Abstract
MEIS1::NCOA1/2 fusions have been identified in spindle cell tumors of the gynecologic and genitourinary tracts, as well as in two cases of intraosseous spindle cell rhabdomyosarcomas. These tumors typically exhibit an infiltrative growth pattern characterized by short fascicles of monomorphic, plump spindle cells. Their immunoprofile is nonspecific, often demonstrating focal and variable expression of ER, PR, CD10, and cyclin D1. Depending on their location, these tumors are frequently diagnosed as low-grade endometrial stromal sarcomas or undifferentiated uterine or renal sarcomas. While they generally exhibit low malignant potential with multiple local recurrences, two cases with high-grade morphology and lung metastases have been reported. Here, we describe a case of pelvic low-grade spindle cell sarcoma in a 19-year-old woman characterized by strong diffuse ER/PR expression and focal CD10 positivity. Next-generation sequencing revealed a MEIS1::NCOA1 fusion without additional genetic alterations. She presented with extensive local disease throughout the abdomen, while the uterus and adnexa appeared normal intraoperatively.