To characterize both malignant and benign renal masses by CT and MRI, it is necessary to use dynamic studies after contrast administration and to apply different protocols and sequences ^[6,7].

In CT studies there is a great variety of protocols for the evaluation of renal masses, although there are 3 mandatory phases that should not be omitted, which are: the basal phase, the cortico-medullary phase and the nephrographic phase. The following table summarizes the types of phases that exist, their acquisition times and their usefulness ^(Tab.1).

In MRI protocols we can count on different sequences, being the most used:- T2W coronal or sagittal -Axial fast spin echo T2W-Diffusion (DWI)-T1 axial gradient echo in phase and opposite phase-T1 fat-sat gradient echo without IV contrast and dynamic, with basal, cortico-medullary and nephrographic phase. 

Once we have acquired the CT or MR images with the appropriate protocol without and after contrast administration, we will use the cortico-medullary phase to quantitatively define the degree of uptake of the renal mass, which is necessary for its characterization.  

In MRI we will say that a mass has enhancement:

-High (if its enhancement is equal to or higher than that of the renal cortex). 

-Moderate (if its enhancement is approximately 50% of the renal cortex enhancement) 

-Low (if its enhancement is approximately 25%-30% of that of the renal cortex).

Regarding CT, a ratio between the attenuation of the mass with respect to the renal cortex has been proposed :

-Intense enhancement: >0.75

- Moderate enhancement: 0.40-0.75

- Low enhancement: <0.40

Benign tumors 

Angiomyolipoma (AML)^{(Fig. 1)}

- Most common benign renal tumor. 

- Mesenchymal tumors containing macroscopic fat, vessels and smooth muscle.

- Sporadic and single, being more common in women aged 40-60 years. In case they are multiple, tuberous sclerosis must be suspected.

- They do not usually present calcifications.

- The vascular component is usually dysplastic with formation of pseudoaneurysms that may bleed spontaneously (Wünderlich Sd.),especially in tumors >4 cm.

Classification:

Classic AML:high fat content (easy to diagnose).

-They are well-defined, heterogeneous, hypervascular cortical masses.

-CT: <20 HU (fat)

-MRI (macroscopic fat): Hyperintense in T1 and T2 ; Hypointense in sequences with fat saturation ; Loss of signal in T1 out of phase (more sensitive).

Lipid-poor AML (5%): difficult to differentiate from CRC.

-They are exophytic lesions of small size, rounded and without capsule.

-≤25% fat component.

-MRI (intracellular fat): homogeneous and hypointense in T2 (similar to papillary RCC); hyperintense in T1; signal reduction in opposite phase (also visible in RCC).

-Hypervascular: early enhancement and rapid washout  

-Density can be measured by ROI on plain CT, arterial phase, venous phase and delayed phase. Typically shows enhancement with values greater than 1.5 in the following calculation: (arterial -precontrast)/(delayed -precontrast).

Oncocytomas^(Fig.2)

- Encapsulated cortical tumor composed of eosinophilic cells of the epithelium of the collecting tubules, presenting histological resemblance to chromophobe RCC.

- Males aged 60-80 years

- Hypervascular, difficult to differentiate from RCC.

- Useful sign: central fibrous scar of stellate morphology (1/3-1/2 of cases) of low attenuation.

- MRI: Hypointense (or isointense) in T1 sequences; hyperintense in T2 sequences; no signal drop in ADC maps.

- T1+contrast: homogeneous enhancement, less intense with respect to the renal parenchyma. 

This table summarizes the difference between oncocytoma and clear cell renal carcinoma ^{(Tab. 2)}

Malignant tumors

-90% of solid renal masses are malignant lesions

-Average age at diagnosis: 50- 70 years old

-More frequent in males-Its diagnosis is usually incidental, although there is the typical triad: hematuria, palpable renal mass and flank pain (10% of cases)

Clear cell carcinoma (RCC) ^{(Fig. 3)}

- Most frequent renal malignant tumors (75%).

- Most are sporadic, but may be associated with syndromes such as Von Hippel-Lindau and tuberous sclerosis.

- Heterogeneous: necrosis component, cystic degeneration (hyperintense in T2) and/or hemorrhage foci (hyperintense in T1). 

- Hypointense (or isointense) in T1 sequences. 

- Hyperintense in T2-weighted sequences.- Sometimes a tumor “pseudocapsule” is observed, hypointense in T1 and T2 sequences secondary to the mass effect it exerts on the adjacent renal parenchyma.

-Coarse and irregular calcifications: hyperdensity in CT.

-Intracytoplasmic fat: signal drop in the opposite phase sequences.

-Hypervascular: intense, heterogeneous and rapid enhancement in cortico-medullary (more sensitive and specific) and nephrographic phases, however, in later phases (excretory phase) they are hypointense.

-If intralesional necrosis is present, it will not enhance after contrast administration.

-It is the subtype with the highest metastatic potential (also hypervascular).

Papillary carcinoma ^{(Fig 4)}

-They represent 10% of malignant renal masses. 

- Bilateral and multifocal 

- Slow growing, less aggressive and with better prognosis than clear cell carcinoma (survival up to 90% at 5 years). 

- Homogeneous mass, and hypointense in T2.

 - May present hemosiderin: foci with signal drop in in-phase T1 sequences in relation to out-of-phase sequences. Hyposignal in T2 and faint hyperdensity in CT.

 - Low intracellular lipid content.

- Hypovascular: very faint and progressive enhancement.

- Hypointense fibrous capsule on T1- and T2-weighted sequences.

Chromophobe carcinoma ^{(Fig. 5)}

- Less frequent subtype of CRC (5-10%).

- Good prognosis survival at 5 years of 78-93%.

- They are larger, well-defined solid tumors, with lobulated and homogeneous borders, since necrosis or cystic degeneration is not usually seen.

- They derive from collecting tubules, as do oncocytomas, hence their histologic and radiologic similarities.

- Hypo- or isointense in T2 - Intermediate vascularization: intermediate enhancement in corticomedullary phase, between clear cell and papillary.Their washout appears significantly smaller and slower compared to oncocytoma.

- Like oncocytomas they may have a central “cartwheel” scar and calcifications within.

- No microscopic fat content: they show no signal loss on the opposite phase image.

-Lower ADC values, even more than those of clear cells.

If once the optimal CT or MRI images have been acquired we have doubts in the characterization of the renal masses according to their behavior and radiological appearance, we can rely on the following algorithms supported by the scientific literature: CT-Score ^[8]and MRI Clear Cell Likelihood Score^[1,3,4].

These algorithms serve us to predict the risk of clear cell renal disease with a score of 1-5 points, where 5 would be a high probability of clear cell cancer. The CT-score algorithm ^(Fig.6) is the simplest and is used as an alternative to MRI to predict the diagnosis of clear cell renal disease. It takes into account two variables: the ratio of attenuation of the mass with respect to the cortex (explained in previous sections) and the heterogeneity of the lesion according to the Likert scale^(Fig.7).

The multiparametric MRI Clear Cell Likelihood score (ccLS) algorithm ^{(Fig 8)} takes into account several variables and is more complex, although the radiological knowledge for its understanding has already been presented throughout this paper. It is considered very useful for noninvasively identifying the clear cell subtype and decreasing the number of diagnostic biopsies of renal masses.