Clinical Outcome of Low-Grade Myofibroblastic Sarcoma in Japan: A Multicenter Study from the Japanese Musculoskeletal Oncology Group

SIMPLE SUMMARY: Low-grade myofibroblastic sarcoma (LGMS) is one of the rarest sarcomas. We aimed to clarify the clinical outcomes of patients with LGMS. Twenty-two patients underwent surgical treatment for the primary tumor and two underwent radical radiotherapy (RT). The best overall response in the two patients who underwent radical RT was one complete response and one partial response. Local relapse-free survival was 91.3% at 2 years and 75.4% at 5 years. Relapsed tumors were treated with surgery in two cases and radical RT in three cases. None of the patients experienced a second local relapse. Disease-specific survival was 100% at 5 years. Wide excision is recommended due to its tendency to local relapse. However, RT was considered a viable option in unresectable cases or in cases where surgery may cause significant functional impairment. ABSTRACT: This retrospective multicenter study aimed to analyze the clinical features and prognosis of 24 patients diagnosed with LGMS between 2002 and 2019 in the Japanese sarcoma network. Twenty-two cases were surgically treated and two cases were treated with radical radiotherapy (RT). The pathological margin was R0 in 14 cases, R1 in 7 cases, and R2 in 1 case. The best overall response in the two patients who underwent radical RT was one complete response and one partial response. Local relapse occurred in 20.8% of patients. Local relapse-free survival (LRFS) was 91.3% at 2 years and 75.4% at 5 years. In univariate analysis, tumors of 5 cm or more were significantly more likely to cause local relapse (p < 0.01). In terms of the treatment of relapsed tumors, surgery was performed in two cases and radical RT was performed in three cases. None of the patients experienced a second local relapse. Disease-specific survival was 100% at 5 years. A wide excision aimed at the microscopically R0 margin is considered the standard treatment for LGMS. However, RT may be a viable option in unresectable cases or in cases where surgery is expected to cause significant functional impairment.