A comparative immunohistochemical study of uterine smooth muscle neoplasms with emphasis on the epithelioid variant

We evaluated the immunophenotypes of 22 spindled and 36 epithelioid uterine smooth muscle neoplasms (SMNs) and 16 extrauterine nongastrointestinal spindled smooth-muscle neoplasms for various markers. The epithelioid neoplasms were subdivided into two histological groups designated true and intermediate, the former showing typical epithelioid features and the latter showing epithelioid features that could be explained by cross-sectioning of blunt spindled cells. Desmin, muscle-specific actin, and smooth muscle actin were equally sensitive in detecting muscle differentiation in all these neoplasms. The true epithelioid variants were more frequently keratin positive but less frequently positive for vimentin, CD34 or the muscle markers, compared with their spindled counterparts. The intermediate epithelioid variants more closely resembled the spindled neoplasms in their immunostaining for muscle markers, vimentin, and CD34 but like the true epithelioid variants were relatively frequently positive for keratin. CD34 was positive in 36% of the spindled and 6% of the true epithelioid uterine SMNs, in most cases faintly. Antikeratin AE1 was positive more frequently than CAM5.2, with 18% of the spindled and 35% of the true epithelioid neoplasms being AE1 positive. The immunophenotype of uterine SMNs, including the epithelioid variant, permits their distinction from carcinomas based on their frequent reactivity for muscle markers in spite of their high rate of keratin positivity. They show sufficient overlap in immunoreactivity with endometrial stromal sarcomas to preclude definitive differentiation from them on immunohistochemical features alone.