Endometrial stromal sarcoma--the new genetic paradigm

Endometrial stromal sarcoma (ESS) is a gynaecological sarcoma that is composed of cells that resemble those of proliferative-phase endometrial stroma. The 2014 World Health Organization tumour classification system separates ESS into low-grade and high-grade types, which are histologically, genetically and clinically distinct from undifferentiated uterine sarcoma (UUS). Low-grade ESSs frequently contain chromosomal rearrangements that result in JAZF1-SUZ12 fusion or equivalent genetic fusions. Although most low-grade ESSs show classic histological features that closely resemble those of proliferative-phase endometrial stroma, there are several histological variants that are associated with the same genetic fusions as seen in the classic type. High-grade ESS is defined by the presence of YWHAE-NUTM2A/B (YWHAE-FAM22A/B) fusions. High-grade ESSs are clinically more aggressive than low-grade ESSs, but are associated with a lower mortality rate than UUSs. The histological and immunophenotypic features of these different types of ESS, and their diagnostic considerations, are the subjects of this review.