of the cases studied were predominantly epithelioid, with small remnants of conventional glomus tumor at the periphery. The three other lesions were purely epithelioid. Epithelioid glomus tumors are of particular importance because they may be mistaken for other lesions histologically. Both benign and malignant epithelial lesions may be considered in the differential diagnosis; spindle-cell lesions, such as schwannoma, leiomyoma, hemangiopericytoma, and others, are in the histological differential diagnosis. Immunohistochemical and ultrastructural studies indicated that epithelioid glomus tumors had characteristics identical to those of conventional glomus tumors: the cells showed features consistent with smooth muscle derivation. The epithelioid areas frequently exhibited cytological atypicality--features that we believe to be a manifestation of cellular degeneration or senescence (analogous to "ancient" change in schwannomas or symplastic change in leiomyomas) rather than evidence of neoplastic progression. Simple surgical excision seems to have been curative (mean duration of follow-up of 4 years in the three cases in which such information was available). To our knowledge, no similar cases have previously been reported.