Enlarging Bilateral Perinephric Masses in Nephrotic Syndrome

A 62-year-old man with biopsy-proven focal segmental glomerulosclerosis presented with nephrotic syndrome and enlarging bilateral perinephric masses. Contrast-enhanced CT showed large low-density lesions surrounding and compressing both kidneys, raising concern for retroperitoneal malignancy. Needle biopsy revealed bland spindle cells with low proliferative activity in myxoid stroma. Because he declined repeat biopsy, dual-tracer PET/CT was performed, showing diffuse mild-to-moderate ¹⁸ F-FDG uptake (SUVmax, 6.5) and moderate ⁶⁸ Ga-FAPI uptake (SUVmax, 8.5), without focal markedly increased uptake. Integration of these imaging and pathologic findings supported the diagnosis of perinephric myxoid pseudotumor of fat and guided conservative management.