Mesenchymal tumors of the uterus. VI. Epithelioid smooth muscle tumors including leiomyoblastoma and clear-cell leiomyoma: a clinical and pathologic analysis of 26 cases.

Twenty-six cases of atypical smooth muscle tumors of the uterus, including leiomyoblastoma, epithelioid leiomyoma, clear-cell leiomyoma, and plexiform tumorlet, are presented. The characteristic microscopic feature serving as the basis for inclusion in this study is the rounded to polygonal shape of the majority of cells instead of the elongated blunt-ended shape of smooth muscle cells seen in the typical leiomyoma. Mixtures of epithelioid, clear-cell, and plexiform patterns occurred with sufficient frequency to indicate that such patterns are variants of a single entity. A transition to typical smooth muscle cells was found in most instances, confirming the smooth muscle origin of these tumors. Features related to a favorable prognosis include the presence of clear cells, an expansile tumor margin, extensive hyalinization, and absence of extensive necrosis as seen microscopically. Until additional evidence has been accumulated it is proposed that neoplasms having five or more mitotic figures per ten high-power fields (HPF) be tentatively termed epithelioid leiomyosarcoma or leiomyosarcoma with epithelioid features and those with less than five mitotic figures per ten HPF, epithelioid leiomyoma.