Bosniak 2019 Renal Cyst Calculator

Describe the cystic renal mass with the 2019 definitions – wall and septa thin (≤ 2 mm), minimally thickened (3 mm), thick (≥ 4 mm) or irregular; nodules ≥ 4 mm with obtuse margins or any size with acute margins – and the calculator returns the Bosniak class, what drove it, and the follow-up dates for class IIF.

Disclaimer:

Implements the Bosniak classification, version 2019 (Silverman et al., Radiology 2019). Provided "as is" without warranty of any kind, express or implied, including accuracy, completeness or fitness for a particular purpose. For educational purposes only. The user is solely responsible for verifying results against the source guideline and for all clinical decisions.

Modality:

Contrast-enhanced renal mass protocol performed

Content:

Wall:

Septa:

Nodule: convex protrusion ≥ 4 mm with obtuse margins, or any size with acute margins Calcification (any type) Wall, septa or nodule enhance (≥ 20 HU at CT, ≥ 15 % at MRI)

Exam date: Only used to date the IIF follow-up.

Location (optional):

Missing a calculator? Suggest one · Report a problem with this one

How it works

The 2019 version replaced the vague "minimal", "thin" and "thick" of the original with measurements, and defines a nodule by size and the angle it makes with the wall. The class is set by the most worrying feature present:

Class	Defining features (CT and MRI)	Malignancy	Management
I	Thin (≤ 2 mm) smooth wall; homogeneous simple fluid (−9 to 20 HU, or CSF-like signal); no septa or calcification. Wall may enhance.	≈ 0 %	None
II	Thin smooth wall with a few (1–3) thin septa (may enhance) or calcification of any type; or a homogeneous mass that is ≥ 70 HU unenhanced, 21–30 HU portal venous, > 20 HU and non-enhancing, or too small to characterise; at MRI, homogeneous and markedly T1- or T2-hyperintense.	< 1 %	None
IIF	Smooth minimally thickened (3 mm) enhancing wall or septa; or many (≥ 4) smooth thin enhancing septa; at MRI also a mass heterogeneously hyperintense on unenhanced fat-suppressed T1.	low (≈ 5–10 %)	Imaging at 6 and 12 months, then yearly to 5 years
III	One or more enhancing thick (≥ 4 mm) or irregular (≤ 3 mm obtuse-margined convex protrusions) walls or septa.	≈ 50 %	Urology: surgery, ablation or surveillance
IV	One or more enhancing nodules: ≥ 4 mm convex protrusion with obtuse margins, or any size with acute margins.	≈ 90 %	Urology: treatment

Enhancement is ≥ 20 HU at CT or ≥ 15 % signal increase at MRI. Thickened or nodular components that do not enhance, heterogeneous non-enhancing masses, and masses seen only without contrast cannot be classified; MRI with subtraction is the usual next step.
Do not apply the system to infectious, inflammatory or vascular lesions, or to masses that are more than about a quarter solid.
Malignancy figures are the approximate rates quoted in the classification paper and later series; class IIF rates vary widely between studies.

References

Silverman SG, Pedrosa I, Ellis JH, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: An Update Proposal and Needs Assessment. Radiology. 2019;292(2):475–488. doi:10.1148/radiol.2019182646 · free full text
Schieda N, Davenport MS, Krishna S, et al. Bosniak Classification of Cystic Renal Masses, Version 2019: A Pictorial Guide to Clinical Use. RadioGraphics. 2021;41(3):814–828. doi:10.1148/rg.2021200160