Perinephric myxoid tumour of fat and IgG4-related disease: when retroperitoneal fat conceals two truths. A case report of a tricky diagnosis.

The perinephric space is a retroperitoneal compartment that can harbour various types of lesions. Among these, fat-containing lesions represent a specific subset that narrows the differential diagnosis. Perinephric myxoid pseudotumor of fat (PMPFT) is a rare and less-known entity within this category; its imaging features may overlap with other conditions such as well-differentiated liposarcoma and IgG4-related disease. We report the case of a 73-year-old man in whom bilateral perinephric fat-containing lesions were incidentally discovered during an abdominal ultrasound. Further imaging with CT and MRI was performed to better characterise the lesions. Two biopsies were subsequently undertaken; only the second one, performed laparoscopically, yielded diagnostic material, leading to a diagnosis of PMPFT. However, one year later, follow-up CT revealed an increase in size of the left perinephric mass, which also contained more evident soft tissue foci. The patient underwent a third biopsy that demonstrated an IgG4-positive plasma cell infiltrate, introducing a diagnostic dilemma.