New and emerging entities of uterine mesenchymal tumours in the era of molecular testing

Uterine mesenchymal tumours represent a group of heterogeneous tumours, the classification of which is evolving, especially in the context of a broader use of molecular testing. Some entities can be diagnosed based on the combination of morphological and immunohistochemical features. In other entities, such as endometrial sarcomas with KAT6B/A::KANSL1 fusion, the correct diagnosis cannot currently be achieved without molecular testing. In this review, we focus on some of the new entities in which molecular testing plays an essential role for diagnosis, including tumours with KAT6B/A::KANSL1 fusion, tumours with MEIS1::NCOA2 fusion, and tumours with fusion involving the KDM2B gene. In addition, uterine tumours with whorling and GREB1:CTNNB1 fusion and endometrial stromal tumours with CTNNB1 mutation will be discussed. Other tumours with recurrent molecular alterations not specific to a single entity (such as tumours with PLAG1 fusion or RAD51B fusion) are also mentioned. In summary, molecular testing is a very important part of the classification of uterine mesenchymal tumours. The growing number of emerging entities helps to correctly classify tumours into clinically significant categories. However, these entities should be viewed with caution until their clinical significance is supported by robust data, and molecular findings should always be correlated with morphological features.