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Low‐grade endometrial stromal sarcoma with an extensive epithelial‐like element

A case of low‐grade endometrial stromal sarcoma with extensive epithelial‐like element (ELE) is reported. This tumor was composed of classical endometrial stromal sarcoma (CESS) showing diffuse proliferation, and ELE occupying approximately 72% of the tumor mass. On immunohistochemistry, ELE was negative for sex‐cord differentiation markers, and was positive for myogenic markers used in our investigation, and had a particularly prominent positivity for α‐smooth muscle actin within the ELE. Therefore, it was considered that ELE showed no true sex cord feature, but smooth muscle differentiation. Moreover, ELE was also positive for CD10, suggesting that it was derived from CESS. It has been reported that there is a distinct clinical behavior between endometrial stromal tumors with abundant ELE and those with limited ELE. In the present case, the Ki‐67 labeling index was markedly higher in CESS than in ELE. Therefore, a difference in cell proliferative activity between ELE and CESS might underlie a different clinical prognosis.