Endometrial stromal tumor with whorling and GREB1::CTNNB1 fusion: expanding the knowledge on a recently described entity.

Endometrial stromal tumors with whorling and CTNNB1 rearrangement (ESTW-CTNNB1) are recently described gynecological mesenchymal neoplasms with unique clinicopathological and molecular features. To date, only four of these tumors have been reported. Herein, we describe a case of ESTW-CTNNB1 in a 67-year-old woman. The tumor measured 10 cm and had well-defined margins. On histology, the neoplasm was mostly composed of whorls of epithelioid cells immersed in a loose fibroblastic background, with a delicate vasculature and focal sex cord-like pattern. Tumor cells were mildly atypical, and mitotic index was < 1/10HPF. The tumor showed areas of ischemic necrosis and hydropic changes; no coagulative tumor cell necrosis was observed. On immunohistochemistry, tumor cells were diffusely positive for β-catenin (nuclear), CD10, WT1, calretinin, SATB2, estrogen, and progesterone receptor and negative for CDX2, CK8/18, and p63, with Ki67 = 10%. Molecular analysis revealed a GREB1::CTNNB1 fusion. In conclusion, we reported we 5th case of ESTW-CTNNB1, confirming the previously described features and highlighting positivity for SATB2.