Bosniak v2019 Reporting: Cystic Renal Mass CT and MRI Examples

Bosniak v2019 reporting: scope and examination adequacy

This guide uses the Bosniak Classification of Cystic Renal Masses, version 2019. The classification is designed for cystic renal masses evaluated with CT or MRI. A cystic mass contains less than approximately 25% enhancing tissue. Do not use the system for infectious, inflammatory, or vascular lesions, or for a solid renal mass with central necrosis.

Bosniak class describes imaging features and estimated malignancy risk; it is not a histologic diagnosis or a complete management algorithm. Mass size and growth rate are not Bosniak v2019 classification criteria. Report size separately and compare with prior imaging, but do not use size alone to assign a class.

Modality and adequacy checklist

Before assigning a class, check that:

• The finding is a renal mass with a cystic appearance and the differential does not favor infection, inflammation, vascular etiology, or a necrotic solid mass.
• The study is an appropriate contrast-enhanced renal CT or MRI. Record whether enhancement can be assessed and which phases or sequences were acquired.
• At CT, the unenhanced and postcontrast appearances are available as needed to determine enhancement and characterize calcification or hyperattenuating contents.
• At MRI, review T2-weighted images, precontrast and postcontrast T1-weighted images, and subtraction images when intrinsic T1 hyperintensity could obscure enhancement.
• The mass is adequately covered and motion or subtraction misregistration does not undermine assessment of thin walls, septa, or nodules.
• Prior examinations are reviewed for interval morphologic change. Growth can be clinically relevant, but it is not itself a Bosniak v2019 category feature.
• A standard grayscale ultrasound finding is not assigned a Bosniak v2019 category. Ultrasound may detect a lesion, but CT or MRI criteria are needed for this classification.

If enhancement or internal architecture cannot be evaluated reliably, state that the mass is incompletely characterized and identify the specific limitation. Do not convert “not seen” into “nonenhancing.”

Feature worksheet

Use an observation-level worksheet before selecting the class. Describe the findings that determine the highest applicable category.

A wall or septum must enhance to count as an enhancing feature. When enhancement is equivocal, review the relevant source images, subtraction images, and acquisition quality before assigning a category.

Bosniak v2019 feature summary

The following summary highlights useful reporting distinctions. Apply the original v2019 definitions and tables when classifying a mass.

If a mass has features from more than one class, assign the highest class supported by the v2019 criteria. Use “cyst” for Bosniak I and for Bosniak II masses confidently characterized as cysts. “Cystic renal mass” is more accurate for IIF, III, and IV findings.

Editable report framework

Examination: Renal mass protocol CT without and with intravenous contrast / MRI of the kidneys without and with intravenous contrast.

Indication: [Clinical question and relevant history.]

Comparison: [Date and modality; prior size and morphologic features, or none available.]

Technique and limitations: [CT phases or MRI sequences; contrast status; enhancement assessment; motion, calcification, or other limitation.]

Findings:
Kidneys: [Renal size and relevant background findings.]

Mass #1: [Side, pole/location, and size in three dimensions or largest diameter.] [Cystic composition; wall and septal number, thickness, smoothness, and enhancement; calcification; enhancing nodule or solid component.] [Describe renal sinus, collecting system, renal vein, and adjacent structures if relevant to the examination.] Bosniak Classification, version 2019: [I / II / IIF / III / IV].

[Repeat the mass block for each lesion requiring characterization.]

Other findings: [Relevant renal, adrenal, nodal, vascular, or abdominal findings within the examination scope.]

Impression:
1. [Mass location and size; Bosniak class and version; concise features supporting the class.]
2. [Additional renal finding or specific limitation.]
3. [Comparison or follow-up context only when supported by the clinical question and local pathway.]

Fictional worked reports

CT example: Bosniak IIF

Fictional teaching case. A multiphase renal mass protocol CT shows a predominantly cystic lesion in the left upper pole. The case illustrates that size is recorded but does not determine the Bosniak class.

Examination: Renal mass protocol CT of the abdomen without and with intravenous contrast.

Indication: Characterization of an indeterminate left renal lesion detected on prior imaging.

Comparison: CT dated [date], when the lesion measured 3.0 cm.

Technique: Unenhanced and postcontrast renal protocol CT with diagnostic image quality. Enhancement is assessable.

Findings:
A well-defined 3.2 cm cystic mass is present in the upper pole of the left kidney. Enhancing tissue comprises less than approximately 25% of the mass. The mass has four smooth enhancing septa, each 2 mm or thinner, and a smooth thin wall. No enhancing thick or irregular septum, enhancing nodule, or solid component is identified. There is no renal sinus or renal vein involvement. The small interval size change does not alter the morphologic features.

Bosniak Classification, version 2019: IIF.

Impression:
1. 3.2 cm left upper-pole cystic renal mass with four smooth thin enhancing septa, Bosniak v2019 class IIF.

The class is supported by four or more smooth thin enhancing septa. The lesion’s 3.2 cm size is documented separately and does not upgrade or downgrade the category.

MRI example: Bosniak IV

Fictional teaching case. A right renal mass is evaluated with a renal MRI protocol. Subtraction images confirm enhancement of a mural nodule.

Examination: MRI of the kidneys without and with intravenous contrast.

Indication: Characterization of a right renal cystic mass.

Comparison: CT dated [date].

Technique: Multiplanar MRI including T2-weighted, precontrast T1-weighted, multiphase postcontrast T1-weighted, and subtraction images. Diagnostic image quality.

Findings:
A 2.4 cm predominantly cystic mass is present in the lower pole of the right kidney. Enhancing tissue comprises less than approximately 25% of the mass. The mass contains an enhancing 5 mm convex mural protrusion with obtuse margins, confirmed on subtraction images. No renal vein involvement or collecting system invasion is identified.

Bosniak Classification, version 2019: IV.

Impression:
1. 2.4 cm right lower-pole cystic renal mass with an enhancing 5 mm mural nodule, Bosniak v2019 class IV.

Bosniak IV describes a cystic mass with an enhancing nodule under v2019 criteria. The class is not a tissue diagnosis; clinical management is determined with the treating team and patient context.

Reporting uncertainty and common pitfalls

• Applying Bosniak to the wrong lesion: The classification is for cystic renal masses. Do not use it for a solid necrotic tumor, infection, abscess, vascular lesion, or a lesion assessed only by standard ultrasound.
• Treating calcification as the category: Calcification of any morphology can occur in Bosniak II. If dense calcification obscures enhancement on CT, state that limitation and consider whether MRI is needed to assess occult enhancing structures.
• Calling a septum enhancing without a reliable comparison: Confirm enhancement on source images and subtraction images when appropriate. Misregistration can create a false appearance of enhancement.
• Confusing irregular thickening and a nodule: Under v2019, an obtuse convex protrusion up to 3 mm is irregular thickening (class III); an obtuse protrusion of at least 4 mm or any acute-margin protrusion is a nodule feature (class IV).
• Using mass size to select the class: Size and growth rate are not classification criteria. Record them separately for comparison and clinical context.
• Equating a class with pathology: Bosniak classes express imaging-based risk categories. They do not establish histology or guarantee a particular outcome.
• Pasting a fixed follow-up or treatment plan into every report: The v2019 proposal describes a general imaging follow-up pattern for IIF masses, but the classification is not a management algorithm. Recommendations should reflect local guidance, patient factors, and the clinical team’s plan.

For solid renal tumor terminology and imaging clues, see the WHO 2022 renal cell tumor classification guide. That framework addresses solid renal tumor types and is separate from Bosniak cystic-mass classification.

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. Article (opens in a new tab)
• Krishna S, Schieda N, Pedrosa I, et al. Update on MRI of cystic renal masses including Bosniak version 2019. Journal of Magnetic Resonance Imaging. 2021;54(2):341–356. Article (opens in a new tab)

Educational note: This article supports structured reporting. Apply the named version to technically adequate CT or MRI and interpret the findings in the patient’s clinical context.