(P 309) RETROPERITONEAL LIPOMAS: MULTICENTER CLINICAL EXPERIENCE WITH AN EXTREMELY RARE DISEASE ENTITY

Objective: The differential diagnosis for a large lipomatous mass in the retroperitoneum is predominantly liposarcoma. Retroperitoneal lipoma is an extremely rare benign entity with bland histomorphology and absence of MDM2 amplification. To date, the largest reported case series of these tumors included 19 patients. In this study, we sought to leverage multicenter collaboration through the Transatlantic Australasian Retroperitoneal Sarcoma Working Group (TARPSWG) to describe collective experience with retroperitoneal lipoma and inform clinical management.

Methods: Data were collected from participating TARPSWG centers for cases of pathology-verified retroperitoneal lipoma from 2010 to 2020. Patient demographics and clinicopathologic factors were summarized. Management algorithms were studied, and clinical outcomes were determined.

Results: 43 patients with retroperitoneal lipoma from 6 centers across Europe, North America, and Asia were included in the study. On initial presentation, 23 (54%) patients were symptomatic, and 20 (46%) were asymptomatic with incidentally found tumors. Among all study patients, needle biopsy was performed on 20 patients (46%), more frequently in those who were asymptomatic (15, 75%) versus those with symptoms (5, 25%, p = 0.0004, Fisher's exact test). In total, 19 (44%) patients underwent upfront surgery while 24 (56%) patients were initially observed. The presence of symptoms was the most common indication for upfront surgery. In the initially observed patients, this management approach continued for a median of 25.5 months (range: 1-116). In fact, 13 (54%) were continuously observed for 2 or more years. Ultimately, an additional 8 initially observed patients proceeded to surgery, most commonly due to continued growth in tumor size or development of symptoms. Among the total of 27 patients, concomitant organ resection was performed in 6 (22%). On the resection specimen, only 1 patient (4%) had a change in final diagnosis to well-differentiated liposarcoma. With a median follow-up of 49 months, 1 of 27 (4%) patients had local recurrence.

Conclusion: Retroperitoneal lipomas are benign tumors for which initial observation may be appropriate in select patients. Surgery can be offered for tumor growth or the development of symptoms. A deeper comparison of retroperitoneal lipoma versus well-differentiated liposarcoma is warranted to better understand differences in disease biology that likely underlie clinical behavior.