Hibernoma - Causes, Symptoms, and Treatment

What Are Hibernomas?

Hibernoma is basically a tumor of brown adipose tissue or brown fat. The common location of tumors includes the thigh, back, and shoulder. Myxoid and spindle cell hibernoma is an uncommon type seen in the back of the neck and shoulder. Hibernomas are benign fatty tissue cancer and cannot transform into malignant (cancerous) types. Hibernomas are made up of brown fat. Hibernoma is a rare, benign lipid tissue tumor that refers to brown fat morphologically. Most hibernomas occur in adults in the brown fatty tissue sites.

What Is the Etiology of Hibernomas?

Hibernomas are fatty tissue tumors with histopathologic origin from fetal brown fat tissue. These tumors are generally not seen in infants. These tumors can occur due to changes in the differentiation pathway of brown fat, like translocations of chromosome 11. Hibernomas are also associated with multiple endocrine neoplasia type 1.

What Is the Histopathology of Hibernomas?

Hibernomas have different histopathology depending on the histological subtypes:

• Typical hibernomas.

• Lipoma-like hibernoma.

• Myxoid hibernoma.

• Spindle cell hibernoma.

The typical neoplasms appear yellow, light brown, or gray in color and lobular in appearance. They appear different because of differences in lipid content. Microscopically, these tumors are large and multi-vacuolated. They have mature fatty tissue along with the branching capillary network. The lipoma-like hibernomas contain white fat. The typical variant has more than 70 % of brown fat. Following are the features of hibernomas.

• A variable portion of brown fat cells.

• Brown fat cells have granular and multi-vacuolated cytoplasm and uni-vacuolated white fat cells.

• Eosinophilic and polygonal brown fat cells, spindled cells, and myxoid stroma.

• Cytochrome pigments.

• No mitotic activity and small multi-vacuolated brown fat cells.

What Are the Symptoms of Hibernoma?

Hibernoma does not cause any symptoms. There are generally no other symptoms other than a lump surrounding fatty tissue. As the hibernoma grows, symptoms can include fever, chills, night sweats, fatigue, weight loss, or weight gain. The following features should be considered while evaluating hibernoma. Size, fixed or mobile nature of the mass, skin ulceration, lymphadenopathy, additional subcutaneous neoplasms, warm to touch due to vascularity.

Symptoms also depend on the tumor's location. An abdomen tumor causes pain, swelling, fullness after light meals, constipation, difficulty breathing, and blood in the stool. A tumor in the extremities causes weakness in the limb, as well as pain and swelling. Patients with hibernomas are asymptomatic. Patients generally have pressure, pain, or discomfort. Hibernomas are mobile and pliable. Compress the adjacent nerves, and associated pain can be seen. Features on history include the following:

• The rapid growth of a soft tissue mass.

• Axillary or inguinal pain indicating associated lymphadenopathy.

• Past history of malignancy.

• Family history of malignancy.

• Unexplained weight loss.

What Are the Risk Factors of Hibernoma?

Genetic changes in fat cells lead to growth that is out of control. The exact triggers are unknown. Risk factors include the following.

• Past radiation treatment.

• Personal history of cancer.

• Family history of cancer.

• Damage to lymphatics.

• Prolonged exposure to toxic chemicals like vinyl chloride.

How to Diagnose Hibernomas?

Imaging plays an important role in evaluating hibernomas.

1. Radiography:

Radiographs are limited to evaluating hibernomas. Radiographs can reveal the alteration of tissue plains when hibernomas are identified on imaging. The lesions are radiolucent muscle and bone with a radiodensity similar to surrounding fat tissue. Depending on the size and location, subcutaneous soft tissue prominence is seen. Hibernomas displace soft tissue structures or are superimposed air-filled organs like lungs. Intramuscular or hibernomas involving the mediastinum, abdomen, and pelvis cannot be seen on radiographs. Intraosseous hibernomas are seen as lytic lesions of the affected bone.

2. Sonography:

Sonography is effective in diagnosing hibernomas of soft tissues. Sonography helps in the evaluation of intramuscular hibernomas along with deeper lesions. These neoplasms are encapsulated and well-circumscribed, along with mild hyperechogenicity. Few vessels can be seen on color Doppler imaging.

3. Computed Tomography (CT):

CT scans show images hypoattenuating to muscle due to their vascularity and low percentage of white fat vacuoles.

4. Magnetic Resonance Imaging (MRI):

MRI shows encapsulated and well-circumscribed hibernomas. They are heterogeneous to surrounding subcutaneous fat. Differentiating hibernomas from lipomas on MRI is a less diagnostic dilemma, as lipomas are more homogenous in MRI scans due to loss of signal on fat suppression.

5. Positron Emission Tomography (PET) Imaging:

Due to the composition of brown fat, hibernomas show increased fluorodeoxyglucose-18 (FDG) activity on PET imaging.

How to Treat Hibernomas?

Treatment depends on a number of factors, like the size and location of the primary tumor; sometimes, they can interfere with blood vessels or vital structures and the spread of cancer. The main treatment option is surgery. The surgery aims to remove the entire tumor growth along with a margin of healthy tissue. This cannot be done if the tumor has spread into vital structures. Also, radiation and chemotherapy can be used in order to shrink the tumor before surgery.

• Radiation therapy uses energy beams in order to kill cancer cells. It can be used post-surgery to destroy remaining cancer cells.
• Chemotherapy is a treatment option that uses strong drugs with the ability to kill cancer cells.
• Regular follow-up is required, including physical exams and imaging tests, like computed tomography CT scans or magnetic resonance imaging MRI.
• Clinical trials can be carried out as well. They give newer treatments that are not yet available otherwise. Hibernomas and atypical lipid tumors cannot be differentiated. They present with local invasion, metastasis, and regional metastasis. Image-guided or open biopsies are considered when imaging is not definitive. Surgical excision is the definitive treatment.

What Is the Differential Diagnosis of Hibernomas?

Benign subcutaneous lesions that can be considered under differential diagnosis include the following.

• Lipomatous lesions.

• Fat necrosis

• Hemangioma

• Fibromatosis

• Primary malignancies

• Metastatic disease

• Abscess

Malignant subcutaneous lesions that can be considered in the differential diagnosis include the following.

• Liposarcoma.

• Lymphoma.

• Merkel cell carcinoma.

• Subcutaneous metastases.

• Dermatofibrosarcoma protuberans.

• Angiosarcoma, which is radiation-induced.

Conclusion:

Hibernomas are difficult to differentiate from other lipid neoplasms, but identifying some imaging characteristics, location, and patient demographics improves preoperative evaluation. Considering the benign nature, accurate diagnosis affects treatment plans and surveillance strategies.