[Perinephric myxoid pseudotumor of fat: a clinicopathological study of five cases].

Objective: To investigate the clinicopathological characteristics, as well as diagnostic and differential diagnostic criteria of perinephric myxoid pseudotumor of fat (PMPF). Methods: Five cases of PMPF were retrospectively collected from the departmental files of Zhejiang Provincial People's Hospital (4 cases) and Ningbo Clinical Pathology Diagnosis Center (1 cases) from January 2020 to December 2023, the clinical, morphological, and immunohistochemical characteristics were analyzed. Fluorescence in-situ hybridization (FISH) was utilized to detect the amplification status of MDM2 and the rearrangement status of DDIT3. Results: There were 3 male and 2 female patients, with a median age of 57(44,70) years (range 33-77). All 5 lesions were related to other diseases of the abdominal or urinary system: 3 were associated with papillary renal cell carcinoma, xanthogranulomatous pyelonephritis, and aggressive fibromatosis of the abdominal cavity; two had a history of previous abdominal surgery (one with nephrolithotomy and the other with radical prostatectomy). Grossly, all 5 cases presented as mass-forming lesions involving the perirenal and/or renal sinus fat, with a median maximum diameter of 6.5 cm (2.6, 12.0), and range 2.2-16.0 cm. All five cases showed similar microscopic appearances: the lesions were ill-circumscribed and consisted of mature adipocytes and bland stellate and spindled stromal cells setting in a prominent fibromyxoid stroma. Occasionally, vesicular pseudolipoblastic stromal cells with cytoplasmic distension by myxoid secretions were present. All the five lesions lacked cells containing bizarre or hyperchromatic nuclei. Mitosis was not found. An arborizing, thin-walled vasculature and rope-like collagen deposition were present. A variably intense mixed inflammatory cell infiltrate was observed in all 5 cases, with lymphoid follicle formation in 3 cases. By immunohistochemistry, the adipocytes expressed S-100 protein while the spindle cells did not in all 5 cases. The spindled cells expressed CD34, p16 and smooth muscle actin in 3/5, 2/5, and 1/5 cases, respectively, with the stains being focally in all the cases; the Ki-67 proliferation index was<2%. The other detected markers were all negative, including CDK4, MDM2, desmin, HMB45, Melan A, NY-ESO-1, ALK, and MUC4. The IgG-positive plasma cells were all less than 5 per high-power field, and IgG4-positive cells were absent in all lesions. The FISH analysis showed negative for MDM2 gene amplification or DDIT3 gene rearrangement. None of the 5 patients showed disease recurrence or progression related to PMPF during a follow-up from 15 to 54 months (mean: 37 months). Conclusions: PMPF is a rare and novel non-neoplastic lesion that is often associated with or secondary to other abdominal or urinary system diseases. It can mimic well-differentiated liposarcoma on both imaging and morphology. Careful histological observation combined with immunohistochemical and molecular genetic analyses can aid in its diagnosis and differential diagnosis.