Hibernoma is a rare soft tissue tumor originating from brown fat. Named for its resemblance to the brown fat found in hibernating animals, this tumor can be mistaken for a malignant lesion before tissue diagnosis.11 A case report has been presented along with a review of the clinical, radiographic, and pathologic features of hibernoma.
A 40-year-old woman presented with a painless right medial thigh mass that had been enlarging progressively over a 1-year period. Her medical history was significant for hypertension only. She reported no trauma to the area and exhibited no systemic signs of weight loss, fatigue, fever, chills, or night sweats. Physical examination revealed a soft nontender right medial thigh mass measuring 18 × 20 cm. No overlying warmth or erythema was noted. The neurovascular status was intact, and no lymphadenopathy was found.
The patient initially was referred to a vascular surgeon because of concern that the right groin mass might represent a femoral artery aneurysm. Subsequent workup included unenhanced computed tomography (CT), which revealed a 10 × 14 cm mass arising from the adductor compartment (Fig 1). Fibrous strands were noted within the mass; however, its primary appearance was that of fat density.
Concern about the increasing size of the mass prompted referral to an orthopaedic oncologist, who proceeded with magnetic resonance imaging (MRI) of the area (Fig 2). The MRI showed a large, well defined mass in the region of the adductor magnus with displacement of the vastus medialis anteriorly. The lesion appeared to be well contained within the fascial planes with marked posterior displacement of the gracilis and semimembranosus and semitendinosus muscles. Laterally, the mass extended to the periosteum of the femur without evidence of invasion or erosion. The femoral vessels were displaced anteriorly and laterally and were not encased by the mass. In general, the lesion exhibited a high signal intensity consistent with a lipomatous tumor. However, some areas of inhomogeneity consisting of decreased signal intensity were seen in the mid and lateral portions of the mass. These findings were suspicious for liposarcoma.
A marginal resection was performed with dissection through the overlying adductor magnus, and the tumor was removed through its pseudocapsule with excision of minimal normal tissue. Grossly, the 840-g tumor was a well circumscribed, yellowish fatty mass with a thin, translucent membrane covering. Microscopically, the mass contained vessels of various calibers coursing through it along with delicate fibrovascular septa. The tumor comprised irregularly shaped lipocytes with centrally located, darkly staining nuclei and finely vacuolated to granular eosinophilic cytoplasm (Fig 3). Atypia or mitotic activity was not identified, and no invasion into adjacent structures was seen. The final diagnosis was that of a benign brown fat tumor, or hibernoma. The patient developed a postoperative hematoma, which resolved uneventfully.
Merkel9 first described a tumor of brown fat in 1906, referring to this lesion as a pseudolipoma. Enzinger and Weiss credited Gery with coining the term hibernoma in 1914, when he recognized the tumor's resemblance to brown fat found in hibernating animals.3 This is a relatively rare tumor, with only about 100 cases reported in the literature. Despite reports of cases believed to be malignant, no histologic criteria have been established for malignancy in hibernoma.5,7,11 Therefore, it generally has been accepted that this tumor is benign and amenable to marginal resection through the pseudocapsule of the lesion with minimal removal of the surrounding normal tissue.
The peak incidence of hibernoma is in the third decade of life, with a slight predominance of females over males. Clinical presentation is that of a painless, enlarging mass. Rigor et al11 reported a 900-g hibernoma, which was believed to reach such large proportions because of its retroperitoneal position. However, it is unusual for a tumor of the size reported here to escape detection in a location as conspicuous as the medial thigh. The examination usually reveals a firm, freely movable nontender mass. The overlying skin may be warmer to the touch than the adjacent skin secondary to the hypervascularity of the tumor. This is in contrast to the findings in lipoma, in which the overlying skin may actually be cooler than the surrounding skin.1,11
Angiography has been advocated as a helpful diagnostic tool. It has been suggested as an effective means of differentiating the highly vascular hibernoma from hypovascular lesions, such as lipoma, fibroma, and neurofibroma, as well as a way in which to define the vascular anatomy before surgical resection.8 Angiography will be less sensitive in differentiating hibernoma, angiolipoma, and low grade liposarcoma. However, a lack of true neovascularity and arteriovenous shunting suggests a benign process.2,10 Despite its utility, angiography has been replaced in most cases by cross sectional imaging studies in the diagnosis of soft tissue tumors. Although specific tissue characterization is not possible, CT and MRI can define significant fat content in a lesion. Computed tomography demonstrates a low density lesion of -100 Hounsfield units, or lower MRI and contrast enhanced CT reveal areas of focal inhomogeneity, thereby allowing for exclusion of a simple lipoma in the differential diagnosis.2,5,6,10
Brown adipose tissue is a specialized form of fat with a high degree of vascularity and distinct morphologic characteristics that differentiate it from white adipose tissue. The cells are polygonal and multivacuolated, with granular cytoplasm and a centrally located nucleus.8 The ultrastructural appearance is unique, with a relatively high concentration of mitochondria and a paucity of organelles, such as endoplasmic reticulum and Golgi's complex.4,7 The function of brown adipose tissue is unknown, although its involvement in the regulation of metabolism and thermogenesis has been proposed.8 This tissue is abundant in hibernating animals but has also been described in nonhibernating species, including humans. The interscapular area, neck, mediastinum, and retroperitoneum are the most common areas of distribution. Brown fat accounts for about 1% of the adult human body mass.4,11
The current case report and discussion highlighted the important features of a rare soft tissue tumor. Inhomogeneity on enhanced CT and MRI is suggestive of a malignant process. However, a lipoma variant, such as hibernoma, must be maintained in the differential diagnosis. The rarity and unusual histologic features of hibernoma make diagnosis difficult for all but the most experienced musculoskeletal pathologist. The hypervascularity of this tumor makes preoperative biopsy ill advised. Therefore, the surgeon should consider excisional biopsy to obtain adequate diagnostic material and curative resection in the case of this benign fatty tumor.11
Meticulous hemostasis must be attained through identification of the feeding vessels, thus postoperative hematoma or extracompartmental contamination is avoided if the lesion is identified as malignant.
1. Dale PA, Frassica FJ, Reiman HM, Pritchard DJ: Hibernoma: A case report. Orthopedics 10:1587-1590, 1987.
2. Deseran MW, Seeger LL, Doberneck SA, Eckardt JJ: Case report 840. Skeletal Radiol 23:301-302, 1994.
3. Enzinger FM, Weiss SW: Soft Tissue Tumors. London, CV Mosby 234-241, 1983.
4. Gaffney EF, Hargreaves HK, Semple E, Vellios F: Hibernoma: Distinctive light and electron microscopic features and relationship to brown adipose tissue. Hum Pathol 14:677-687, 1983.
5. Hertzanu Y, Mendelson DB, Louridas G: CT findings in hibernoma of the thigh. J Comput Assist Tomogr 7:1109-1111, 1983.
6. Kransdorf MJ, Moser RP, Meis JM, Meyer CA: Fatcontaining soft-tissue masses of the extremities. Radiographics 11:81-106, 1991.
7. Levine GD: Hibernoma: An electron microscopic study. Hum Pathol 3:351-359, 1972.
8. McLane RC, Meyer LC: Axillary hibernoma: Review of the literature with report of a case examined angiographically. Radiology 127:673-679, 1978.
9. Merkel H: On a pseudolipoma of the breast. Beitr Pathol Anat 39:152-157, 1906.
10. Nigrisoli M, Pietro R, Picci P, Pignatti G. Case Report 489, Skeletal Radiol 17:432-435, 1988.
11. Rigor VU, Goldstone SE, Jones J, et al: Hibernoma: Case report and discussion of a rare tumor. Cancer 57:2207-2211, 1986.