Bosniak calculator
Answer a few questions about a cystic renal mass on CT or MRI to get its class under version 2019, then copy the summary.
A radiologist has not yet reviewed this calculator. It is for education and drafting, not for clinical decisions. It follows the Bosniak classification, version 2019. Check every result against the guideline.
Answer each question to see the class. If a mass has features of more than one class, the highest class applies, so the questions start at the top.
Follows the Bosniak classification, version 2019 (Silverman et al, Radiology 2019). The calculator runs in your browser, and your answers are never sent anywhere. Version 2019 is a proposed update, and it predicts malignancy rather than setting management. It is for cystic masses once infectious, inflammatory and vascular causes and necrotic solid masses are excluded, and its reporting wording is not for patients with a renal cell carcinoma syndrome.
The classes
- Class I
- Simple cyst: thin smooth wall, homogeneous simple fluid, no septa or calcifications.
- Class II
- Few thin septa, or one of several homogeneous masses. Benign or likely benign.
- Class IIF
- Smooth 3 mm enhancing wall or septa, or 4 or more thin enhancing septa. At MRI, also heterogeneous hyperintensity on unenhanced fat-saturated T1.
- Class III
- Enhancing walls or septa that are 4 mm or thicker, or irregular.
- Class IV
- One or more enhancing nodules.
Under review
A radiologist has not yet checked these points. Until then the calculator behaves as described here.
- Portal venous phase CT only. A well-defined, thin-walled, homogeneous mass of -9 to 20 HU with no septa or calcifications is given Class I. Table 2 of version 2019 defines Class I that way and names no phase. Its Class II type for this phase starts at 21 HU.
- Features that can hide enhancement at CT. Abundant thick or nodular calcifications, and heterogeneous nonenhancing masses, are sent to MRI before any class, as the Table 2 footnote suggests. The calculator asks about enhancing nodules (Class IV) and thick or irregular enhancing walls or septa (Class III) first, because MRI could only raise those. A Class III mass with one of these features keeps Class III, with the MRI advice added. A mass with a Class IIF feature and one of these features is still sent to MRI with no class.
- Calcification without septa. A thin-walled mass with calcification and no septa is given Class II. Table 2 names calcification of any type in the septated type and the over 20 HU type only.
Common questions
- What changed in Bosniak version 2019?
- It defines its terms with numbers: thin is 2 mm or less, minimally thickened is 3 mm, thick is 4 mm or more, few is 1 to 3 septa and many is 4 or more. It adds MRI criteria, adds new homogeneous masses to Class II, and treats calcification of any type as a Class II feature.
- What is the difference between a nodule and irregular thickening?
- A nodule is an enhancing convex protrusion of any size with acute margins, or 4 mm or more with obtuse margins, and it makes the mass Class IV. Enhancing irregular thickening is 3 mm or less with obtuse margins, and it makes the mass Class III. Measure at right angles to the wall or septum. If the protrusion is on both sides, add the two distances and leave out the thickness of the wall or septum itself.
- What if a mass has features of more than one class?
- The highest class applies. A cystic mass with few thick enhancing septa is Class III, not Class II. The calculator asks about the higher classes first.
- Why does the calculator say no class yet?
- Some masses need a better study before a class is assigned. Abundant thick or nodular calcifications, and heterogeneous nonenhancing masses at CT, can hide enhancing elements, so MRI is suggested first. A single-phase study can place certain homogeneous, well-defined masses in Class II, and a portal venous phase CT can show a Class I simple cyst. Class IIF, III and IV need a renal mass protocol study.
- When does the Bosniak classification not apply?
- It is for cystic masses, meaning less than about 25 percent is enhancing tissue. Exclude infectious, inflammatory and vascular causes and necrotic solid masses first. The reporting wording is for the general population, not patients with a renal cell carcinoma syndrome.
- Are the 2 mm and 3 mm limits exact?
- No. The authors call the numbers guideposts rather than absolute values, and accept that 1 mm differences may not be measurable in practice. They also say radiologists may use clinical judgment to deviate from the classification.
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