Oncocytoma: Understanding Your Pathology Report

Section Editor: Trevor Flood MD FRCPC
July 19, 2026

Oncocytoma is a benign (noncancerous) tumor of the kidney. It is built from large cells with abundant pink cytoplasm called oncocytes, which are packed with mitochondria, the parts of a cell that make energy. Oncocytoma is one of the most common noncancerous kidney tumors. It grows slowly, does not spread to other organs, and rarely causes serious problems.

The most important thing to understand about oncocytoma is not about danger, because it is benign. It is about certainty. Under the microscope, oncocytoma closely resembles a kidney cancer called chromophobe renal cell carcinoma, along with a small group of related tumors, and telling them apart is the central task of your pathology report. Much of what follows explains how that distinction is made and why it can be made with confidence only after the entire tumor has been examined.

This article will help you understand the findings in your pathology report for oncocytoma of the kidney, what each term means, and why it matters for your care.

What causes oncocytoma?

In most cases the cause of an oncocytoma, a common noncancerous kidney tumor, is not known. These tumors usually develop on their own, with no specific event or exposure to explain them, and they are found most often in older adults and somewhat more often in men. When researchers examine the genes of these tumors, they commonly find loss of an entire chromosome 1 or changes involving a chromosome region called 11q13, but these findings are not something a patient needs to act on. Oncocytes, the cells that make up the tumor, are thought to come from a specific type of cell lining the kidney’s collecting tubules.

Inherited conditions associated with oncocytoma

Most oncocytomas are not inherited. The main exception is Birt-Hogg-Dubé syndrome, an inherited condition caused by a change in a gene called FLCN that a person is born with. People with Birt-Hogg-Dubé syndrome tend to develop several kidney tumors, often in both kidneys and at a younger age than usual, and the tumors are typically oncocytomas, chromophobe renal cell carcinomas, or tumors with features of both (described below). Two findings outside the kidney point toward the syndrome: small bumps on the skin of the face and neck, and cysts in the lungs. Because of this link, a person who has more than one oncocytoma, oncocytomas in both kidneys, or an oncocytoma at a young age may be asked about these features and, in some cases, referred for genetic assessment.

What are the symptoms of oncocytoma?

Most oncocytomas of the kidney cause no symptoms and are found by chance during imaging done for another reason, such as a CT scan, MRI, or ultrasound. When an oncocytoma grows large enough, it may cause:

Pain in the back or the side of the abdomen.
Blood in the urine, which can make the urine look pink, red, or brown.
A lump that can be felt in the abdomen (uncommon).

Many of these symptoms are caused far more often by conditions other than a tumor, which is why tests are needed to make the diagnosis.

What if more than one oncocytoma is found?

Sometimes more than one oncocytoma is found in the same kidney, or in both kidneys. When many oncocytomas develop throughout the kidneys, the condition is called oncocytosis. Having several tumors does not mean they are cancerous; oncocytosis is still benign. But multiple oncocytomas, like oncocytomas at a young age, raise the possibility of Birt-Hogg-Dubé syndrome, so they are a reason to consider genetic assessment and to monitor the kidneys over time.

How is the diagnosis made?

The diagnosis of oncocytoma is made when a pathologist examines the tumor under a microscope, usually after the whole tumor has been removed by surgery. The tumor is first seen as a mass in the kidney on imaging, but imaging cannot reliably tell an oncocytoma apart from a kidney cancer, because several tumors look alike on a scan. This is one reason many oncocytomas are confirmed benign only after removal.

A point that surprises many people is that oncocytoma usually cannot be diagnosed with confidence from a needle biopsy. The features that separate an oncocytoma from the cancer it resembles are spread unevenly across the tumor, and a small biopsy sample may not capture them, or may show an area that overlaps with chromophobe renal cell carcinoma. A biopsy can suggest an oncocytic tumor, but the final distinction is usually made only after the pathologist examines the entire tumor. For this reason, a kidney mass that turns out to be an oncocytoma is often removed before the benign diagnosis is certain.

Under the microscope, an oncocytoma is composed of large, round cells with abundant, deeply pink (oncocytic) cytoplasm, arranged in small nests and islands within a pale, loose, sometimes scar-like stroma. The nuclei are round, regular, and evenly spaced, without the wrinkled, raisin-like shape and clear halos that mark chromophobe renal cell carcinoma. A central star-shaped scar is sometimes seen in larger tumors, both on imaging and in the removed specimen. Because the appearance overlaps with other tumors, a test called immunohistochemistry, which uses special stains to detect proteins inside cells, is often used. Oncocytoma is typically positive for PAX8 (confirming it started in the kidney) and CD117, and, crucially, shows CK7 in only rare scattered cells. This last point is the key to the diagnosis: chromophobe renal cell carcinoma shows CK7 in most of its cells, so the pattern of CK7 staining is one of the main features used to tell the two apart. When the distinction is still difficult, a test that looks at the tumor’s chromosomes, using a method called fluorescence in situ hybridization (FISH), may be added.

Tumors that look similar

Understanding what an oncocytoma is often means understanding what it is not. Several tumors composed of pink, oncocytic cells lie close to the oncocytoma, and your report may mention one of them.

Chromophobe renal cell carcinoma — The most important tumor to separate from oncocytoma. It is a cancer, though usually one with a favorable outlook, distinguished by wrinkled nuclei, clear halos around the nuclei, and CK7 staining in most cells.
Hybrid oncocytic tumor — A tumor with features of both oncocytoma and chromophobe renal cell carcinoma. These occur especially in people with Birt-Hogg-Dubé syndrome and in oncocytosis, and they behave in a benign way.
Low-grade oncocytic tumor and eosinophilic vacuolated tumor — Two recently recognized tumors made of oncocytic cells. Both are distinguished from oncocytoma by their own patterns of CK7 and CD117 staining, and both behave in an indolent, benign-like way. Your report may name one of these if the tumor did not fit oncocytoma exactly.

The reason this matters to you is reassuring: the tumors most often mistaken for oncocytoma are themselves either benign or slow-growing, with a favorable outlook, so even when the exact label requires extra testing, the overall picture is usually good.

Surgical margins

A surgical margin is the edge of the tissue removed during surgery for an oncocytoma. Your pathologist examines these edges under the microscope to see whether any tumor cells reach them.

Negative margin — No tumor cells are seen at the cut edge, which suggests the whole tumor was removed.
Positive margin — Tumor cells are present at the cut edge, meaning some tumor may remain.

Because oncocytoma is benign, a positive margin does not carry the urgency it would for a cancer. It does not put other organs at risk. It may slightly raise the chance that the tumor could regrow at that site, so your treatment team may recommend imaging follow-up, but it rarely calls for more surgery on its own.

Findings that can look concerning but do not change the diagnosis

Occasionally an oncocytoma shows a feature that sounds alarming in a report: extension of tumor into the fat around the kidney, tumor cells within a small blood vessel, or scattered cells with larger, irregular nuclei. In an oncocytoma, none of these changes the benign diagnosis or the excellent outlook. Extension into surrounding fat and involvement of small vessels have both been described in oncocytomas that subsequently behaved in a completely benign manner. Scattered enlarged nuclei are a recognized feature of oncocytoma related to age-related change in the cells, not a sign of cancer. If your report mentions one of these findings alongside a diagnosis of oncocytoma, it means the pathologist saw the feature, considered it, and still concluded the tumor is benign.

Is oncocytoma staged?

Oncocytoma is not given a stage. Staging systems such as the TNM system are used for cancers to describe how far they have grown and spread. Because oncocytoma is benign and does not spread, there is no stage to assign. Its outlook is excellent regardless of size.

What is the prognosis?

Prognosis means the expected course of a disease. For oncocytoma, the outlook is excellent. It is benign, it does not spread to other parts of the body, and surgery to remove it is curative. Once the diagnosis of oncocytoma is confirmed on the removed tumor, no further cancer treatment is needed. The main reasons for any ongoing follow-up are not about this tumor spreading, but about watching for new tumors in people with more than one oncocytoma or with Birt-Hogg-Dubé syndrome, and about kidney function after surgery.

What happens after the diagnosis?

Once oncocytoma has been confirmed, the findings on your pathology report, above all the confirmation that the tumor is a benign oncocytoma rather than a cancer, shape the next steps. Care usually involves a urologist and a radiologist.

Surgery — Most oncocytomas are diagnosed after the tumor has been removed, because the benign diagnosis usually cannot be confirmed beforehand. Removal, often by a partial nephrectomy that preserves the rest of the kidney, is curative. In some situations, particularly a small tumor confidently identified on biopsy in an older person or someone with other health problems, active surveillance with regular imaging may be discussed instead of surgery.
Follow-up — After a confirmed oncocytoma is removed, extensive follow-up is generally not required. Imaging may be continued for a time to check kidney function and, when relevant, to watch the other kidney, particularly if more than one tumor was present.
Genetic assessment — If you have more than one oncocytoma, tumors in both kidneys, an oncocytoma at a young age, or the skin or lung features described earlier, a referral to a genetics clinic may be offered so that Birt-Hogg-Dubé syndrome can be looked for and family members supported if needed.

Questions to ask your doctor

How was my tumor confirmed to be an oncocytoma rather than a chromophobe renal cell carcinoma?
Were special stains such as CK7 and CD117, or chromosome testing, used to make the diagnosis?
Was my diagnosis made from the whole removed tumor, or from a biopsy?
Did my report mention any findings, such as extension into fat, that look concerning, and do they change anything?
Was more than one tumor found in my kidney or in the other kidney?
Could this be related to Birt-Hogg-Dubé syndrome?
Should I be referred for genetic counseling, and should my family be assessed?
Were the surgical margins clear, and do I need any follow-up because of them?
What are the chances the tumor could grow back?
Do I need any follow-up imaging, and if so, how often and for how long?