CT of parapelvic cystic renal cell carcinoma

Parapelvic renal cysts are benign ubiquitous lesions of the renal sinus frequently demonstrated on CT and ultrasonography. Occasionally a renal parenchymal mass will grow so that its epicenter is within the renal sinus. We report two cases of surgically proven cystic renal cell carcinoma centered in the renal sinus with no apparent connection to the renal parenchyma. Cystic renal cell carcinoma must be excluded whenever a parapelvic cyst with atypical features is encountered on cross-sectional imaging studies.