The goal of this manuscript is to provide a comprehensive overview of the use of immunohistochemical methods in diagnosing mesenchymal tumours of the uterus. The main points discussed include, especially, the issue of determining smooth muscle differentiation, the differential diagnosis between smooth muscle and endometrial stromal tumours, and the diagnosis of inflammatory myofibroblastic tumour. The role of immunohistochemical examination in the diagnosis of some of the only recently definedentities such as YWHAE-altered high grade endometrial stromal sarcoma (HG-ESS), BCOR-altered HG-ESS, tumours with NTRK fusion, and SMARCA4-deficient sarcomas is also discussed. The last aspect of this work is an analysis of the significance of immunohistochemical methods in the determination of the biological behaviour of leiomyocellular tumours. The issue of their molecular classification for those lesions associated with the presence of recurrent molecular aberrations is also discussed.
Keywords: endometrial stromal tumors; immunohistochemistry; mesenchymal uterine tumors; molecular classification; smooth muscle tumors; undifferentiated uterine sarcoma.