Renal cell carcinoma originating in a renal cyst in a 12-year-old girl.

We report a 12-year-old girl with renal cell carcinoma originating in a cyst of the left kidney. Ultrasonography revealed 2 small hyperechoic masses in the luminal side of a cyst. Although hypervascularity was not detected in the cyst by computed tomography, the possibility of malignancy could not be ruled out because of the presence of 2 solid masses. Therefore, partial left nephrectomy was performed. On histopathologic examination, the 2 solid masses within the cyst were found to be renal cell carcinoma. This patient remains disease-free at 4 years after partial nephrectomy.