(P 310) ATYPICAL SPINDLE CELL PLEOMORPHIC LIPOMATOUS TUMORS OF THE RETROPERITONEUM ARE HETEROGENOUS: A REPORT OF FOUR DISPARATE CASES

Objective: The vast majority of retroperitoneal fatty tumors are MDM2-amplified liposarcomas. Atypical spindle cell pleomorphic lipomatous tumors (ASPLT) have recently been described outside of the retroperitoneum, characterized by absence of MDM2 amplification, loss of Rb1, and CD34 positivity. The presentation and characteristics of these rare tumors have not been described in the retroperitoneum. Here, we present four cases of retroperitoneal ASPLT.

Methods: The Vanderbilt University Medical Center pathology database was queried for atypical lipomatous tumors of the retroperitoneum from 2010 to 2025. Clinical and pathologic data were extracted from the medical record.

Results: There were 3 males and 1 female, with a median age of 60 years (range 56-62). One patient presented with a 70cm left retroperitoneal fatty mass with extension into the inguinal canal. Two patients presented with bilateral but asymmetrically large (>40cm) retroperitoneal tumors. The final patient presented with three tumor foci: left retroperitoneum, diaphragm and in the subcutaneous tissue of the back – all three lacked lipomatous signal characteristics on pre-operative imaging. Pre-operative biopsies were suspicious for well-differentiated liposarcoma despite the absence of MDM2 amplification. Margin-negative resections were performed in all cases. All four patients are without recurrence of resected tumors at a median follow-up of 18 months. Invasion of encased organs was not observed.

Conclusion: APSLTs are rare benign lipomatous MDM2-non-amplified tumors. These cases highlight their heterogeneity in the retroperitoneum, since they may be bilateral or multifocal, and even lack a fatty appearance on pre-operative imaging. Immunostains for Rb1 and CD34 on pre-operative biopsy could be diagnostic and potentially allow surveillance or organ-sparing resection, although the natural history of these tumors remains uncertain.