131P Rethinking grade 1 myxoid liposarcoma: Is it truly a low-risk disease?

Background: Myxoid liposarcoma (MLPS) is traditionally classified into low-and highgrade (HG) based on the round cell (RC) component, with >5% RC indicating HG disease.In parallel, the FNCLCC grading system, routinely applied to sarcomas, is also used in MLPS and may assign an alternative grading system.Although MLPS is considered an intermediate-grade malignancy, FNCLCC grade 1 MLPS represents a distinct subgroup with indolent behavior but a persistent risk of local and distant recurrence.However, long-term outcomes of this low-grade subset remain poorly defined.The aim of this study was to characterize the clinical features and survival outcomes of pts with grade 1 MLPS.Methods: This retrospective study included all pts with histologically confirmed grade 1 MLPS treated at Gustave Roussy between January 1990 and March 2024 from the institutional database.Clinical data, including demographics, primary tumor site, and disease stage, were collected from electronic medical records.Survival outcomes were estimated using the Kaplan-Meier method.Results: 178 pts with grade 1 MLPS were included, with a median age of 43 yrs and 62.9% were males.Median tumor size was 10 cm (3 cm; 30 cm).Four had metastatic disease (2.3%) with lung lesions in 3 pts.Most common tumor sites were in lower limbs (78.1%), most in hips (108 pts, 61.2%) and knees (17 pts, 9.6%).They rarely occurred in upper limbs (3pts, 1.7%) or trunk (17 patients, 9.6%).Most pts had surgery (92.7%), with 50% receiving RT (21.9% in neoadjuvant (NAT).Chemotherapy was given to 25 pts (14.6% with NAT in 11.2%), while isolated limb perfusion was done in 9 (5.1%).With a follow-up of 84 mo, 25.3% (43 pts) had local relapse and 23.6% (41 pts) developed metastatic disease.Median time to local and distant relpase were 26.74 mo and 40.42 mo.Median overall survival was not reached, while estimated survival at 1, 5, and 10 years were 94.6% (95% CI: 89.9-97.2),81.4% (95% CI: 74.0-86.9),and 69.0%(95% CI: 59.3-76.8). Conclusions:In this cohort, grade 1 MLPS demonstrated a generally favorable prognosis with 25.3% and 23.6% risk of local and distant relapse, respectively.Identifying prognostic factors within this indolent subtype may help refine treatment strategies in localized disease and guide the long-term surveillance of affected patients.