This guidance applies to incidental renal masses in asymptomatic adult patients (18 years of age or older). The incidental renal mass is one that is initially detected on an imaging study performed for an indication other than the assessment of urinary tract disease. It should not be applied to patients with medical conditions or genetic syndromes that predispose them to renal neoplasms or to those with a primary malignancy that has a reasonable possibility of metastasizing to the kidneys, such as lung cancer, lymphoma, or melanoma. It does not apply to infiltrating renal processes. If the renal mass presents with lymphadenopathy or other metastases or are so overwhelmingly likely to be a renal cell carcinoma (RCC), it should be directly referred for management.

This guidance believes a cystic renal mass can only be fully characterized on CT or MRI without and with IV contrast. So the Bosniak classification for cystic renal mass is listed under the section of nonfat masses discovered on CT/MRI without and with contrast—>cystic. For direct link to the Bosniak classification, please click here. 

On unenhanced CT, a homogeneous mass of 70 HU or greater is almost always a hyperdense Bosniak II cyst and needs no further evaluation.

Does the renal mass contain gross fat?

Yes.        

No.      

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Notes:
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A peripherally calcified mass in the renal hilum could represent a renal artery aneurysm.

The size of the mass is the largest diameter on any plane.

RCC=renal cell carcinoma. TSTC=too small to characterize. AML=angiomyolipoma.

Criteria for defining enhancement in a renal mass:

CT increase in attenuation after IV contrast: >=20 HU, definite enhancement. >10 to <20 HU, equivocal for enhancement.  (possibly caused by beam hardening or intra-renal location) <=10 HU, no enhancement.

MRI criteria: >=15% increase in signal intensity after IV contrast or visible signal intensity on subtraction images.

Please measure regional density of a renal mass with ROI instead of single pixel.

An ROI value <-10 HU within the mass indicates the presence of gross fat in general, most likely AML.

A homogeneous mass with density value between -20 to -10 HU is most likely an AML with a small possibility of representing a low density cyst. In this case, MR or ultrasound should be considered for further evaluation.