Imaging appearances of perinephric myxoid pseudotumor of fat (PMPF)

This study aimed to identify the imaging feature of perinephric myxoid pseudotumor of fat (PMPF) in a large cohort.

Institutional radiology and pathology databases were queried for PMPF for the period from January 2010 to December 2023. Of the 22 identified individuals, two were excluded due to nonavailability of computed tomography (CT) or magnetic resonance (MR) images and five due to lack of pathological confirmation. The study cohort comprised of 15 individuals (males = 10), with median age of 59 years (range: 26-87 years). Three abdominal radiologists in consensus reviewed all relevant CT, magnetic resonance imaging (MRI) and positron emission tomography (PET) images.

Seventeen tumuors were analysed since two individuals had bilateral tumuors. The tumuors were at the renal hilum (n=7) or perinephric space (n=10). Two imaging patterns were seen: hazy ill-defined predominantly fatty mass (n=9) or a soft tissue mass (n=8) with variable fat content (0-90%). At presentation, the tumuors ranged in size from 1.5 to 16.5 cm (median: 6.2 cm). The perihilar tumuors encased central renal vessels without occlusion or thrombus. Ureteric encasement and mild hydronephrosis were seen in 11 and 4 tumuors, respectively. Most (9 of 13) of the affected native kidneys and all transplant kidneys (n=5) showed parenchymal poor contrast enhancement. Tumuors with greater than 12-month follow-up (n=11) showed a median growth of 15%.

PMPF may be considered in the diagnosis of tumuors with soft tissue and fat components situated at the renal hilum or perinephric space. The delayed contrast enhancement, relative lack of mass effect for size, and poor function of affected kidney are diagnostic features.