Perinephric Myxoid Pseudotumor of Fat: A Series of 13 Cases and Literature Review.

Perinephric myxoid pseudotumor of fat (PMPF) is a recently described and rare retroperitoneal mass-forming lesion whose clinical significance chiefly involves mimicry of a variety of soft tissue tumors. For unknown reasons, it commonly occurs in male patients with underlying non-neoplastic renal diseases and/or type 2 diabetes (DMT2). A total of 55 cases have been reported in the literature. Recently, we have encountered 13 such masses with peculiar histologic features; thus, we sought to investigate our experience and review the clinicopathologic characteristics of the literature. Our series confirms that PMPF frequently occurs in adult male patients (11/13, 85%), with an average age of 66 years, and commonly co-occurs with renal disease, such as DMT2 (2/13, 15%), end-stage renal disease (ESRD) (5/13, 39%), renal cysts (4/13, 31%), concurrent or prior renal neoplasia (2/13; 15%), and myeloma/lymphoma (2/13; 15%). Histologic evaluation shows lipomatous masses commonly showing variable amounts of fat necrosis, myxoid degeneration, lymphoplasmacytic inflammation and lacking atypical hyperchromatic stromal spindle cells. Unusual histologic features include extramedullary hematopoiesis (1/13, 8%), hemosiderin deposition (4/13, 31%), and small wisps of mature smooth muscle (6/13, 46%). All cases tested were negative for MDM2 and did not show an increased ratio of IgG4⁺/IgG⁺ plasma cells. Our study confirms the clinical and pathologic features of PMPF and expands its histologic spectrum, underscoring the importance of this entity as a benign pseudotumor which should be included in the differential diagnosis of other fat-containing retroperitoneal masses, particularly well-differentiated liposarcoma.