Sonographically detected malignant transformation of a simple renal cyst.

The clinical course is reported of a simple renal cyst which developed into a septated renal cyst, and finally to a cystic renal cell carcinoma. A 49-year-old man, who had been diagnosed as having a renal cyst, was found by repeated ultrasonography over 6 years to have solid components developing within the cyst. Radical nephrectomy was performed, and pathological examination confirmed cystic renal cell carcinoma (RCC). This case clearly shows a natural history of malignant transformation from a simple renal cyst, and emphasizes that careful follow-up of renal cysts, especially of complicated renal cysts, is mandatory for successful treatment of RCC.