Renal cell carcinoma presenting as a huge simple renal cyst

Simple renal cysts are the most common renal masses, accounting for roughly 65-70% of cases. Transformation of a simple renal cyst into a renal cell carcinoma (RCC), however, is extremely rare. We reported a 48-year-old man with a right huge simple renal cyst from which about 800 ml of dark yellowish fluid with elevated protein and lactic dehydrogenase (LDH) levels was aspirated. Cytological examination disclosed negative for malignant cells. Nephrectomy was performed because of intractable flank pain 3 months after aspiration. The pathology examination confirmed RCC. While this is an extremely unusual finding, one must not exclude the possibility of malignancy in an apparently simple cyst.