Well-Differentiated Liposarcoma of the Psoas Muscle: A Distinct Anatomic Subset with Highly Favorable Outcomes

BACKGROUND: Retroperitoneal (RP) liposarcoma (LPS) is characterized by histologic subtype and grade. This study sought to define outcomes for a novel subset of well-differentiated (WD) RP LPS defined by anatomic location in the psoas muscle. METHODS: Data were reviewed for all patients with primary WD LPS who underwent curative-intent complete resection at our sarcoma referral center from March 2006 to September 2021. For the study patients, clinicopathologic data were summarized. Crude cumulative incidences of local recurrence (LR) and disease-specific death (DSD) were estimated. RESULTS: The 254 patients who met the inclusion criteria were stratified by location of their tumor: 12 RP psoas patients (group A), 123 RP non-psoas patients (group B), and 119 extremity patients (group C). Tumor size was greater in groups A (median, 20.0 cm) and B (median, 25.0 cm) versus group C (median, 15.0 cm; standardized mean difference [SMD], 0.747). Four patients (33 %) in group A underwent concomitant resection of muscle or nerve. In group B, 120 patients (97.5 %) had at least one organ resected (SMD, 5.606). During a median follow-up period of 8.8 years, no LR occurred in group A. The 5-year incidence of LR was 16.3 % in group B and 2.9 % in group C (p < 0.001). No DSDs occurred in group A or C, whereas the 5-year incidence of DSD was 1.9 % in group B. CONCLUSIONS: In psoas WD LPS, local control rates and survival are highly favorable after surgery and approximate those seen for patients with extremity disease. With further multicenter validation, a more conservative approach to management may be indicated for this anatomically defined subset of RP liposarcoma.