Metastatic High-Grade Endometrial Stromal Sarcoma With ZC3H7B::BCOR Fusion in Pleural Fluid: A Case Report.

Pleural fluid involvement by high-grade endometrial stromal sarcoma (HG-ESS) is exceedingly rare, with only one previously reported case. We report a case of ZC3H7B::BCOR fusion-positive HG-ESS metastatic to the pleural fluid in a 32-year-old female who presented with pelvic pain and a large uterine mass suspicious for leiomyosarcoma on MRI. Additional imaging showed pulmonary and vertebral lesions suspicious for metastases. A total abdominal hysterectomy with bilateral salpingo-oophorectomy revealed HG-ESS, with RNA sequencing confirming a ZC3H7B::BCOR fusion. Despite trials of chemoradiation, she developed worsening metastatic burden and eventually presented with a large pleural effusion. Pleural fluid cytology demonstrated three-dimensional clusters of spindled to epithelioid tumor cells with high nuclear-to-cytoplasmic ratios and coarse chromatin, an architectural pattern not seen on the primary resection. Correlation with the patient's known primary uterine tumor and its immunophenotypic features helped establish the diagnosis of metastatic HG-ESS. Malignant effusions secondary to sarcoma account for fewer than 6% of cytologic fluid samples. This case expands the limited cytologic literature on metastatic HG-ESS involving pleural fluid and emphasizes that metastatic HG-ESS may display cytomorphologic features that differ from the primary tumor, making correlation with the patient's clinical history and prior resection essential for accurate diagnosis.