Bosniak Renal Cyst Classification System

Disclaimer: The author makes no claims of the accuracy of the information contained herein; this information is for educational purposes only and is not a substitute for clinical judgment.

Original Criteria

Category Criteria Management
  • Hairline-thin wall
  • No septa, calcifications, or solid components
  • Water attenuation
  • No enhancement
No follow-up
  • Few hairline-thin septa with or without perceived (not measurable) enhancement
  • Fine calcification or a short segment of slightly thickened calcification in the wall or septa
  • Includes uniformly high-attenuating lesions (≤3 cm) that are sharply marginated and do not enhance
No follow-up
  • Multiple hairline-thin septa or minimal smooth thickening of walls or septa, with or without perceived (not measurable) enhancement
  • Calcification may be thick and nodular
  • No enhancing soft-tissue components
  • Includes totally intrarenal nonenhancing high-attenuating renal lesions (>3 cm)
Follow-up *
3 Thickened irregular or smooth walls or septa, with measurable enhancement. Surgery *
4 Criteria of category 3 but also containing enhancing soft-tissue components adjacent to or separate from the wall or septa. Surgery *

2019 Proposed Update

Category Criteria
1 All well-defined with thin (<=2 mm) smooth walls:
  • Homogeneous simple fluid (-9 to 20 HU)
  • No septa or calcifications
  • The wall may enhance
2 All well-defined with thin (<=2 mm) smooth walls:
  • Cystic masses with thin (<=2 mm) and few (1-3) septa; septa and wall may enhance; may have calcification of any type

  • Homogeneous hyperattenuating (>=70 HU) masses at non-contrast CT
  • Homogeneous nonenhancing masses > 20 HU at renal mass protocol CT, may have calcification of any type
  • Homogeneous masses -9 to 20 HU at noncontrast CT
  • Homogeneous masses 21 to 30 HU at portal venous phase CT
  • Homogeneous low attenuation masses that are too small to characterize

  • Homogeneous masses markedly hyperintense at T2-weighted imaging (similar to CSF) at noncontrast MRI
  • Homogeneous masses markedly hyperintense at T1-weighted imaging (approx 2.5 x normal parenchymal signal intensity) at noncontrast MRI
  • Cystic masses with a smooth minimally thickened (3 mm) enhancing wall, or smooth minimal thickening (3 mm) of one or more enhancing septa, or many (>=4) smooth thin (<=2 mm) enhancing septa

  • Cystic masses that are heterogeneously hyperintense at unenhanced fat-saturated T1-weighted imaging
3 One or more enhancing thick (>=4 mm width) or enhancing irregular (displaying <=3 mm obtusely marginated convex protrusions) walls or septa
4 One or more enhancing nodules (>=4 mm convex protrusion with obtuse margins, or a convex protrusion of any size that has acute margins)
* 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.