Recent advances in imaging technology, and in particular the wide use of ultrasonography (US) and computed tomography (CT), have led to an increase in the numbers of incidentally discovered renal masses in seemingly asymptomatic patients. Although renal cell carcinoma (RCC) is by far the most lethal urologic malignancy, benign tumors constitute a significant proportion of incidental solid renal masses (Prasad et al. 2008b; Silverman et al. 2008; Berland et al. 2010). As benign solid renal masses are increasingly found, characterization of the lesion, especially as benign vs. malignant, as well as the decision of optimal treatment method, has been increased and become more important. In fact, a significant proportion of the surgically removed solid renal masses are histologically benign (Kutikov et al. 2006; Beland et al. 2007; Maturen et al. 2007; Vasudevan et al. 2006; Silverman et al. 2008), especially lesions <3 cm in diameter, and unnecessary surgery is performed.

According to the American College of Radiology Incidental Findings Committee (Berland et al. 2010), when a solid renal mass is incidentally identified, pseudotumors (e.g., pyelonephritis) and fat-containing angiomyolipomas should be excluded. Incidental solid renal masses <1 cm in diameter should undergo follow-up (CT or MR imaging at 3–6 months, 12 months, and then yearly) until 1 cm or 1.5 cm in patients with comorbidities or limited life expectancy. Incidental solid renal masses 1–3 cm in diameter should undergo surgery in the general population or surgery or observation in patients with comorbidities. In particular, if the solid renal mass is hyperattenuating and homogeneously enhancing, MR imaging and/or percutaneous biopsy should be considered to diagnose angiomyolipoma with minimal fat. In incidental solid renal masses > 3 cm, again surgery in the general population, or surgery or observation in patients with comorbidities is suggested. In particular, biopsy should be considered to exclude angiomyolipoma with minimal fat and to confirm renal cell carcinoma.

Incidental cystic renal masses with a Bosniak I or II pattern (Bosniak 1986), corresponding to simple cysts with hairline thin wall or with fine wall calcifications or with short segment of slightly thickened calcifications in the wall or septa or also hyperdense cysts <3 cm, should be ignored (Berland et al. 2010). Incidental cystic renal masses classified as Bosniak IIF, with minimal smooth thickening of wall or septa that may show perceived (not measureable) enhancement, and with thick calcifications and no enhancing soft-tissue components, should undergo follow-up by CT or MR imaging at 6 and12 months, then yearly for 5 years. The other incidental cystic renal masses should undergo surgery provided that cystic masses ≤1.5 cm that are not clearly simple cysts or that cannot be characterized completely may not require further evaluation in patients with comorbidities and in patients with limited life expectancy.

Anyway, the relatively high incidence of benign masses removed at surgery warrants a change in the management algorithm for patients with suspicious RCC, and proceeding to surgery, even with a minimally invasive approach such as partial nephrectomy, should be avoided whenever possible. Anyway, benign and malignant tumors may be impossible to differentiate by imaging. Moreover, pathologists advise caution when interpreting tumors, specifically those with oncocytic features or hybrid or collision tumors (Rowsell et al. 2007). Under these clinical circumstances, the need for less invasive and less extensive treatment strategies, such as careful follow-up with CT, US, or percutaneous biopsy, becomes critical treatment issues.

Different strategies have been proposed for the clinical management of enhancing renal masses. Percutaneous image-guided renal mass biopsy is suggested to preoperatively characterize renal masses and to establish definitive diagnoses (Silverman et al. 2006). Based largely on advances in cytological techniques, percutaneous biopsy can now be used to diagnose benign neoplasms and thus prevent them from being treated unnecessarily (Sahni and Silverman 2009). Percutaneous biopsy has been suggested also for renal masses without radiologic evidence of malignancy (Lebret et al. 2007). Minimally invasive management, such as partial nephrectomy, cryotherapy (Gill et al. 2000), or radio-frequency ablation (Fotiadis et al. 2007), should be considered for tumor removal, while conserving unaffected normal renal parenchyma.

Benign and malignant renal tumors may present a similar vascular architecture at color and power Doppler US and similar appearance on CT. In order to characterize a solid renal mass, the contrast-enhanced CT examination should be multiphasic including the unenhanced scan, the corticomedullary phase scan to recognize the vascularization of the tumor, and the nephrographic phase scan to recognize the enhancement level and all the outline of the lesion. The most important aspect in analyzing a renal mass is to show the presence or absence of enhancement within it (Israel et al. 2006), even though it is not considered a sign of malignancy. An increase of 20 HU attenuation (Israel and Bosniak 2005) within a renal lesion after the intravenous administration of contrast agent has been generally accepted as a threshold for contrast enhancement. If the change in lesion density is between 10 and 20 HU (equivocal enhancement) or if the change in lesion density is <10 HU (absent enhancement), the lesion may be solid benign or malignant (typically papillary-cell carcinoma) or cystic.

In this case the renal lesion should be considered indeterminate, and further evaluation with magnetic resonance (MR) imaging with image subtraction technique (Hecht et al. 2004) should be performed. MR imaging examinations are performed with a phased-array body coil. Each examination should include coronal and transverse T2-weighted turbo spin-echo (TSE) sequence; T2-weighted turbo spin-echo sequence with fat suppression including spectral presaturation with inversion recovery (SPIR), or spectral adiabatic inversion recovery (SPAIR), or short-tau inversion recovery (STIR) sequences; axial dual-echoin-phase and opposed-phase gradient echo (GRE) T1-weighted sequence; and axial or coronal 3D fat-suppressed spoiled GRE (SPGR)T1-weighted sequence obtained before and after the intravenous administration of contrast material. Dual echo in-phase and opposed-phase gradient echo (GRE) T1-weighted sequence; and axial or coronal 3D frequency-selective fat-suppressed fast spoiled GRE (SPGR) T1-weighted sequence obtained before and after the intravenous administration of contrast material. Dual-echo inphase and opposed-phase GRE T1-weighted sequence is helpful in assessing for the presence of intratumoral fat. Images are obtained at specific echo times at which water and fat protons are inphase and out of phase with each other (at 1.5 T, 4.2 and 2.1 ms, respectively). A loss of signal intensity on the opposed-phase images in comparison with the in-phase images occurs when both fat and water are present within a voxel as in the case of microscopic fat.

Except for angiomyolipoma, MR imaging sequences are not sensitive or specific in the characterization of renal masses as benign or malignant. In most cases, a renal solid enhancing mass corresponds to a malignancy, even though many benign lesions may present a hypervascular pattern after contrast administration. Image subtraction consists in the subtraction of the unenhanced from the gadolinium-enhanced fat-suppressed T1-weighted images, and it represents a reliable method of showing enhancement within a renal mass (Hecht et al. 2004). If subtraction is not available or image misregistration is present, the percentage of enhancement can be calculated. If a renal mass or a portion of a renal mass enhances more than 15 % after a scan delay of 2–4 min, it is highly likely to be a solid mass (Ho et al. 2002). It is also possible to identify enhancement visually within a renal mass with a side-by-side comparison of the unenhanced and contrast-enhanced images (Israel et al. 2006). This is straightforward in hypervascular masses, while in hypovascular masses and in those masses that are hyperintense on unenhanced T1-weighted images, side-by-side comparison is usually not reliable.

In this chapter we will describe the most frequent benign renal tumors, while the more rare histotypes are described in chapter “Rare and secondary tumors of the kidney and renal pseudotumors.”