A Rare Case of Atypical Pelvic Retroperitoneal Lipoma Mimicking Liposarcoma; A Case Report and Review of Literature.

Retroperitoneal lipomas are exceptionally rare tumors that mimic liposarcomas. We present a case of a woman in her 30s undergoing fertility investigations, with an incidental finding of a retroperitoneal tumor on MRI. PET and MRI scans suggested this may be a liposarcoma, with widespread effects on the surrounding iliac vessels. Collaboration between the orthopedic sarcoma service, gynecological oncology, and vascular surgery was required to excise the tumor without damaging the surrounding vessels. The patient underwent an exploratory midline laparotomy, with mobilization of the ascending colon along the Toldt line to gain access to the tumor. The morbid adherence of the tumor to neurovascular structures added significant complexity to this procedure; however, complete tumor resection was achieved with mobilization and excision of the obturator nerve only. The initial histological diagnosis was well-differentiated liposarcoma; however, subsequent molecular analysis showed no evidence of MDM2 amplification, suggesting a final diagnosis of benign lipoma. This case exemplifies diagnostic challenges in lipomatous retroperitoneal tumors, as well as the importance of inter-disciplinary collaboration in surgery to achieve the best surgical and oncological outcomes.