Perinephric myxoid pseudotumor of fat: a distinctive pseudoneoplasm most often associated with non-neoplastic renal disease

In 2009, Tanas et al reported unusual changes in the perinephric fat, mimicking well-differentiated liposarcoma. We report 11 perinephric masses showing similar changes but chiefly arising in patients with non-neoplastic renal disease. Tissue from 11 perinephric masses was retrieved, and immunohistochemistry for IgG/IgG4 and fluorescence in situ hybridization (FISH) for MDM2 amplification was performed. Clinical information was obtained. Cases occurred in 10 males and 1 female (43-84 years of age; median, 63.5 years). Ten patients presented with perinephric masses (size range, 2-28 cm), and one was an incidental finding. Four patients had bilateral or multiple masses. Underlying renal disease included diabetes mellitus (n = 3), end-stage kidney (n = 2), diabetes and end-stage kidney disease (n = 1), chronic pyelonephritis (n = 1), and non-invasive high-grade papillary urothelial carcinoma of the renal pelvis (n = 1). Three patients were not known to have renal disease. Most tumors were submitted as “well-differentiated liposarcoma.” The masses consisted of mature fat, myxoid stroma, moderately variable spindled to stellate cells and a mixed inflammatory cell infiltrate. Enlarged, hyperchromatic stromal cells were absent. IgG4-positive plasma cells and MDM2 amplification were absent in all tested cases. Clinical follow-up (11 patients; range, 1-120 months; median, 24 months) showed absent or stable disease in 9 patients; 2 died of unrelated causes. This distinctive pseudoneoplasm usually occurs in association with non-neoplastic renal disease, although similar changes may be identified in the perinephric fat of patients with renal carcinoma. Morphologic evaluation and FISH for MDM2 amplification should allow its distinction from liposarcoma and other mimics. •Perinephric myxoid pseudotumor of fat usually occurs in patients with non-neoplastic renal disease.•This lesion mimics well-differentiated liposarcoma and IgG4-disease.•Morphology, clinical correlation and appropriate ancillary testing should allow for the correct diagnosis.