Hibernoma is a very rare tumor originating from brown adipose tissue, mainly developing in the paravertebral region, on the neck, in the armpits, in most cases benign, but there is also a malignant form. It appears as an elastic, spherical, slow-growing formation on the skin with a diameter of 1-13 centimeters, painless; there are no inflammatory manifestations. Diagnosis of hibernoma is based on examination by a dermatologist or oncologist, as well as on histological examination of the neoplasm tissues. Treatment of this tumor is only surgical, other methods (for example, X-ray therapy) are ineffective.
General information
A hibernoma (brown lipoma, lipoma of lipoblasts) is a neoplasm consisting of fat cells (adipocytes) rich in mitochondria and lipochrome. It was first described in 1914 by oncologist Gehry. In the vast majority of cases, its growth is slow and benign, sometimes there is a malignant variety. Given the fact that cases of degeneration of benign hibernoma are unknown, many researchers do not connect these two forms. Basically, hibernoma affects middle–aged and older women, forms in places of rudimentary deposits of brown fat – along the vertebrae, on the neck, in the armpits, rarely occurs on the lower back and in the buttocks. Isolated cases of the occurrence of this neoplasm in children are described. Since brown adipose tissue is well developed in animals that fall into hibernation, this is reflected in the name of hibernoma – from the Greek “hibernus” – winter.
Causes
Despite the fact that there is a widespread opinion about brown adipose tissue as a source of hibernoma, not all dermatologists agree with this. There are indications that this neoplasm develops from germinal adipose tissue. Histochemical studies of hibernoma in different patients sometimes reveal contradictory data, perhaps we are talking about clinically very similar, but different in origin tumors from adipose tissue. Despite the fact that deposits of brown adipose tissue are present not only under the skin, but also in the intermuscular layers, almost always the hibernoma develops only under the skin. To date, there is no reliable explanation for this fact.
The proof that the hibernoma originates from the cells of brown adipose tissue is the presence in its cells of a large number of mitochondria with a complex internal structure similar to brown adipocytes. This gives the cytoplasm of these cells a granular appearance. Presumably, the reason for the growth and differentiation of brown adipose tissue cells is a certain enzyme defect. The search for genetic and hereditary factors in the development of hibernoma has not yielded unambiguous results. Among other reasons for the development of this neoplasm, the influence of metabolic disorders, female sex hormones, and embryonic disorders is assumed.
Classification
Histological studies of hibernoma biopsies revealed certain differences in its structure in different patients. Processing of data and biopsy results of a large number of patients allowed us to identify four main pathohistological forms of this tumor:
- Lobular type is the most common variant of hibernoma. Differentiation of cells can be quite diverse, mainly there is a tendency of development from multivacuolar to monovacuolar forms. Individual ordinary adipocytes are noted.
- Myxoid type – the vast majority of cells of such a hibernoma are foamy, that is, multivacuolar. In addition to them, there are individual cells that are much smaller than adipocytes, do not contain fatty inclusions and have an eosinophilic cytoplasm.
- Lipoma–like type – most cells have a monovacuolar structure similar to ordinary adipocytes.
- Fusiform cell type – multivacuolar foamy cells of the hibernoma are randomly arranged among fusiform, similar in structure to the elements of the fusiform lipoma.
As a rule, there is no direct correlation between the cellular structure and the symptoms or clinical manifestations of hibernoma.
Symptoms
Having begun its growth, the hibernoma proceeds asymptomatically for a very long time, only after reaching a certain size it becomes noticeable and palpable for the patient. Usually it is a dense and elastic skin node with a diameter of 3 to 13 centimeters, covered with unchanged skin. The favorite localization of hibernoma is near the spine, on the neck, in the armpits, sometimes in the inguinal and gluteal areas. In rare cases, the neoplasm may somewhat hinder a person’s movements (when located in mobile areas of the body) or put pressure on nearby vessels or nerves. This can cause pronounced disorders, which will require removal of the hibernoma to eliminate.
Diagnosis
In dermatology and oncology, the diagnosis of hibernoma is made on the basis of examining the patient and performing a biopsy followed by histological examination of tumor tissues. Sometimes the macroscopic structure of the neoplasm is additionally studied after its removal. When examined on the surface of the skin of a patient with a hibernoma, an elevation of a spherical shape with a diameter of 3-13 centimeters is determined, dense to the touch, but painless. The skin in the area of the tumor is not changed, the survey reveals that its growth is very slow and takes many years, there were no sharp jumps in size increase.
Conducting a fine needle biopsy followed by microscopic examination reveals a lot of foamy cells with large mitochondria. Sometimes, with some pathohistological forms of hibernoma, ordinary adipocytes with one large fat vacuole and a peripherally located nucleus may occur. In contrast, adipocytes of hibernoma and brown adipose tissue have many small vacuoles in the cytoplasm and a centrally located nucleus. When cells are stained with Sudan 3, cell inclusions turn yellow.
Macroscopically, a hibernoma is an encapsulated dense tumor that is sufficiently well vascularized. From the connective tissue shell of the neoplasm, partitions go inside, giving it a lobular structure. The hibernoma is not soldered to the surrounding tissues and is easily separated from them when removed. Differential diagnosis should be carried out with other forms of lipomas, brown liposarcoma (sometimes called malignant hibernoma), xanthoma.
Treatment and prognosis
The treatment of hibernoma is exclusively surgical, no conservative measures will help eliminate this neoplasm. Depending on the location and size of the tumor, the operation is performed under local anesthesia or general anesthesia. After removal of the hibernoma, its examination and additional histological examination are performed to confirm the diagnosis. The prognosis of the disease is generally favorable with a benign form of neoplasm, relapses after complete elimination of the tumor almost never occurs. There were also no cases of the development of hibernoma in other parts of the body.