Being told that a scan shows a mass on your kidney can be worrying, especially when you had no symptoms. Many renal masses are found by chance on scans done for another reason, and not all of them are cancer. This guide explains, based on European (EAU), American (AUA) and UK (NICE, NHS) sources, what the finding may mean, which tests follow, and the options doctors discuss.
Medically reviewed by Prof. Dr. Samir Ahmed Al-Samarrai, consultant urologist. Last reviewed: · Last updated:
What does a mass on the kidney mean?
A renal mass is any lump or area in the kidney that differs from normal tissue. It is not a diagnosis: the key questions are whether it is fluid-filled (cystic) or solid, and whether it has features that suggest cancer.
Many masses are found incidentally. According to the EAU guidelines on renal cell carcinoma, about 60% of patients overall were diagnosed incidentally, and this rose to 87% for small, early-stage tumours. Masses can be benign: the AUA renal mass guideline states that benign masses make up approximately 15-20% of surgically removed tumours smaller than 4 cm. If cancer is confirmed, see our kidney cancer guide.
Solid or cystic: the Bosniak classification
Cystic masses are graded with the Bosniak classification, based on the wall, internal septa, calcification and enhancing solid parts. Simple cysts are common and usually harmless; see kidney cysts. The EAU guidelines describe the 2019 version as follows:
| Bosniak category | What it looks like | Usual approach (as EAU describes) |
|---|---|---|
| I | Simple cyst with a thin, smooth wall, no septa or calcification | Benign |
| II | A few thin septa, with or without calcification | Benign |
| IIF | Slightly thickened enhancing wall or septa, or several thin enhancing septa | Follow-up imaging for up to five years; some turn out to be malignant |
| III | Thick or irregular walls or septa that enhance with contrast | Surgery or active surveillance |
| IV | One or more enhancing solid nodules | Surgery; most are malignant |
According to a meta-analysis cited by the EAU, the rate of malignancy was 26% for category IIF, 80% for category III and 88% for category IV.
Benign possibilities: angiomyolipoma and oncocytoma
Not every solid mass is cancer. Two common benign tumours are:
- Angiomyolipoma (AML): made of fat, blood vessels and smooth muscle. The Canadian Cancer Society notes that it often occurs in people with tuberous sclerosis, and that tumours larger than 4 cm can cause sudden bleeding into the abdomen. Options include surveillance, arterial embolisation (blocking its blood supply) or surgery.
- Oncocytoma: a benign tumour that can grow quite large and may occur in one or both kidneys. It can sometimes be found alongside a cancerous tumour.
The EAU notes that CT or MRI cannot reliably tell an oncocytoma or a fat-free angiomyolipoma apart from a cancer, which is one reason a biopsy is sometimes advised.
How a kidney mass is diagnosed
Assessment includes your history, an examination, kidney function blood tests and a urine test. Imaging is the key step:
- Ultrasound: often where the mass is first seen; it helps tell a cyst from a solid lump. See urology ultrasound.
- Multiphase contrast CT: the EAU strongly recommends contrast-enhanced CT of the abdomen and chest to characterise and stage a renal mass.
- MRI or contrast-enhanced ultrasound: the EAU recommends these when CT is unclear.
- Renal mass biopsy: a thin needle takes a small tissue sample under imaging guidance. The EAU advises it for masses that remain unclear on imaging, for selected people considering surveillance, and before ablation. It is generally not used for cysts without a significant solid part.
Treatment and follow-up options
The choice depends on the mass, your age, general health and kidney function. The EAU guidelines describe:
- Active surveillance: regular scans for small, early masses that do not need immediate treatment, often in older or frailer patients. In one study quoted by the EAU, the median growth was 0.09 cm per year.
- Watchful waiting: for very frail people with limited life expectancy.
- Thermal ablation: destroying the tumour with heat (radiofrequency) or freezing (cryoablation), usually through a needle in the skin. The EAU suggests it mainly for people who need treatment but are not fit for surgery, with a biopsy first. NICE guidance says percutaneous cryotherapy should only be offered after assessment by a specialist urological cancer multidisciplinary team.
- Partial nephrectomy: removing only the tumour and a margin of healthy tissue. The EAU recommends offering it for small (T1) tumours because cancer control is comparable to removing the whole kidney while more kidney function is preserved.
- Radical nephrectomy: removing the whole kidney, considered when partial removal is not feasible. See our nephrectomy guide.
How the decision is made
Your urologist weighs the scan findings, any biopsy result, other health conditions and how well both kidneys work. The EAU notes that evidence on ablation is mostly retrospective, so its limits should be discussed. If your kidney function is reduced, a kidney specialist (nephrologist) may share your follow-up. Ask what each option involves and what follow-up is planned.
When to seek urgent care or see a doctor
Most kidney masses cause no symptoms. The NHS advises an urgent appointment if you notice:
- Blood in your urine, or urine that is cloudy or smells
- Persistent pain in your back or under the ribs
- A lump or swelling in the back, under the ribs or in the neck
- Pain when passing urine, or a sudden need to pass urine more often
Go to an emergency department straight away if you have a known kidney mass (especially an angiomyolipoma) and develop sudden severe pain in your side or abdomen, or feel faint or dizzy, as these may be signs of bleeding. Also see a doctor for unexplained weight loss, persistent fever or extreme tiredness.
Frequently asked questions
Does a mass on the kidney always mean cancer?
No. A renal mass may be a cyst, a benign tumour such as an angiomyolipoma or oncocytoma, or a cancer. According to the AUA guideline, benign masses make up approximately 15-20% of surgically removed tumours smaller than 4 cm. Further imaging and sometimes a biopsy help clarify the cause.
What is the difference between a cyst and a solid mass?
A cyst is filled with fluid, while a solid mass is made of tissue. Simple cysts are very common and usually harmless. Complex cysts are graded with the Bosniak classification, and solid masses that take up contrast usually need further assessment by a urologist.
Will I need a biopsy?
Not always. The EAU guidelines advise a biopsy when imaging cannot clarify the mass, for selected people considering active surveillance and before ablation. A biopsy is usually not needed if surgery is planned regardless of the result, or for cysts without a solid component.
Can a small kidney mass just be watched?
For some people, yes. The EAU guidelines describe active surveillance with regular scans for small, early masses in people who do not need immediate treatment, particularly older or frailer patients. Treatment is considered if the mass grows or changes. Your doctor decides whether this suits you.
Will I lose my whole kidney if I need surgery?
Not necessarily. For small tumours, the EAU guidelines recommend partial nephrectomy, which removes only the tumour and keeps the rest of the kidney working. Removing the whole kidney is considered when a partial operation is not feasible.
What symptoms should make me seek help quickly?
According to the NHS, blood in the urine, persistent pain in the back or under the ribs, or a new lump need an urgent appointment. Sudden severe side or abdominal pain with faintness, especially with a known angiomyolipoma, needs emergency care.
Sources
- EAU Guidelines on Renal Cell Carcinoma: Diagnostic Evaluation
- EAU Guidelines on Renal Cell Carcinoma: Disease Management
- Renal Mass and Localized Renal Cancer: Evaluation, Management, and Follow-up (AUA Guideline)
- Percutaneous cryotherapy for renal cancer (NICE HTG269)
- Symptoms of kidney cancer – NHS
- Non-cancerous tumours and conditions of the kidney – Canadian Cancer Society
Book a consultation
Prof. Samir Al-Samarrai and Dr. Amer Mousa assess urological conditions at Prof. Al-Samarrai Medical Center, Dubai Healthcare City, and discuss the options that suit your case. Medical consultations are confidential.
Read more: Urologist in Dubai: doctors, services, insurance and booking
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