Low grade endometrial stromal sarcoma with epithelial differentiation mimicking endometriosis

Low grade endometrial stromal sarcoma (ESS) is a difficult diagnostic entity as the tumour cells closely resemble normal endometrial stromal cells with minimal nuclear atypia and mitotic activity. The site of occurrence in the uterine corpus is a useful suggestion for this tumour type, but when present at extrauterine sites it may be difficult to diagnose. In addition, a variant with epithelial differentiation has been recently described which mimics both endometriosis and low grade adenosarcoma. We describe the case of a 52-year-old female who presented with a pelvic tumour showing very minimal cytological atypia mimicking endometriosis, but diagnosed as low grade ESS with epithelial differentiation. We also discuss the possible role of a characteristic translocation t(7;17)(p15;q21), resulting in the fusion of the JAZF1/JJAZ1 genes, in aiding the histological diagnosis.