Renal cell carcinoma with peri-renal fat necrosis radiographically mimicking extra-capsular tumor extension

Peri-renal fat necrosis is rarely encountered, especially in the absence of previous surgery, trauma, or pancreatic disease. We report a patient being followed for chronic renal insufficiency by renal ultrasound in whom a renal mass was detected. Imaging studies were suggestive of renal cell carcinoma with extra-capsular extension. Histologic study confirmed renal cell carcinoma but the radiographic impression of tumor extension into surrounding adipose tissue was explained by peri-renal fat necrosis and fibrosis. To the best of our knowledge, this is the second reported case of peri-renal fat necrosis associated with renal cell carcinoma. We propose a possible mechanism for the association.