Perinephric Myxoid Pseudotumor of Fat: Case Series, Systematic Review of the Literature, Clinical Classification, and Treatment Implications.

Perinephric myxoid pseudotumor of fat (PMPF) is a rare and highly heterogeneous disease consisting of mature fat, myxoid stroma with spindled or stellate stromal cells histologically. Here, we present 2 cases of PMPF occurring in an allograft and a native kidney, and a systematic review that included 120 previously reported cases was subsequently conducted. Notably, 89% of PMPF cases with documented medical history had comorbidities. The most common comorbidities were urologic neoplasm (45%), chronic kidney disease (23%), allograft kidney (16%), and urinary tract obstruction (5%). We propose a classification of PMPF based on comorbidities and discovered that patients with concomitant urologic neoplasm or obstruction tended to have significantly smaller tumors, and active surveillance of PMPF after treating the primary disease might be an appropriate approach. In contrast, for patients with kidney transplants, chronic kidney disease, or no identifiable primary disease, the tumors were generally larger; therefore, surgical intervention should be considered. Regardless of the classification method used, for tumors exhibiting a slow growth rate, active surveillance should be considered. Our findings aimed to facilitate more accurate differential diagnosis and optimize treatment options.