[Perinephric myxoid pseudotumor of fat].

Myxoid pseudotumors rarely develop in the perirenal adipose tissue, they are incidental findings most commonly. Myxoid pseudotumors may mimic other benign and malignant tumors of this region; therefore, being aware of these lesions is particularly important for the pathologist. In our report, we present the case of a 38-year-old man who developed end-stage renal disease due to bilateral vesicoureteral reflux and left-sided hydronephrosis, for which renal allograft transplantation was performed. 19 years after the transplantation, graftectomy became necessary. During histological processing, in addition to the histological features of end-stage renal disease, we observed a circumscribed spindle cell proliferation with myxoid areas in the renal hilum. Based on the absence of cytological atypia and the immunohistochemical findings, the lesion was diagnosed as a perinephric myxoid pseudotumor of fat tissue. This disease of unknown pathogenesis develops in association with both tumorous and non-tumorous kidney disorders. According to the current data, it is considered a benign tumor-like lesion, and no recurrences have been reported so far following excision. This rare entity most commonly resembles well-differentiated liposarcoma, and its distinction relies on morphology, immunoprofile, and MDM2 FISH analysis. FISH reaction did not show MDM2 gene amplification in the tumor cells in the current case of perinephric myxoid pseudotumor. Orv Hetil. 2025; 166(31): 1224–1229.