[IPSILATERAL SYNCHRONOUS FUMARATE HYDRATASE-DEFICIENT RENAL CELL CARCINOMA AND MULTILOCULAR CYSTIC RENAL NEOPLASM OF LOW MALIGNANT POTENTIAL: A CASE REPORT AND LITERATURE REVIEW].

A 75-year-old man was being followed up at a nearby clinic for hypertension and chronic renal failure. The patient was referred to our department as abdominal ultrasound revealed a left renal tumor. Plain computed tomography (CT) showed a 50 mm complex renal cyst in the upper pole of the left kidney. Plain magnetic resonance imaging showed a cystic mass with numerous septa. Partial thickening of the septa was suspected, and the lesion was classified as Bosniak IIF or III. As the patient had renal dysfunction, regular imaging study of the tumor lesion was performed to determine the timing of surgery. In the following year, plain CT revealed a new renal tumor 20 mm in diameter located lateral to the known tumor, with the mass having a tendency to increase. The patient underwent a laparoscopic radical left nephrectomy after the introduction of hemodialysis. Histopathological examination revealed that the tumor located in the medial upper pole of the left kidney was a multilocular cystic renal neoplasm of low malignant potential and that the new tumor located lateral to the known tumor was fumarate hydratase-deficient renal cell carcinoma. Simultaneous occurrence of fumarate hydratase-deficient renal cell carcinoma and multilocular cystic renal neoplasm of low malignant potential in the ipsilateral kidney is extremely rare. We report our case with a review of the literature.