Abstract: Objective To investigate the clinical characteristics, diagnostic process, surgical treatment and peri-operative management of retroperitoneal dedifferentiated liposarcoma (DDLPS) and to improve awareness of this type of rare giant tumor while providing a reference for future clinical diagnosis and treatment. Methods A case of a 57-year-old male patient admitted to the Department of Urology, Peking Union Medical College Hospital in September 2025. The patient′s electronic medical records were retrospectively reviewed, including outpatient and inpatient records, laboratory and radiology results, and treatments during hospitalization. Results Physical examination revealed a giant palpable mass in the left abdomen. Radiology imaging showed significant displacements of the spleen, pancreas, left kidney, and left adrenal gland by the tumor and a compressed left ureter, resulting in upper urinary tract hydronephrosis. After en bloc resection, the tumor measured up to 45 cm in maximum diameter and weighed approximately 12.5 kg. Pathological fluorescence in situ hybridization (FISH) demonstrated amplification of murine double minute 2 (MDM2), supporting the diagnosis of DDLPS. The patient recovered uneventfully after surgery, was discharged on postoperative day 11, and remained in generally good condition at the 1-month follow-up. Conclusions The case demonstrated complex imaging features, since DDLPS with polymorphic calcification is relatively rare. For retroperitoneal masses of this size, diagnosis should be established based on a combination of imaging findings, pathological examination, and MDM2 amplification testing. Adequate preoperative assessment and complete surgical resection are also essential for relieving compression-related symptoms and improving prognosis.

Key words: dedifferentiated liposarcoma, retroperitoneal tumor, rare disease, murine double minute 2(MDM2) amplification