Differential and Ancillary Testing in the Diagnosis of Perinephric Myxoid Pseudotumor of Fat: A Case Report and Review of the Literature

Perinephric myxoid pseudotumor of fat is a newly described entity and may mimic adipocytic soft tissue tumors, such as well-differentiated liposarcoma or fibrosclerosing lesions associated with immunoglobulin G4 (IgG4)–related disease. To our knowledge, a total of 13 cases have previously been reported in the English literature, of which 11 cases are from 1 case series. We report the 14th case of perinephric myxoid pseudotumor of fat. A 70-year-old woman with a medical history of rheumatoid arthritis, diabetes mellitus type 2, hypertension, hyperlipidemia, and hypothyroidism presented with a long-standing retroperitoneal mass and worsening abdominal distension and pain. Computed tomography scan showed a large 21-cm right retroperitoneal mass and gallstones. Histologic sections show a lipomatous lesion with fat necrosis, areas of myxoid change, and lymphoplasmacytic infiltrate. No atypical stromal cells were identified. Well-differentiated liposarcoma and IgG4-related disease were considered; however, immunohistochemical stains were negative for MDM2, and IgG4 and IgG did not show an increased ratio of IgG4+/IgG+ plasma cells. In addition, fluorescent in situ hybridization was performed and showed no evidence of MDM2 amplification. Perinephric myxoid pseudotumor of fat is an extremely rare, fat-containing perirenal mass. The exact etiology is unclear but thought to result from perinephric fat irritation secondary to underlying nonneoplastic renal disease. Because of its location, overlapping morphologic features with liposarcoma, and treatment implications, accurate diagnosis and awareness by pathologists are crucial. Further refinements in imaging studies may assist in the diagnosis and prevent potential overtreatment. MDM2 amplification by fluorescence in situ hybridization can help to distinguish this benign lipomatous pseudotumor from liposarcoma.