Pelvic spindle cell sarcomas harboring MEIS1::NCOA2 fusion and novel gene amplifications in 10q23-26 region: a potential predictor for tumor progression.

MEIS1::NCOA1/2 fusion undifferentiated spindle cell sarcomas generally arise from genitourinary and gynecologic tracts and mostly exhibit low-grade malignancies according to limited cases. Here, we report two cases from pelvic cavities. Case 1 showed low-grade morphology of monotonous spindle cells with moderate atypia in short fascicular or storiform patterns, while case 2 exhibited marked atypia, brisk mitosis, and significant necrosis. Notably, both cases identified intracytoplasmic eosinophilic globules. Next-generation sequencing analysis observed MEIS1::NCOA2 rearrangements in both cases, but only case 2 detected additional 10q23-26 amplifications and CTNNB1 mutation (c.94G > T/p.D32Y). Case 1 developed twice local recurrences in 3 years, while case 2 metastasized to the liver and gastroduodenal interstice and died 7 months after intraabdominal surgery. To the best of our knowledge, it is the first report about MEIS1::NCOA2 fusion sarcoma with distant metastasis to the abdomen. The extra 10q23-26 amplifications and CTNNB1 mutation may indicate potential predictors for malignancy in this study.