Hibernoma: Understanding Your Pathology Report

Section Editor: Bibianna Purgina MD FRCPC
September 24, 2026

A hibernoma is a noncancerous tumor made of brown fat. Brown fat is a specialized type of fat that produces heat. It is plentiful in newborns and young children, and most adults have only small amounts.

A hibernoma is not a cancer. It grows slowly, stays in one place, does not spread to other parts of the body, and does not turn into cancer. The name comes from the brown fat that hibernating animals use to keep warm.

Hibernomas are usually found in adults between about 20 and 50 years old. The most common locations are the thigh, shoulder, upper back, and neck. Most sit just under the skin, although some develop deeper in the body.

This article explains how a hibernoma is diagnosed and what your pathology report findings mean.

What is the difference between a hibernoma and a lipoma?

Both are noncancerous tumors made of fat, and both are cured by removal. The difference is the type of fat.

A lipoma is made of ordinary white fat, the type found throughout the body in adults. A hibernoma is made of brown fat, which contains many small droplets of fat inside each cell and far more of the structures that produce energy and heat. That difference is what a pathologist sees under the microscope.

What are the symptoms of a hibernoma?

Most hibernomas cause few symptoms. The usual finding is a soft, painless lump that grows slowly over months or years.

A large hibernoma can press on nearby structures and cause discomfort. Some are found by chance on a scan done for another reason.

What causes a hibernoma?

A hibernoma develops when the tumor cells lose a small part of chromosome 11, in a region known as 11q13. This region contains two genes, MEN1 and AIP, and losing both appears to be the key step in the development of this tumor.

This change happens in the tumor cells during a person’s lifetime. It is not inherited and cannot be passed on to children.

One of these genes, MEN1, is also the gene involved in an inherited condition called multiple endocrine neoplasia type 1. A small number of case reports describe a hibernoma in a person with that condition, but hibernoma is not a recognized feature. A hibernoma alone is not a reason to suspect an inherited condition.

How is the diagnosis made?

The diagnosis of a hibernoma is made after a pathologist examines tumor tissue under the microscope. The tissue may come from a biopsy or from surgery that removes the whole tumor.

Under the microscope, a hibernoma is made of brown fat cells surrounded by a rich network of small blood vessels. The brown fat cells look granular or bubbly because the cytoplasm, which is the body of the cell, is divided into many small droplets. Pathologists describe these cells as multivacuolated. Ordinary white fat cells are usually mixed in as well.

Features that would suggest a cancer are absent. There are very few mitotic figures, which are cells in the process of dividing, and no atypical cells.

Why can a hibernoma look worrying on a scan?

Brown fat burns a lot of energy, and PET scans measure how much sugar a tissue takes up. A hibernoma therefore lights up brightly on a PET scan, sometimes more brightly than a cancer would.

On MRI, a hibernoma can also look different from ordinary fat and may show prominent blood vessels. For these reasons, doctors sometimes mistake a hibernoma for a fatty cancer before examining the tissue. Examining the tissue is what settles the question, and a strong PET result does not mean the tumor is cancerous.

What other tumors can look like a hibernoma?

Some hibernomas contain many cells with small fat droplets that resemble immature fat cells. These cases can look like an atypical lipomatous tumor, a fatty tumor that can come back after surgery.

Two tests help separate them. An immunohistochemistry stain for S100 is positive in the fat cells of a hibernoma. A test called fluorescence in situ hybridization (FISH) counts copies of the MDM2 gene. A hibernoma has the normal two copies, while an atypical lipomatous tumor has extra copies.

Tumor size and margins

Tumor size is the greatest dimension of the hibernoma, measured in centimeters (cm). Hibernomas can reach a large size, particularly when they develop deep in the thigh. Size does not change the noncancerous nature of this tumor.

A margin is the edge of tissue cut by the surgeon. A negative margin means no tumor cells are seen at the cut edge. For a hibernoma, a positive margin does not carry the meaning it does for cancer, and further surgery is usually not needed.

What is the prognosis?

The outlook is excellent. A hibernoma is noncancerous, does not spread, and does not become cancer. Once removed, it rarely comes back, and recurrence is unusual even after incomplete removal.

Routine long-term follow-up is usually not needed after the tumor is removed and the diagnosis is confirmed.

What happens after the diagnosis?

Treatment depends on the size and location of the tumor and on whether it causes symptoms.

Surgical removal. Removing the tumor is the usual treatment and is curative. Because a hibernoma has a rich blood supply, the surgeon plans the operation accordingly.
Observation. A small hibernoma that causes no symptoms and has been confirmed by biopsy may be watched instead.
No further treatment. Chemotherapy and radiation therapy have no role for this tumor.

Questions to ask your doctor

Was the whole tumor removed?
How was the hibernoma separated from a fatty cancer?
Was MDM2 testing performed, and what did it show?
My scan looked worrying. Does the pathology result change that?
Is there a chance this tumor could come back?
Do I need any follow-up imaging?
Does having a hibernoma mean anything for my family?