Benign kidney tumours

In medical terms, a tumour just means an area of localised tissue growth. The term covers both malignant tumours (cancer) and benign growths. Benign tumours grow only locally, displacing surrounding tissue. Cancer, by contrast, destroys neighbouring tissues and can form secondary tumours (metastases) in other organs.

The most common benign kidney tumours are simple kidney cysts. These are fluid-filled sacs surrounded by a thin wall which grow in the kidney. About one person in three over the age of 60 will have one or more kidney cysts. Simple kidney cysts are harmless and only need treating if they grow very large, causing pain or other problems. They are usually easily visible on ultrasound.

In the case of solid kidney tumours, only about 10% are benign. The only way of determining this with certainty, however, is to remove the tissue and have it examined by a pathologist. To avoid unnecessary risk, in case of doubt such growths should therefore always be surgically removed.

The most common benign kidney tumour, making up around 3–7% of all kidney tumours, is an oncocytoma. Despite its misleading name, this tumour is not malignant. It is not possible to distinguish these tumours from malignant kidney cancer with certainty using imaging procedures alone. Diagnosis is therefore made after surgery by a pathologist, who examines the tumour tissue under a microscope.
Tumours occasionally turn out to be a type of growth called an ‘angiomyolipoma’ which, due to their high fat content, are relatively easy to distinguish from malignant tumours using imaging procedures.
Other benign tumours of the kidney, such as renal adenomas and mesenchymal tumours, are extremely rare.

As discussed above, it is rarely possible to differentiate with certainty between benign and malignant tumours in advance. Where there is doubt, the entire tumour should therefore always be removed. Whilst it is possible in principle to take a biopsy (which involves taking a tissue sample using a needle), this is only performed in exceptional cases, as there is a risk of misdiagnosis and, for malignant tumours, of spreading tumour cells into the puncture wound created by the needle. A biopsy should, however, be considered if the patient has pre-existing conditions which make surgery too high a risk.