CD34-positive pleomorphic uterine sarcoma with NUDT3::RAD51B fusion.

Sarcomas with RAD51B fusions are rare, recently recognized neoplasms that predominantly arise in the uterus. They showed heterogeneous phenotypic features and are typically associated with aggressive biological behaviors. To date, only 14 cases of RAD51B-rearranged sarcomas have been reported in English literature, including 6 perivascular epithelioid cell tumors, 6 leiomyosarcomas, and 2 undifferentiated sarcomas (both uterine in origin). We present an additional case of uterine sarcoma with RAD51B rearrangement. The patient was a 71-year-old woman with a 5.0-cm polypoid mass in the uterine cavity. Histologically, the tumor was composed of spindle to pleomorphic cells arranged in storiform and fascicular patterns within variably myxoid and collagenous stroma. The neoplastic cells had vesicular to hyperchromatic nuclei with occasional multinucleation, with a mitotic rate of 3/10 high-power fields. Notable stromal features included staghorn shaped blood vessels, thick band-like collagen deposition, and prominent chronic inflammatory infiltrates. Immunohistochemically, the tumor cells showed diffuse and strong positivity for CD34, P53, and P16, with focal expression of pan-TRK and smooth muscle actin. Targeted RNA-sequencing revealed NUDT3::RAD51B fusion, which was confirmed by fluorescence in situ hybridization of RAD51B rearrangement. The patient had been free of disease at 22 months' follow-up. This case further illustrates the phenotypic heterogeneity of RAD51B-rearranged sarcomas and expands their clinicopathological and molecular genetic spectrum.