A rare case of low-grade endometrial stromal sarcoma with myxoid differentiation and atypical bizarre cells.

Abstract Endometrial stromal sarcoma (ESS) is a rare mesenchymal tumor with characteristic histological appearances, consisting of diffuse infiltrate of small uniform endometrial stromal cells with a multinodular arrangement and distinct vascular pattern. Less common variants of ESS include "mixed endometrial stromal and smooth muscle tumors", "endometrial stromal tumors resembling ovarian sex cord tumors" and "endometrial stromal neoplasms with endometrial glands", and "aggressive endometriosis". Rarely do endometrial stromal tumors have a prominent fibrous or myxoid appearance which causes confusion and possible misdiagnosis as myxoid leiomyosarcoma. In this report we present a very unusual subtype of ESS in a 32-year-old woman. The tumor revealed atypical pleomorphic bizarre cells which were stained positive only with vimentin and CD10 in an abundant myxoid matrix. A low-proliferative rate was established with MIB-1 staining. To our knowledge such appearance has not been previously reported.