Perinephric myxoid pseudotumor of fat: A very rare entity that can mimic a renal cyst and retroperitoneal liposarcoma on imaging

We present a case of perinephric myxoid pseudotumor of fat, a rare benign entity that often occurs in patients with non-neoplastic renal disease. In our case, an 80 year old man with end-stage renal disease was imaged over the course of 5 years during evaluation for renal transplantation. Imaging identified a left perinephric mass whose appearance over time and on different imaging modalities variably suggested a simple cyst, cystic neoplasm, and liposarcoma. Contrast enhanced examination was necessary to discern the solid nature of this mass, and ultimately, tissue sampling with histopathologic evaluation and molecular testing were required to make the diagnosis of myxoid pseudotumor of fat and exclude the imaging mimics.