Both CT and MRI are used to detect, characterize, and stage renal tumors. The high spatial and contrast resolution of CT and MRI makes detection of masses fairly easy. Characterization is however more challenging because there are some difficulties in differentiating benign from malignant tumors and cystic complex masses. Benign tumors that enter into differential diagnosis with malignant tumors are oncocytoma and lipid-poor angiomyolipoma (Israel and Bosniak 2005). Renal oncocytoma is a benign tumor which presents with soft tissue density on CT, mainly isoattenuating to the normal renal parenchyma on non-contrast-enhanced CT. Following contrast administration, renal oncocytoma is usually hypervascular and therefore hyperdense in the arterial phase and relatively hypodense compared to the renal ­parenchyma in the ­nephrographic phase (Fig. 33.7). In typical cases, a central hypodense area with stellate morphology, representing the central scar, is visible. However, this typical feature, highly characteristic of renal oncocytoma, is visible only in a minority of cases and mainly in larger oncocytomas. Similarly, at MRI renal oncocytomas appear hypointense on T1-weighted images and hyperintense on T2-weighted images. Following contrast administration these tumors are hyperintense in the arterial phase and relatively hypointense compared to the renal parenchyma in the nephrographic phase (Schatz and Lieber 2003; Zagoria 2000). The detectability of the central scar on MRI is the same as on CT, so the differential diagnosis remains difficult also with MRI. Recently a number of papers have been dedicated to the possibility of differentiating oncocytomas from renal cell carcinoma using diffusion-weighted images and apparent diffusion coefficient (ADC) values. Oncocytomas tend to have higher ADC values than renal cell carcinomas; a possible explanation is the higher ­cellularity of malignant renal tumors compared with benign renal tumors. Although this information is of interest and represents an additional finding useful for the differential diagnosis, it must be noted that there is some overlap between the ADC values of oncocytomas and renal cell carcinomas (Doğanay et al. 2011; Rosenkrantz et al. 2010).

Lipid-poor angiomyolipomas also enter into differential diagnosis with malignant tumors (Bj and Wong-Yoll-Cheong 1997; Pozzi Mucelli and Locatelli 2002; Jinzaki et al. 1997). Typical angiomyolipomas are easily characterized with ultrasound, CT, and MRI because they contain fat (Pozzi Mucelli and Locatelli 2002): fat is responsible for the hyperechogenicity on ultrasound, low density on CT, and high signal intensity on T1-weighted MRI. Furthermore, fat can be specifically suppressed on selective fat-saturated sequences in MRI. Lipid-poor angiomyolipomas are rare and represent about 10 % of all renal angiomyolipomas. Their diagnosis is difficult since the density on CT is the same as renal cell carcinomas, both on non-contrast- and contrast-enhanced scans. Also, the signal intensity in MRI of lipid-poor angiomyolipomas and renal cell carcinomas is similar, although it has been suggested that chemical shift sequences, with in- and out-of-phase images, can show a signal drop on out-of-phase images due to the minimal amount of fat that is not detectable with other imaging methods (Burdeny et al. 1997; Israel et al. 2005; Olltwater et al. 1997).

CT and MRI have important roles in staging of renal tumors (Moch et al. 2009). CT is usually preferred in the staging of this neoplasm, because of the possibility of evaluating the lungs, while the results of CT and MRI for staging the abdomen are similar. Concerning local extension, a careful evaluation of the size and the site of the tumor should be performed, together with assessment of the involvement of the renal fascia, the renal vein, the inferior vena cava, and the ­surrounding organs (adrenal glands, liver, spleen, colon) (Fig. 33.8). The evaluation of the extent includes also the presence of nodes and metastases, mainly in the liver and lungs, but, although rare, also in other organs, such as the pancreas (Figs. 33.9 and 33.10). This assessment is very important because imaging findings influence the type of surgery. Nowadays, since a growing number of renal tumors of small size are detected, these lesions are treated conservatively with tumorectomy, partial nephrectomy, or by means of ablation techniques, rather than with radical nephrectomy which is performed for larger, infiltrating tumors (Tan et al. 2012; Zagoria et al. 2011; Rodriguez et al. 2000; Gill et al. 2000).