Kidney cyst found by chance: Bosniak follow-up

The main guidance

Radiologists use the Bosniak classification to sort cystic kidney masses by their chance of being cancer. Its 2019 update added MRI, set exact measurements for terms such as "thin" and "few", and moved more masses into the lower-risk classes. (Bosniak 2019) The European Association of Urology (EAU) renal cell carcinoma guideline, 2026 edition, uses the 2019 classes and adds what each class usually means for follow-up. (EAU 2026)

Who it applies to

Bosniak is for cystic masses once infection, inflammation, blood vessel causes and dead tissue inside a solid tumor have been ruled out. (Bosniak 2019) It is not used for people with a hereditary cystic kidney disease or a kidney cancer syndrome such as von Hippel-Lindau disease; follow-up then comes from the condition's own guidance. (Edney 2021) For many cysts in both kidneys, see screenings for people with polycystic kidney disease.

The Bosniak 2019 classes

Bosniak 2019 classes and the usual follow-up
Class	What the scan shows (simplified)	Usual follow-up
I	Simple cyst: thin smooth wall, even fluid (-9 to 20 HU on CT, like spinal fluid on MRI), no septa or calcium	None; benign
II	Few (1 to 3) thin septa, calcium of any type, or an even mass that fits set benign density rules, including some too small to measure	None; benign
IIF	Many (4 or more) thin enhancing septa, or a wall or septa 3 mm thick, or a mixed bright mass on a T1 MRI	Repeat imaging, up to five years (EAU)
III	One or more enhancing walls or septa 4 mm or thicker, or irregular	Beyond screening; EAU says over 50% are malignant
IV	One or more enhancing nodules	Beyond screening; EAU says most are malignant

Sources for the table: the Bosniak 2019 update (Bosniak 2019) and EAU Table 5.1. (EAU 2026) If a mass has features of more than one class, the highest class is used. (Bosniak 2019)

Which scan

CT without contrast: a smooth, even mass of -9 to 20 HU, or 70 HU or more, is highly likely to be a benign cyst. Bosniak 2019 puts the first group in class II with no more imaging needed. (Bosniak 2019)(Edney 2021)
Ultrasound: Bosniak 2019 says ultrasound can confirm a class I simple cyst, but its role in giving other classes is uncertain. (Bosniak 2019)
MRI: the EAU guideline says MRI may be preferable for complex cysts (IIF to III) because it picks up enhancement better than CT. (EAU 2026) Bosniak 2019 also suggests MRI for masses with heavy calcium or a mixed look on CT, before a class is given. (Bosniak 2019)

Repeat checks and when

Bosniak I and II: no follow-up. (Bosniak 2019)
Bosniak IIF: repeat imaging for up to five years. (EAU 2026) About 10% change on later scans, and those that change have about an 85% chance of being malignant; a change in shape or the growth of solid parts matters more than overall size. (Bosniak 2019)
Bosniak III and IV: these go beyond screening; the EAU guideline says over 50% of class III and most class IV masses are malignant. (EAU 2026)

When a kidney lump is solid

Bosniak 2019 counts a mass as cystic when less than about 25% of it is enhancing tissue; above that, it is a solid mass and Bosniak does not apply. (Bosniak 2019) For a solid mass, the EAU guideline says enhancement is the key sign separating cancer from benign masses, measured on CT before and after contrast, with a rise of 15 HU or more counting as enhancement. A CT in several phases, or MRI or contrast ultrasound when CT is unclear, is the next step. (EAU 2026)

Routine age-based screening

A kidney cyst found by chance does not replace the routine screenings for your age, and it is not a test of kidney function. For kidney function, see kidney test results: eGFR and uACR. For other findings on a scan, see lung nodule found by chance and adrenal mass found by chance.

Where they differ:

How often each class is cancer. The 2019 update, drawing on older studies, gave a range of 0% to 38% for IIF, about half for III and about 90% for IV. (Bosniak 2019) The EAU guideline cites a later meta-analysis of masses classed with Bosniak 2019 that found 9% for II, 26% for IIF, 80% for III and 88% for IV, and says the IIF and III risk may be higher than under the old system. (EAU 2026)
Status of the 2019 update. The EAU guideline says the 2019 version still needs more validation and asks that the scan report name which version was used. (EAU 2026)
Bosniak III. Bosniak 2019 says it is unclear whether class III masses are best watched or treated in most people. (Bosniak 2019) The EAU table lists either treatment or active surveillance. (EAU 2026)

Your doctor decides what your result means for you.

Questions people ask

Is a kidney cyst found by chance usually serious?

Usually not. MedlinePlus says simple kidney cysts become more common with age and are usually harmless. Bosniak 2019 counts class I and II cysts as benign. (MedlinePlus) (Bosniak 2019)

Which kidney cysts need no follow-up?

Bosniak I simple cysts and Bosniak II cysts. Bosniak 2019 says these are generally ignored. A smooth, even mass of -9 to 20 HU on a CT without contrast counts as Bosniak II and needs no more imaging. (Bosniak 2019) (Edney 2021)

What does Bosniak IIF mean?

The F stands for follow-up. These cystic masses are probably benign and are watched with repeat scans. The EAU guideline lists follow-up for up to five years, and notes that some turn out to be malignant. (Bosniak 2019) (EAU 2026)

Why do Bosniak IIF masses need repeat scans?

Bosniak 2019 says about 10% of IIF masses change on later scans, and those that change have about an 85% chance of being cancer. A change in shape or new solid parts matters more than a change in size. (Bosniak 2019)

Does Bosniak apply if I have a genetic kidney condition?

No. Bosniak is not used for people with a hereditary cystic kidney disease or a kidney cancer syndrome; those follow condition-specific guidance. (Edney 2021)