Low-Grade Fibromyxoid Sarcoma and Related Subtypes: A Systematic Review and Pooled Analysis of 773 Cases

Background: Low-grade fibromyxoid sarcoma (LGFMS) is a rare malignant fibroblastic tumor that often appears deceptively benign. Accurate diagnosis is challenging due to its variable morphology and low mitotic activity. This systematic review provides a comprehensive overview of LGFMS and its subtypes. Methods: A systematic search of PubMed and Embase up to September 2025 identified 273 studies, complemented by four institutional cases from Amsterdam UMC. Individual patient data were pooled to analyze clinical presentation, diagnostic approaches, treatment modalities, and outcomes. Results: In total, 773 patients were included, with a median age of 35 years and equal gender distribution. Tumors were predominantly deep-seated (80%), most commonly located in the thigh or pelvis. MUC4 positivity (96%) and FUS-CREB3L2 fusion (47%) were the most consistent diagnostic markers. Surgery was the mainstay of treatment (98%), with R0 resection achieved in 36% of cases and R1 in 15%. Adjuvant therapies, including chemotherapy and radiotherapy, were rarely used and showed limited efficacy. After a median follow-up of 3.0 years, 19% developed local recurrence and 21% developed metastases. R0 resections were associated with significantly better recurrence-free survival than R1 resection (p < 0.05). Conclusions: LGFMS exhibits indolent histology but potential for late recurrence and metastasis, warranting prolonged radiological follow-up and multicenter studies to evaluate adjuvant strategies.