The Rare Gynecologic Sarcoma Study: Molecular and Clinicopathologic Results of A Project on 379 Uterine Sarcomas

The Rare Gynecologic Sarcoma study involved 23 institutions from 10 countries focusing on myxoid leiomyosarcoma and non-smooth muscle uterine sarcomas. Here, we present the main results of the study, including the comparison between the original and final diagnosis, the frequency and type of molecular aberrations, and the clinicopathologic outcomes. A total of 379 cases were included, with available results for next-generation sequencing (NGS) RNA in 338 of 379 cases and NGS DNA in 335 of 379 cases. According to the original diagnoses, the study included 204 cases of low-grade endometrial stromal sarcoma (LG-ESS), 75 cases of high-grade endometrial stromal sarcoma (HG-ESS), 74 cases of undifferentiated uterine sarcoma (UUS), 17 cases of myxoid leiomyosarcoma, and 9 cases of unclassifiable sarcoma. The results of our second reading showed that 29% (110/379) of all the tumors had been originally misdiagnosed. After the reclassification, the final diagnoses were 147 cases of LG-ESS, 69 cases of HG-ESS, 58 cases of UUS, 3 cases of LG-ESS with high-grade transformation, 7 cases of perivascular epithelioid cell tumor, 9 cases of uterine tumor resembling ovarian sex cord tumor, 8 cases of tumors with a KAT6B/A::KANSL1 fusion, 2 cases of tumors with an NTRK fusion, 29 cases of undifferentiated carcinoma, and 47 tumors with smooth muscle differentiation. The molecular testing showed that LG-ESS harbor a recurrent fusion in 75.9% and HG-ESS in 43.7% of cases. The results of our study emphasize the diagnostic, prognostic, and predictive significance of molecular testing in mesenchymal uterine tumors.