What Is A Hibernoma?

Overview

A hibernoma is a rare, non-cancerous soft tissue tumour made of brown adipose tissue (brown fat). Although they are benign (non-cancerous), hibernomas often resemble malignant tumours like liposarcomas. 

They are most commonly found in the shoulders, thighs and back. These tumours grow slowly and do not spread to other parts of the body.¹

Hibernomas are similar to lipomas as they are both benign soft tissue tumours. Lipomas are the more common type of benign soft tissue tumour and are usually white in colour.

This article will explain everything you need to know about hibernomas, including the different types of hibernoma, the symptoms, diagnosis and treatment options.

Types of hibernoma

There are different types of hibernomas that are formed by different cells and a biopsy is required to identify the type. The types of hibernomas include typical lobular hibernoma, lipoma-like hibernoma, myxoid hibernoma and spindle-cell hibernoma.¹

• Typical lobular hibernoma: this accounts for 80% of hibernoma cases and consists of tumours that are yellow, light brown or grey in colour. This type of hibernoma mainly appears on the thighs, chest or trunk and may also form within muscles¹
• Lipoma-like hibernoma: this type contains white fat with brown specks, making it resemble a lipoma. This tumour mainly appears on the thighs¹
• Myxoid hibernoma: this is a less common type of hibernoma that is usually found on the scalp or shoulders of people assigned male at birth (AMAB)¹
• Spindle-cell hibernoma: this is a very rare type which accounts for only 2% of hibernomas. Tumours usually form on the scalp or the back of the neck¹

Causes of hibernomas

The main purpose of brown fat is to regulate your body temperature in cold conditions, burn calories and store energy. To produce heat, brown fat breaks down blood sugar and molecules of fat. This process is known as thermogenesis.

There is less brown fat than white fat in your body and the amount decreases as we age. Brown fat makes up 2% to 5% of a newborn’s body weight compared to adults, who have an even smaller amount of brown fat. The continued growth of brown fat can result in a hibernoma.

Research shows that almost all hibernomas have breakpoints in chromosome 11q, which inactivates the tumour suppressor genes MEN1 and AIP. Therefore, it appears that gene mutations cause the development of hibernomas.¹

As well as this, benign soft tissue tumours such as hibernomas and lipomas are more common in people who have multiple endocrine neoplasia (MEN) syndromes. These conditions cause tumours to grow in glands in your endocrine (hormone) system.

Signs and symptoms of hibernomas

Hibernomas are usually between 5-10cm, but hibernomas that form within the abdominal cavity can be as large as 10cm. The colour of the fat can range from a light tan to a reddish-brown. This colour will not be seen unless it is surgically removed.

Hibernomas are usually found underneath the skin on your thighs but they can also appear on the shoulder, back, chest, arm, neck, scalp, vulva and scrotum. Around 10% of hibernomas form within soft tissues, muscles or ligaments in the musculoskeletal system. They can also develop in the lungs, larynx (voice box) and pelvis.¹

The main symptom of hibernoma is a lump underneath the skin that:

• Moves around
• Is round
• Is warm to touch
• Is soft or rubbery

Hibernoma rarely causes pain but if the tumour causes compression of nearby nerves, you may experience some pain.¹

Concerning symptoms include:¹

• Rapid growth of the tumour
• Armpit or inguinal (groin) pain, which may indicate swollen lymph nodes
• Unexpected weight loss
• Personal or family history of cancer

These more concerning symptoms could suggest a more serious condition, such as a liposarcoma or another malignant tumour.¹

Management and treatment for hibernomas

Treatment of hibernomas depends on where the tumour is located in the body and if the patient is symptomatic or asymptomatic. If they have no symptoms, no treatment is required.

The most effective treatment is surgical removal of the hibernoma, which doctors may recommend even if there are no symptoms. Removal of the tumour means the doctor can confirm the hibernoma diagnosis and ensure the tumour is non-cancerous. The tumour is unlikely to return after it is removed.¹

Complications

Complications may arise if a hibernoma grows too large and invades nearby nerves, tissues and organs. If a tumour compresses nearby nerves, it can lead to neuropathy and cause nerve pain which can appear as:

• Knee pain
• Carpal tunnel syndrome
• Thoracic outlet syndrome
• Lower back pain/sciatica

Complications following surgery can include delayed wound healing, scarring and formation of a haematoma/seroma.¹

Diagnosis

Imaging tests

Imaging tests play a key role in the diagnosis of hibernomas. These tests include X-rays, computed tomography (CT) scans, magnetic resonance imaging (MRI) scans, sonography and positron emission tomography (PET) scans.¹

Needle biopsy 

To confirm a hibernoma diagnosis, a biopsy is needed. Your doctor may suggest a needle biopsy to rule out soft tissue cancer such as liposarcoma. This procedure involves using a thin needle to take a sample of tissue from the tumour, which is then analysed in a lab for diseased cells.¹

Differential diagnosis of hibernomas

On imaging, hibernoma can sometimes be different to distinguish from other tumours. The differential diagnosis of hibernomas includes both benign and malignant tumours.

The benign soft tissue tumours that are similar to hibernoma include:

• Lipoma
• Haemangioma
• Angiolipoma

The malignant soft tissue tumours that are similar to hibernoma include:

• Liposarcoma
• Myxoid liposarcoma
• Rhabdomyosarcoma
• Lymphoma

FAQs

How can I prevent hibernoma?

Hibernomas are rare and there is no way to prevent the gene mutation that causes them. However, you should be aware of any changes in your body that may be signs of a tumour. This is important as although most soft tissue tumours are benign, some are cancerous.¹

How common are hibernomas?

Hibernomas are very rare and they account for less than 1% of benign soft tissue tumours. 

Who is at risk of hibernoma?

You are more at risk if you:¹

• Are between the ages of 20-40 years old
• Are assigned male at birth (AFAB)
• Have multiple endocrine neoplasia (MEN) syndrome

When should I see a doctor?

You should contact your GP if you have:

• A lump underneath your skin
• A chronic cough (this may mean you have a tumour on or near your lungs)
• Unexplained pain e.g. pain in your hands, knees or back

Summary

Hibernoma is a rare type of benign soft tissue tumour made of brown fat. These tumours can form underneath your skin or within your musculoskeletal system. They are commonly found on the thighs but can also appear on the arms, back, shoulders, chest, neck and scalp. The main symptom of hibernoma is a round lump underneath the skin that moves, is warm to the touch and is rubbery or soft. 

Hibernomas are diagnosed using imaging tests such as X-rays, CT scans, MRIs and PET scans. However, to confirm a diagnosis a biopsy is needed where a sample of tissue from the tumour is analysed in a lab. 

The main treatment for hibernoma is surgical excision of the tumour which is helpful in confirming the diagnosis and ensuring it is non-cancerous. After removal of the tumour, it is unlikely to return.