Oncocytoma is a benign tumor arising from proximal tubular epithelial cells. In literature cases of coexistent oncocytoma and RCC (10 %); multicentric, bilateral, or metachronous oncocytomas (4–6 %) [3]; and recurrence or even metastases following surgical resection are reported, suggesting that these neoplasms can become malignant. No reliable pattern on US has been reported to differentiate oncocytomas from RCC. Tumor homogenicity has been considered as suggestive of small oncocytomas, but it is not useful in differential diagnosis because of the considerable overlap among small renal tumors including RCC. Percutaneous biopsy may help in differentiating diagnosis; in difficult cases partial nephrectomy or minimally invasive approaches are also performed.

Renal cysts are a common finding, but any cyst that does not show the typical features of a benign cyst is by definition “complicated” and requires further assessment. US findings that suggest a cystic or pseudocystic renal neoplasm include thick irregular wall, multiple septa, solid mural nodule, heterogeneous echoic content, and the demonstration of signal flow on color Doppler evaluation within the solid components.

In 1986, Morton Bosniak proposed a simple classification that classifies renal cysts into five categories based on their CT imaging appearance criteria, including morphological and enhancement characteristics [4–6]:

Even if the Bosniak classification system was developed on the basis of contrast enhancement findings of cystic renal masses on CE-MDCT [5, 6], CEUS can provide useful information to predict the risk of malignancy and, therefore, for the management of these lesions: surgical treatment or observation.

The problematic aspect is the differential diagnosis between lesions classified in category IIF, which require serial imaging follow-up [8] and category III lesions that require prompt surgery because of potential malignancy. In particular, CEUS is acquiring an increasing role in the assessment of indeterminate cystic lesions (Bosniak IIF and III) by detecting the presence and the enhancement of solid components. Recent comparative studies [9] between CEUS and CT revealed that CEUS imaging was superior to CT in terms of detecting additional septa, thickness of the wall or septa, and solid components. Microbubble contrast agents circulate in the microvessels of septa and walls, and CEUS provides the evaluation of internal structures of cystic renal masses with a higher resolution than CT. In particular, the demonstration of solid components is the key factor in differential with categories III and IV that are considered malignant and must be surgically removed (Fig. 6.3). A recent study [10] also introduced an independent diagnostic classification system for CEUS that was based on the enhancement patterns involving or not involving the peripheral wall, the intracystic septa, and the mural or septal nodule. This approach reveals a high correspondence between CEUS and CT, particularly for lesions of category III undetermined on CT, by proposing CEUS as an alternative to CT in the follow-up of complex renal cysts and in case of contraindications to iodinate contrast.

In conclusion, hyperattenuating lesion at CT with no significant contrast enhancement should be considered as possible hypovascularized tumor (RCC): in these cases, prompt US examination should be performed because hyperdense cysts are typically anechoic in 30–50 % of cases.

Sometimes necrotic RCC may mimic a cystic lesion on CT because of a necrotic content similar to fluid cysts in attenuating values: in these cases, US examination is able to show a heterogeneous echoic content consistent with necrosis, associated to irregular thickening of peripheral walls (Bosniak IV).

The majority of primary renal tumors are renal cell carcinomas (RCCs), whereas angiomyolipomas and oncocytomas represent a small part of renal solid lesions.

Most RCCs are first detected by US, while CT is the gold standard imaging modality for staging and surgical planning. CT can also provide information for the differentiation of RCC subtypes, which is important in planning treatment and closely related with prognosis. Magnetic resonance imaging (MRI) is used as a problem-solving modality and for staging. In doubt cases, percutaneous renal biopsy is a safe and accurate method of sampling the lesion and reaching a final histopathological diagnosis, which is closely related with prognosis.

The 2004 World Health Organization (WHO) classification recognizes several distinct histologic subtypes of RCC: clear cell RCC, papillary RCC, chromophobe RCC, multilocular cystic RCC, collecting duct carcinoma, medullary carcinoma, mucinous tubular and spindle cell carcinoma, hereditary cancer syndromes (von Hippel-Lindau disease, Birt-Hogg-Dubé syndrome, tuberous sclerosis, and other genetic conditions), and unclassified lesions.