First and second year — the floor first, then every step
Only a renal-mass protocol CT or MRI can apply Bosniak 2019: count and measure septa and walls, look for enhancement in HU (CT) or visually/subtraction (MRI), and find any enhancing nodule — its shape decides class IV.
Orient first
Bosniak 2019 is defined on CT and MRI, not ultrasound; ultrasound can call a simple cyst (and CEUS is used in some centres), but a complex cyst needs a protocol study.
MRI and ultrasound have no equivalent of the CT rule: on CT, enhancement is a rise of 20 HU or more between unenhanced and enhanced phases; 10–19 HU is equivocal (pseudo-enhancement, especially in small lesions).
The 2019 revision added quantitative wall/septum thickness (≤ 2, 3, ≥ 4 mm) and defined a nodule by its shape (obtuse ≥ 4 mm or acute any size) — verify against the version in use.
Acquire the study
Unenhanced and nephrographic phase (about 90–100 s) thin-slice CT.
The manoeuvre
Unenhanced attenuation of the fluid (homogeneous −9 to 20 HU = simple fluid; ≥ 70 HU homogeneous = hyperdense cyst class II).
Enhancement: ROI in the same place on both phases; ≥ 20 HU rise = enhancing.
Count septa and measure wall and septal thickness in mm.
Nodules: a focal protrusion of the wall/septum — obtuse margins ≥ 4 mm or acute margins any size = class IV.
What confirms it
A Bosniak 2019 class based on a protocol CT or MRI.
What licenses you to exclude it
A simple cyst by strict criteria needs no follow-up.
The classic misread
Calling pseudo-enhancement in a small cyst next to enhancing parenchyma real.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Normal limits
Renal cyst (simple) · Simple-cyst features (Bosniak I)
anechoic, imperceptible wall, posterior enhancement on US; water attenuation and no enhancement on CT/MRI — Bosniak I, leave alone
A "simple cyst" verdict requires ALL the simple features. Thin-section, an adequate ROI, and no enhancement — a 1 cm high-attenuation cyst is not simple (it is a Bosniak IIF/II conversation). Versioned criterion — verify against the current edition before clinical use.
USG · CT · MRI
Diagnostic criteria
Renal cyst (Bosniak 2019 classes) · Bosniak 2019 classes I–IV — what each class is
I = simple (hairline wall, no septa, no enhancement). II = few hairline septa or fine calcification, or a homogeneously hyperdense cyst ≤3 cm, no measurable enhancement. IIF = more septa, perceived (not measurable) enhancement, thick/nodular Ca, or a hyperdense cyst >3 cm — follow-up class. III = thickened irregular or smooth walls/septa with measurable enhancement — surgical conversation. IV = enhancing nodular soft tissue. Read septal-thickness and protrusion rules from the current 2019 table; do not invent a millimetre from the old edition. The already-registered uro-bosniak entry is the parent how-to-report row
Version 2019 redefined several boundaries and formally extended the system to MRI. Name the version. Versioned criterion — verify against the current edition before clinical use.
CT · MRI
Renal cyst · Bosniak I–IV (2019 class map)
I = simple (hairline wall, no septa, no enhancement). II = few hairline septa or fine Ca, or a homogeneously hyperdense cyst ≤3 cm, no measurable enhancement. IIF = more septa, perceived (not measurable) enhancement, thick/nodular Ca, or a hyperdense cyst >3 cm — follow-up class. III = thickened walls/septa with measurable enhancement. IV = enhancing nodular soft tissue. Same map as the already-registered exp12-uro-bosniak-2019 row — figures must not drift
Version 2019 redefined several boundaries against the original classification and formally extended it to MRI — name the version in the report, and read the septal-thickness definitions from the current table rather than from memory. Versioned criterion — verify against the current edition before clinical use.
CT · MRI
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.