A 65-year-old female presented to the emergency room with an acute onset of right flank pain and gross hematuria that occurred one day earlier. There were no other unusual findings in her past medical history and family history. She was taking antihypertensive medication. No abnormalities were found on the physical examination.

Contrast-enhanced abdominal computed tomography (CT) revealed a large, lobulated contoured fatty mass in the right kidney with a surrounding hematoma. The fatty mass was thought to be a ruptured AML. Contrast-enhanced CT demonstrated a 1.9-cm avidly enhancing mass, which was initially considered a non-fatty component of the AML, in the peripheral portion of the fatty mass (Fig. 1A, B). Because the most likely cause of perirenal hematoma was thought to be rupture of the fatty mass associated with the AML, we performed an emergent angiography, which showed multiple microaneurysms and contrast extravasation. Selective arterial embolization was successfully implemented with microcoils and Gelfoam.

During an 18-month follow-up, additional selective arterial embolizations and a single laparoscopic cryoablation were performed to reduce the size of the large fatty mass in the right kidney. During the cryoablation procedure a biopsy was performed for the fatty renal mass, and AML was diagnosed by histologic analysis. A CT scan performed 31 months after initial embolization showed a newly developed calcification in the enhancing mass.

The size of fatty components of the AML and the renal parenchyma decreased on serial follow-up CT scans. However, the enhancing mass on the initial CT scan, which had previously been thought to be a non-fatty component of the AML, gradually increased in size up to 4.6 cm over the 55-month follow-up period (Fig. 1C). In addition, the mass showed strong contrast enhancement on the corticomedullary phase and wash-out on the excretory phase. There were no apparent findings of any invasion to adjacent structures or lymphadenopathies in the upper retroperitoneum on the follow-up CT scans.

Contrast-enhanced, diffusion-weighted magnetic resonance imaging (MRI) was subsequently performed for further characterization of the enhancing and gradually enlarging mass in the right kidney. The mass appeared hypointense on T1-weighted images and hyperintense on T2-weighted images compared with the surrounding renal parenchyma (Fig. 1D). Strong contrast enhancement on the corticomedullary phase and wash-out on the excretory phase, identical to the previous CT scans, were evident. On diffusion-weighted images, the mass appeared hyperintense on the high b value (800 sec/mm²) image and showed a low diffusion coefficient value on the apparent diffusion coefficient (ADC) map.

Fluorine 18 fluorodeoxyglucose (FDG) positron emission tomography/CT (PET/CT) scan of the lesion showed intensively increased metabolism and no evidence of distant metastasis.

The relatively rapid growth evident in the follow-up CT scans, strong contrast enhancement on the corticomedullary phase and excretory phase wash-out, diffusion restriction on MRI, and hypermetabolism on FDG PET/CT indicated the potential for a malignant tumor, such as RCC. Appropriately, the patient therefore underwent ultrasonography-guided biopsy, and histologic examination confirmed the diagnosis of RCC. Right radical nephrectomy was performed. Surgical pathology revealed a large fatty mass and an adjacent solid mass surrounded by a fibrous capsule. The solid mass was diagnosed as a clear cell carcinoma with Fuhrman grade 3 that did not show adjacent tissue invasion or sarcomatous differentiation (Fig. 1E). The postoperative course was uneventful and the patient was well at the 7-month follow-up after surgery.

A 45-year-old male patient was admitted because of two incidental masses in his right kidney. He did not show any signs or symptoms. Routine preoperative laboratory tests, including urinalysis, were normal. Initial abdominal ultrasonography carried out for general examination revealed two masses in the kidney. The lower pole mass was an approximately 4.0-cm well-defined hyperechoic mass, and the upper pole mass was a 3.7-cm subtle hypoechoic mass. On contrast-enhanced abdominal CT, the lower pole mass was composed of soft tissue components presenting moderate prolonged enhancement and abundant gross fat components (Fig. 2A). In contrast, the upper pole mass showed heterogeneous strong enhancement on the corticomedullary phase and wash-out on the excretory phase (Fig. 2B). The masses were presented in the same kidney but completely separated. The patient underwent laparoscopic right radical nephrectomy. The diagnoses of a clear cell carcinoma for the upper pole mass and an AML for the lower pole mass were confirmed by a pathologic examination. There were no remarkable postoperative complications or tumor recurrences during a 1-year follow-up.