CT and MRI imaging features of atypical spindle cell lipomatous tumor in the retroperitoneum: a case report.

Atypical spindle cell lipomatous tumor (ASCLT) is a rare benign adipocytic neoplasm, and primary involvement of the retroperitoneum is exceedingly uncommon. Because its imaging features overlap with those of other fat-containing tumors, particularly atypical lipomatous tumor/well-differentiated liposarcoma (ALT/WDL), preoperative diagnosis remains challenging. We report a case of primary retroperitoneal ASCLT in a 58-year-old woman whose lesion was incidentally detected during a routine physical examination. Computed tomography (CT) and magnetic resonance imaging (MRI) revealed a small, well-defined fat-containing mass located between the celiac trunk and the superior mesenteric artery, with fat-related signal attenuation on fat-suppressed sequences, no obvious diffusion restriction, and mild reticular enhancement after contrast administration. Complete surgical excision was performed, and postoperative histopathological, immunohistochemical, and molecular findings supported the diagnosis of ASCLT, with no MDM2 amplification. Compared with previously reported retroperitoneal ASCLT cases, the present case provides detailed CT and multiparametric MRI findings for a small, incidentally detected lesion adjacent to major abdominal vessels. Recognition of this imaging pattern may help include ASCLT in the preoperative differential diagnosis and support appropriate surgical planning, while definitive diagnosis still requires pathological and molecular confirmation.