Hibernoma of the Right Back

By Masazumi Saito, MD; Yoshiro Tsuji, MD; Hiroaki Murata, MD; Kyoseki Kanemitsu, MD; Atsushi Makinodan, MD; Takumi Ikeda, MD; Eiichi Konishi, MD; Toshikazu Kubo, MD
ORTHOPEDICS 2007; 30:495
June 2007

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Hibernoma is a rare benign soft-tissue tumor of brown fat. This tumor has not been demonstrated as a malignant process, but has clinical similarities with malignant tumors, such as liposarcoma. It must be considered in a differential diagnosis when inhomogeneity on enhanced computed tomography (CT) and magnetic resonance imaging (MRI) reveal a malignant process.

This article describes a case of a patient with a hibernoma arising from the right back, and the radiographic and pathologic features of this rare tumor.

Case Report
A 31-year-old man presented in August 2003 with a painless soft-tissue tumor in his right back. He did not recall any remarkable recent trauma in this area. His medical history did not reveal significant information. The patient had been in good health recently.

Laboratory investigation showed only alanine aminotransferase (ALT) elevation. Physical examination revealed a round, flat, nontender mass with a diameter of 8 cm in the median of his right scapula. No overlying skin changes or warmth were noted. The mass was moved easily and did not adhere to the chest wall.

Computed tomography revealed a subcutaneous mass in this area measuring 8×8×1 cm. Its density was slightly higher than that of subcutaneous fat. No bone or chest wall invasion was present. The mass was enhanced diffusely by intravenous enhancement, and a linear high density lesion was observed in the mass (Figure 1).

Magnetic resonance imaging revealed a flat and well-defined mass measuring 9×8.5×1 cm. The lesion was partially hypointense to subcutaneous fat on a T1-weighted image, and isointense on a T2-weighted image (Figure 2A). It was not suppressed in a T2-weighted fat suppression image, so it was distinguished from a simple lipoma (Figure 2B).





Figure 1: Enhanced CT demonstrated a subcutaneous well-defined mass of the right back. Its density was higher than subcutaneous fat. The mass was enhanced diffusely and a linear high density lesion was observed in the mass (white arrow) (white bar=5 cm). Figure 2: MRI of the right back. T2-weighted axial image. A well-defined subcutaneous mass is observed in the right back. The mass is isointense to subcutaneous fat (white bar=5 cm) (A). Sagittal T2-weighted fat suppression MRI. The lesion is inhomogeneous and not suppressed in a fat suppression image (white bar=5 cm) (B).


Gallium-citrate scintigraphy revealed no abnormal uptake in the right back.

As liposarcoma was suspected, mainly from these findings, and open biopsy was performed. The mass had a thin capsule, and the tumor was gray-white. The obtained sample was sent for pathological examination.

The tumor was diagnosed as hibernoma. Microscopic examination showed the tumor to be composed of round or polygonal, multivacuolated cells and mature adipose cells, and many small vessels were observed in the tissues. Atypia of nuclei or mitotic activity were not identified (Figure 3). A marginal tumor resection was performed and the tumor was removed with its capsule. Macroscopically, the mass was gray-tan in color and had a thin capsule and lobulation; invasion into adjacent structures was not seen.

No signs of local recurrence were observed 9 months postoperatively, and the patient was asymptomatic.



Figure 3: Appearance of the tumor in light microscopy. It is composed of mature adipose cells (black arrow) and round or polygonal, multivacuolated cells (open arrow). Atypia of nuclei or mitotic activity are not seen. There are small vessels in these tissues (hematoxilin-eosin x 100).


Discussion
A hibernoma is a rare soft-tissue tumor of brown fat origin, with approximately 100 cases in the literature. Merkel1 first described this tumor as a pseudolipoma in 1906. Gery2 first named this tumor hibernoma in 1914 and recognized its morphologic resemblance to the brown fat of hibernating animals. The tumor generally has been accepted as being benign histologically. However, this rare tumor is clinically important because it is indistinguishable from malignant lesions by CT and MRI.

The peak incidence of hibernoma is in the third decade of life, with a slight predominance of women over men.3 Common sites of origin include the interscapular region, axilla, neck, and the mediastinum—all of which are sites of residual brown fat in adults.4 Clinical presentation usually is a painless, slowly enlarging, and nontender mass.

Imaging characteristics of hibernoma have been described in the literature.3-8 On precontrast CT, a low-attenuation mass is characteristic and its density is higher than that of subcutaneous fat. After intravenous enhancement, this tumor was slightly enhanced diffusely, and septa in the mass were highly enhanced.5

Magnetic resonance imaging can reveal the accurate size, location, and extent of this tumor. Hibernoma is described as a high intensity and well-defined mass in both T1- and T2-weighted images. In most cases, it is reported that the intensity is lower than subcutaneous fat on a T1-weighted image4; thus, this tumor can be distinguished from a simple lipoma. Atilla et al6 reported that the density of a hibernoma was not suppressed on short T1 inversion recovery imaging. However, it is difficult to distinguish hibernoma from a well-differentiated liposarcoma from these MRI findings.

Although not performed in our case, angiography has been reported to be a helpful diagnostic tool.3 Angiography reveals hypervascular masses, so it has been suggested as an effective means of differentiating the highly vascular hibernoma from hypovascular lesions (eg, lipoma, fibroma, and neurofibroma). Angiography is less sensitive in differentiating hibernoma, angiolipoma, and low grade liposarcoma. However, a lack of true neovascularity and arteriovenous shunting suggests a benign process.7,8

The pathology of hibernoma has been reported in the literature.3-5 Macroscopically, hibernomas are brown or gray tumors with a thin capsule. Microscopically, the tumor has a lobular appearance and numerous small vessels are present between the lobules. Brown adipose cells—characterized as polygonal, multivacuolated with granular cytoplasm, and by a centrally located nucleus—are characteristic in hibernoma. Mitosis and atypia of nuclei are not observed.

Hibernomas have been regarded as benign tumors with no potential of recurrence and metastasis after resection. Marginal excision is a sufficient and curative surgical procedure. However, preoperative biopsy is recommended because this tumor is clinically indistinguishable from malignant lesions. Hibernoma is a rare benign soft-tissue tumor that can only be diagnosed histologically.

References
Merkel H. On a pseudolipoma of the breast. Beitr Pathol Ana. 1906; 39:152-157.
Weiss Sw, Goldblum JR. Benign lipomatous tumors. In: Enzinger and Weiss’s Soft-tissue Tumors. St. Louis, Mo: Mosby; 2001:624-628.
Lewandowski PJ, Weinder SD. Hibernoma of the medial thigh: case report and literature review.Clin Orthop Relat Res. 1996; 330:198-201.
Kallas KM, Vaughan L, Haghighi P, Resnick D. Hibernoma of the left axilla; a case report and review of MR imaging. Skeletal Radiol. 2003; 32:290-294.
Alvine G, Rothenthal H, Murphey M, Huntrakoon M. Hibernoma. Skeletal Radiol. 1996; 25:493-496.
Atilla S, Eilenberg SS, Brown JJ. Hibernoma: MRI appearance of a rare tumor. Magn Reson Imaging. 1995; 13:335-337.
Deseran MW, Seeger LL, Doberneck SA, Eckardt JJ. Case Report 840. Hibernoma of the right gracilis muscles. Skeletal Radiol. 1994; 23:301-302.
Nigrisoli M, Ruggieri P, Picci P, Pignatti G. Case report 489. Hibernoma of left thigh. Skeletal Radiol. 1988; 17:432-435.
Authors
Drs Saito, Murata, and Kubo are from the Department of Orthopedics and Dr Konishi is from the Department of Pathology, Graduate School of Medical Science, Kyoto Prefectural University of Medicine; and Drs Tsuji, Kanemitsu, Makinodan, and Ikeda are from the Department of Orthopedics, Nishijin Hospital, Kyoto, Japan.

Correspondence should be addressed to: Masazumi Saito, MD, Dept of Orthopedics, Graduate School of Medical Science, Kyoto Prefectural University of Medicine, Kawaramachi-Hirokoji, Kamigyo-ku, Kyoto 602-8566, Japan.

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