Oncocytoma of the kidney

by Jason Wasserman MD PhD FRCPC and Trevor Flood MD
June 14, 2025

An oncocytoma is a non-cancerous (benign) tumour found in the kidney. The tumour is composed of large, pink cells known as oncocytes. Oncocytomas usually grow slowly and rarely cause serious health problems.

What are the symptoms of oncocytoma?

Most oncocytomas in the kidney do not cause noticeable symptoms. They are typically discovered accidentally during imaging tests, such as a CT scan, MRI, or ultrasound, performed for other reasons. However, if an oncocytoma becomes large enough, it can cause symptoms, including:

• Pain in the back or side of the abdomen.

• Blood in the urine, causing the urine to appear pink, red, or brown.

• A lump or mass felt in the abdomen (rare).

Many of these symptoms can also be associated with other conditions, so additional tests are typically required to confirm the diagnosis.

What causes an oncocytoma in the kidney?

In most cases, the exact cause of an oncocytoma is not known. Usually, oncocytomas develop randomly without any specific reason. However, some people have a genetic (inherited) condition called Birt-Hogg-Dubé syndrome, which increases their risk of developing kidney tumours, including oncocytomas.

What if multiple oncocytomas are found in the same kidney?

Sometimes, more than one oncocytoma can be found in the same kidney. When multiple oncocytomas develop, the condition is referred to as oncocytosis. Having multiple tumours does not mean that the tumours are cancerous; oncocytosis is still considered a benign (non-cancerous) condition. However, multiple tumours may require careful monitoring.

How is oncocytoma diagnosed?

Oncocytomas are often discovered by accident during imaging tests done for other reasons. If a kidney mass is detected, your doctor might recommend removing the entire tumour through surgery. After the tumour is removed, it is examined closely by a pathologist under a microscope. The pathologist’s examination provides a definitive diagnosis, confirming whether the tumour is an oncocytoma or another type of tumour.

What does an oncocytoma look like under the microscope?

Under the microscope, an oncocytoma is made up of large, round cells called oncocytes. These cells have a distinctive pink appearance. Pathologists recognize this unique appearance, helping them confirm the diagnosis of oncocytoma.

What is a surgical margin and why is it important?

A surgical margin is the healthy tissue surrounding the tumour that a surgeon removes along with the tumour during surgery. After removal, pathologists examine these margins closely under a microscope to see if any tumour cells remain at the edges.

• Negative margin: No tumour cells are found at the edges of the tissue removed. This means that the entire tumour was successfully removed, reducing the risk that the tumour will grow back.

• Positive margin: Tumour cells are found at the edges of the tissue removed. A positive margin means some tumour cells may remain in your body, potentially requiring further treatment or surgery.

The status of surgical margins helps your doctor decide whether any additional treatment is needed. However, since oncocytomas are non-cancerous tumours, having a positive margin usually does not carry the same significance or urgency as it would with a cancerous tumour. Your doctor will discuss the meaning of your margins with you clearly and advise you on appropriate next steps or monitoring.

Treatment and prognosis of oncocytoma

Oncocytomas are benign tumours. This means they do not spread to other parts of the body (metastasize), and they are generally not life-threatening. Treatment usually involves surgery to remove the tumour, which is typically curative. The outlook for people with an oncocytoma is excellent.

In some cases, doctors may recommend regular follow-up visits and imaging tests to monitor the kidneys, especially if multiple tumours are present or if you have an inherited condition like Birt-Hogg-Dubé syndrome.

Questions to ask your doctor

• Do I need additional tests or procedures after my tumour is removed?

• What are the chances the oncocytoma could grow back?

• Should I have follow-up imaging or regular check-ups to monitor my condition?

• Could this tumour be related to an inherited genetic condition?

• Are my family members at risk, and should they be screened?

• What symptoms or signs should prompt me to seek medical attention?