What is Hibernoma?
Hibernomas are rare tumors made up of brown fat cells. These tumors most commonly appear in the thigh, shoulder, and back areas. Other less common forms of hibernomas, called myxoid and spindle cell variants, are often found in the back of the neck and shoulder. Hibernomas are a type of harmless, fat-related tumor and they don’t turn into cancer. They are named ‘hibernomas’ due to their content of brown fat, which is a type of fat found in hibernating animals. First discovered in 1906 by Merkel, these tumors behave similarly to lipomas, a common kind of fat tissue tumor, but they have distinctive features when looked at through medical imaging and tissue examination.
Hibernomas usually occur in young adults around the age of 38. They are typically:
* Made up of brown fat
* Harmless tumors
* Clearly separated masses
* Generally small
* Slow-growing
When looking at these tumors under a microscope, they are made up of brown fat cells. When doctors are trying to diagnose a hibernoma, they typically consider liposarcomas as another possible diagnosis. These are a type of cancer that develops in fat cells. Hibernomas most commonly occur in the thigh, trunk, and chest, but they can appear in other locations in less than 10% of cases, including inside the abdomen, in the chest, or behind the abdominal cavity. Less than 20% are found in muscle tissue.
What Causes Hibernoma?
Hibernomas are a type of fat tissue tumor that seem to originate from a specific type of fat tissue in unborn babies, known as fetal brown fat tissue. However, these tumors don’t often show up in infants. This might suggest that the brown fat tissue is taking a different route in how it changes and grows.
Interestingly, these tumors often involve a switching of parts between two chromosomes – specifically chromosome 11. Tumor suppressor genes MEN1 and AIP, genes that help keep cells from growing and dividing too rapidly or in an uncontrolled way, are typically found to be at the precise area where this switch occurs. It’s also worth mentioning that hibernomas are often associated with a condition called multiple endocrine neoplasia type 1, which is a disorder that affects your endocrine glands.
Risk Factors and Frequency for Hibernoma
- The highest number of cases typically occurs in individuals in their third decade of life.
- There are differing reports regarding whether males or females are more likely to have this condition, but most research suggests a slight inclination towards males.
- This condition makes up 1.6% of all adipocytic (fat cell) tumors.
- The average age when the disease is diagnosed is 38 years old.
Signs and Symptoms of Hibernoma
Hibernomas, a type of body growth, often don’t cause any symptoms. However, sometimes they can cause a light pressure or discomfort due to the size of the growth. These masses are often movable and have a soft consistency. They may sometimes compress nearby nerves, leading to discomfort. It can be challenging to evaluate hibernomas through medical history and physical examination, especially if the patient doesn’t have any symptoms. Important factors to look for that might suggest a more serious condition, like a liposarcoma or other cancerous tumor, include:
- Fast growth of the mass
- Pain in the armpit or groin area, suggesting swollen lymph nodes
- A personal or family history of cancer
- Unintended weight loss
A physical exam might provide limited information, but some key features to assess when looking at a subcutaneous mass or lump under the skin include:
- The size of the growth
- Whether the mass can be moved or not
- Any breaks or ulcers on the skin over the mass
- Swellings in the lymph nodes
- Assessment for any extra skin lumps
Hibernomas are often warm to the touch due to their blood supply. Beyond this, the physical features of these growths are not specific.
Testing for Hibernoma
When checking for hibernomas – which are a type of tumor – doctors often use imaging. The type of imaging depends on the location and depth of the suspected hibernoma.
X-rays aren’t great for spotting hibernomas as these tumors usually look much like the surrounding tissue. However, sometimes, the tumors may show up as a certain type of brightness on an X-ray. Hibernomas can sometimes be seen on X-rays if they shift some tissue or air-filled structures like the lungs. But, X-rays might not catch hibernomas that are within muscles or located near the center of the body, such as the chest or stomach area. Sometimes, hibernomas can show up in the bones and appear as holes in the bone seen on an X-ray.
Ultrasound imaging works best for hibernomas located just beneath the skin. These tumors usually appear well-defined and enclosed. They often look a lot like the surrounding fat tissue. Sometimes they can show up a little lighter or darker than the fat. The presence of a large blood vessel either within or next to the tumor can often be confirmed using color Doppler imaging, a type of ultrasound that can visualize blood flow. The appearance of hibernomas under ultrasound is similar to both typical and atypical lipomas – another kind of benign fat tissue tumor.
Computed tomography, or CT scans, allow doctors to see hibernomas as areas that are less dense than muscle tissue but slightly denser than the surrounding fat – given that these tumors have increased blood flow and less white fat cells. Sometimes, a prominent blood vessel can be identified in the scan.
Magnetic Resonance Imaging, or MRI, shows hibernomas as well-defined and enclosed. They often look ‘noisy’ in signal and may appear slightly dim or uniform in brightness compared to the surrounding fat on T1 imaging – a specific type of MRI scan. An increased brightness on T2 imaging – another type of MRI scan – is seen due to hibernoma’s increased blood flow compared to the white fat. The tumors may also resist the suppression of fat signal on specific MRI sequences. The level of contrast enhancement in the scan depends on the quantity of brown fat within the hibernoma.
Positron Emission Tomography, or PET scans, show hibernomas as areas with high metabolic activity due to their composition of brown fat, which is more metabolically active than regular fat. These tumors are often discovered unintentionally during PET scans. However, small hibernomas can sometimes be mistaken for spread of malignant disease due to their high metabolic activity. It’s crucial to understand that hibernomas are distinct tumors and differ from adults who have symmetric stores of brown fat, which also shows increased metabolic activity. The brown fat retained in adults usually appears as non-tumorous increased metabolic activity in certain regions such as the neck, behind the collarbone, or along the spine.
Treatment Options for Hibernoma
It can be challenging to distinguish between hibernomas and atypical lipomatous tumors or well-differentiated liposarcomas (ALT/WDLS), particularly if the hibernoma is located within the abdomen and appears mixed or uneven on CT and MRI scans. In simple terms, hibernomas and ALT/WDLS are types of growths or lumps made primarily of fat cells.
Hibernoma refers to a benign (non-cancerous) fatty tumor that has characteristics similar to brown fat, the type of fat that is present in hibernating animals. On the other hand, ALT/WDLS are slow-growing cancers that consist of fat cells and can occur anywhere in the body, but are often found in the abdomen and thigh.
Medical imaging may reveal patterns of blood flow or enhanced vascular structures, which may suggest the presence of a hibernoma rather than an ALT/WDLS. Both types of tumors can have thick separating areas within them that show up on scans. However, ALT/WDLS can be more aggressive, potentially invading local tissues, spreading to other parts of the body (metastasis), or spreading to nearby lymph nodes (regional metastasis).
If the imaging results are not clear enough to establish a diagnosis, a biopsy may be considered. A biopsy is a procedure that requires the removal of a small sample of tissue for further examination. However, a type of biopsy, known as a core needle biopsy, is generally avoided for deep tumors because of the reported risk of creating an unplanned connection between an artery and a vein (arteriovenous shunting).
Surgical removal is the definitive treatment for these tumors. When it comes to differentiating hibernomas from lipomas (benign tumors composed of fat tissue) on MRI, it’s usually less complicated. This is because lipomas usually appear more consistent or homogeneous on MRI and show a known drop in signal on fat suppression, which helps in their identification.
What else can Hibernoma be?
If you find a lump under your skin, it might be a hibernoma. Hibernomas are benign tumors that don’t often lead to serious health complications. However, when a doctor examines the hibernoma, they’ll need to rule out other conditions or diseases that might appear similar.
There are benign (non-harmful) and malignant (harmful) conditions that a hibernoma can be mistaken for.
The harmless conditions include:
- Lipomatous lesions like lipoma and angiolipoma, which have a lot of fatty tissue.
- Fat necrosis, which is the death of fat cells in the body.
- Hemangiomas, which are collections of blood vessel cells that form a lump.
- Fibromatosis, which is a condition where soft tissue grows excessively.
- An abscess, which is a collection of pus caused by an infection.
In addition, physicians should also consider other, more serious conditions. On imaging scans, a hibernoma can sometimes look like a harmful condition called a liposarcoma. Also, if a patient has a history of cancer, the doctor should watch out for signs of cancer spreading, or metastasis. Skin and tissue below the skin often attract spreading cancer cells from diseases like melanoma, breast cancer, colorectal cancer, kidney cancer, and lung cancer.
The serious conditions that a hibernoma can be mistaken for include:
- Liposarcoma, which is a type of fat cell cancer.
- Lymphoma, which is a type of blood cancer.
- Merkel cell carcinoma, which is a rare, aggressive skin cancer.
- Subcutaneous metastases, or when cancer spreads to the tissue beneath the skin.
- Dermatofibrosarcoma protuberans, which is a rare skin cancer.
- Angiosarcoma, a rare cancer that can sometimes be caused by radiation treatment.
What to expect with Hibernoma
The outlook after removing hibernomas, which are generally benign (not harmful) tumors, is excellent. They rarely come back after they’re locally excised or surgically removed. Some long-term symptoms that might occur due to the mass pressing on nearby nerves and blood vessels include mild to moderate levels of pain. These symptoms are typically observed in cases where people show symptoms.
Possible Complications When Diagnosed with Hibernoma
Complications from tumors, another name for neoplasms, typically occur because of their growing size and the effect it has on surrounding areas. One possible symptom is pain caused by the tumor compressing on nerves. Hibernomas, a specific type of tumor, usually do not show any symptoms. After removal of the tumor, some people might experience complications such as delayed healing of the wound, scarring, or formation of a hematoma or seroma, which are both collections of fluid in the body.
Common Side Effects:
- Pain due to nerve compression caused by the tumor
- Delayed wound healing post-surgery
- Scarring after surgery
- Formation of a hematoma or seroma after surgery
- No symptoms (in the case of Hibernomas)
Preventing Hibernoma
If a patient is diagnosed with hibernomas – which are non-cancerous growths – the diagnosis usually comes through imaging tests or a biopsy. We then discuss the best way to manage this condition with the patient. The options can include keeping a close watch on it (surveillance) or removing it entirely through surgery (complete surgical excision).
If the decision is made to remove the hibernoma through surgery, it’s important for patients to understand the specific risks and benefits of the operation. This will depend on where the tumor is located – whether it’s on the surface (superficial) or deeper within the body, and how close it is to other important body structures like nerves and vessels (neurovascular structures) or organs.
When the hibernoma is on or just beneath the skin (superficial or subcutaneous), we’ll talk to patients about what they can expect in terms of how their body will look after the surgery (cosmesis).
