GU

* Surgery or follow-up may not be appropriate in patients with limited life expectancy or comorbidities that increase the risk of treatment.

Follow-up for Bosniak 2F: CT or MR at 6 and 12 months, then yearly for 5 years; however, interval and duration of observation may be varied (e.g. longer intervals if mass unchanged, longer duration for greater assurance, shorter duration for "minimal category 2F" lesions).

Silverman SG, Israel GM, Herts BR, Ritchie JP. Management of the incidental renal mass. Radiology 2008; 249:16-31.

Berland LL, Silverman SG, Gore RM et al. Managing incidental findings on abdominal CT: white paper of the ACR Incidental Findings Committee. J Am Coll Radiol 2010; 7:754-73.

Bosniak MA. The Bosniak renal cyst classification: 25 years later. Radiology 2011; 262: 781-5.

In one study, 10.9% of 156 Bosniak category 2F cystic lesions progressed to malignancy within 6 months to 3.2 years. The authors therefore recommended follow-up of Bosniak 2F lesions for 4 years, though again the duration of follow-up may vary.

Hindman NM, Hecht EM, Bosniak MA. Follow-up for Bosniak category 2F cystic renal lesions. Radiology 2014; 272:757-66.

Pitfalls of classification include inconsistencies of CT Hounsfield unit measurements, pseudoenhancement of simple renal cysts at CT, and lack of standardization for evaluating enhancement on MR.

Israel GM, Bosniak MA. How I do it: evaluating renal masses. Radiology 2005; 236:441-50.

Seppala N, Kielar A, Dabreo D, Duigenan S. Inter-rater agreement in the characterization of cystic renal lesions on contrast-enhanced MRI. Abdom Imaging 2014; 39:1267-73.

Hyperdense cysts less than 3 cm with at least one quarter of the lesion extending outside the renal parenchyma can be considered category 2. Intrarenal hyperdense cysts larger than 3 cm are considered category 2F, partly because the morphology and smoothness of the wall cannot be assessed.

Israel GM, Bosniak MA. Follow-up of moderately complex cystic lesions of the kidney (Bosniak category IIF). AJR 2003; 181: 627-633.

In a retrospective examination of renal cell carcinomas and high-attenuation renal cysts, a homogeneous mass measuring 70 HU or greater at unenhanced CT had a greater than 99.9% chance of representing a high-attenuation renal cyst.

Jonisch AI, Rubinowitz AN, Mutalik PG, Israel GM. Can high-attenuation renal cysts be differentiated from renal cell carcinoma at unenhanced CT? Radiology 2007; 243:445-50.

The proposed 2019 update to the Bosniak classification system enables a greater proportion of masses to enter lower-risk classes.

At portal venous phase CT, well-defined homogeneous masses of 40 HU or less are likely to be benign cysts, but the optimal attenuation threshold is unclear; a threshold of 30 HU was chosen for the proposed update.

The presence of calcification as an isolated feature has little predictive value, so calcifications of any morphology are proposed to be a Bosniak 2 feature.

Silverman SG, Pedrosa I, Ellis JH et al. Bosniak classification of cystic renal masses, version 2019: an update proposal aned needs assessment. Radiology 2019; 292:475-488.