Perinephric myxoid pseudotumor of fat is a very rare, benign, slowly growing tumor, which can mimic renal cysts, retroperitoneal cystic tumors and liposarcomas alike. Due to its rarity and overlapping imaging appearance the diagnosis is almost always made by histopathological analysis 1,2.
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Etiology
Perinephric myxoid pseudotumor of fat is thought to be caused by irritation of the perinephric fat by pre-existing chronic kidney disease. It is most commonly encountered in elderly patients suffering from chronic kidney disease (end stage kidney failure, diabetes mellitus, prior pyelonephritis) 1,2.
Pathology
These tumors predominantly comprise of normal adipocytes mixed with spindle cells and chronic inflammatory cells, with a small amount of fibromyxoid stroma. Cytologic atypia is notably absent 1,2.
Radiographic appearance
CT
Perinephric myxoid pseudotumor of fat typically demonstrates a low density on non-contrast CT, and is initially not easily distinguished from a simple renal cortical cyst. In larger lesions, irregular, more characteristically fat density areas can be encountered. Postcontrast enhancement is variable and not always equivocal 1.
US
The ultrasound appearance is more characteristic, with irregular internal echogenicity resembling hemorrhagic cysts. CEUS may demonstrate irregular, internal enhancement 1.
MRI
These tumors are usually hypointense on T1 and hyperintense on T2-weighted sequences. The internal irregularity can be better depicted by applying fat suppression to the T2W sequence 1.
Treatment and prognosis
These lesions typically demonstrate slow growth and are invariably benign. Due to their extreme rarity, surgical resection is the mainstay of therapy, whilst postoperative histology is typically needed for the final diagnosis 1.
Differential diagnosis
- benign renal cyst
- hemorrhagic/proteinaceous cyst
- lymphocele
- primary retroperitoneal cystic tumors (e.g. mucinous cystadenoma)
- myxoid liposarcoma
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