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Something seen on your kidney? Here's what it means.
Kidney cysts and small kidney masses are often found by chance on an ultrasound. Most simple cysts are harmless. Here is how they are assessed and when anything needs to be done.
● Simple cysts are common● CT or MRI for clarity● Not every mass needs surgery
Dr. Nameirakpam ShantajitMBBS (SPMC Bikaner) · MS Surgery & M.Ch Urology (RIMS Imphal) · MMC Reg. No. MNP 150
Often found by chance
During a scan done for another reason.
Simple cyst — very common, usually harmless, no treatment
Complex cyst — graded on CT/MRI; some need follow-up
Solid mass — needs further assessment
Many cysts in both kidneys — may be polycystic kidney disease
Simple cystsVery common with age
Bosniak gradeGuides what to do with a cyst
Contrast CT/MRIKey test for any mass
Many optionsWatch, ablate or operate
Kidney Cysts
Cysts: mostly harmless
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Simple cysts
Thin-walled, fluid-filled — very common, especially after 50. They almost never become cancer and usually need no treatment or follow-up.
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Complex cysts
Cysts with walls, partitions or calcium are graded (Bosniak system) on CT or MRI. Some need periodic scans; a few need removal. See the grades ↓
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Polycystic kidney disease
Many cysts in both kidneys, often running in families — managed with a kidney specialist, with blood pressure and kidney function checks.
Bosniak classification
Grading a kidney cyst: Bosniak I to IV
When a cyst is not simple, it is graded on a contrast CT or MRI using the Bosniak system. The grade tells us how likely the cyst is to contain cancer — and what to do next. Ultrasound alone cannot grade a cyst fully.
I · II IgnoreIIF FollowIII Discuss — a coin tossIV Treat
Easy to remember: the higher the number, the more "stuff" inside the cyst — and the higher the chance of cancer.
Bosniak IIgnore
Simple cyst
Thin smooth wall, clear fluid, nothing inside.
Chance of cancer~0%
What to do: No follow-up needed.
Bosniak IIIgnore
Minimally complex
A few hair-thin partitions or a fleck of calcium. Includes small "dense" cysts.
Chance of cancer~0%
What to do: Usually no follow-up needed.
Bosniak IIFFollow
“F” = Follow-up
More thin partitions, or a slightly thickened smooth wall. Nothing thick or nodular.
Chance of cancer5–10%
What to do: Repeat scan at 6 and 12 months, then yearly for up to 5 years. Most never change.
Bosniak IIIDiscuss
Indeterminate
Thick or irregular wall or partitions that light up with contrast.
Chance of cancer~50%
What to do: A coin-toss risk. Usually surgery (often saving the kidney); careful scan follow-up may suit some people — decided together.
Bosniak IVTreat
Likely cancer
A solid lump (nodule) inside the cyst that lights up with contrast.
Chance of cancer~90%
What to do: Treated like a kidney cancer — usually partial or radical nephrectomy.
Fluid with a thin wall Calcium Parts that light up with contrast
Good to know
Most kidney cysts found on scans are Bosniak I or II — harmless.
Cancers that arise in cysts are often slow-growing and have a good outlook when treated.
A cyst can be "upgraded" if it changes on follow-up — which is why IIF cysts are watched.
About the figuresPercentages are approximate, from published studies of the Bosniak system (including the 2019 update), and vary between studies. Your own risk depends on your scan, age and health — these figures are for general information, not a diagnosis.
Kidney Masses
Solid masses
A solid lump in the kidney needs careful assessment, as some are kidney cancers — but many small masses grow slowly, and some are benign.
Contrast CT or MRI
Shows whether a mass is solid, its size and its relation to blood vessels.
Blood & urine tests
Kidney function and urine tests.
Discuss options
Based on size, growth, age and health.
Plan
Surveillance, ablation, or surgery — partial (keeping the kidney) or complete removal.
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Not every mass needs immediate surgery
Very small masses, especially in older patients or those with other health problems, may be safely watched with regular scans. Larger or growing masses are usually treated. Options are explained in detail, and specialist cancer care is arranged where needed.
Persistent pain in the side, not explained by a stone.
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A lump
A swelling felt in the tummy or side.
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Weight loss & fever
Unexplained weight loss, tiredness or fevers.
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High blood pressure
Especially with polycystic kidneys.
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Found on a scan
Most cysts and masses cause no symptoms and are found by chance.
Questions
Frequently asked questions
I have a simple kidney cyst. Is it cancer?
Almost never. Simple cysts are very common, especially after 50, and usually need no treatment or follow-up.
What is the Bosniak grade?
A way of grading kidney cysts on contrast CT or MRI. Bosniak I and II are harmless (cancer risk about 0%) and need no follow-up. IIF has a small risk (about 5–10%) and is watched with repeat scans. Bosniak III has about a 50% chance of cancer, and IV about 90% — these are usually treated with surgery, often saving part of the kidney.
Can a harmless cyst turn into cancer?
Simple (Bosniak I) cysts almost never do. A complex cyst can change over time, which is why Bosniak IIF cysts are re-scanned for a few years. If it develops thicker walls or a solid part, it is re-graded and treated if needed.
Does every kidney mass need surgery?
No. Very small masses may be watched with regular scans, especially in older patients. Larger or growing masses are usually treated.
Can part of the kidney be saved?
Often yes — for many smaller tumours, only the tumour is removed (partial nephrectomy) and the rest of the kidney is kept.
Why do I need a contrast CT if I already had an ultrasound?
Ultrasound finds cysts and masses well, but a contrast CT or MRI shows their exact nature and is needed to decide what to do.
I have many cysts in both kidneys. What does that mean?
It may be polycystic kidney disease, which often runs in families. It needs regular checks of blood pressure and kidney function, usually with a kidney specialist.
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