Image Quiz

Painless Shoulder Mass in a Fifty-Six-Year-Old Woman (continued)

Diagnosis: Diagnosis: Hibernoma.

Treatment: A complete marginal excision of the lesion was performed. As of the time of writing, there was no recurrence at the latest follow-up of three months.

Fig. 1-A	Fig. 1-B	For larger view, click on image
Fig. 1-A Coronal computed tomographic image with contrast, illustrating the lesion (large arrow) and a few foci of enhancement within the lesion (small arrow). Fig. 1-B Axial computed tomographic image with contrast, showing the lesion abutting the humerus and subcutaneous fat (white arrows). There is a subtle difference in density between the lesion and the subcutaneous fat.

Fig. 2-A	Fig. 2-B	For larger view, click on image
Fig. 2-A Medium-power photomicrograph showing multivacuolated fat cells admixed with univacuolated adipocytes(hematoxylin and eosin; original magnification, ×20). Fig. 2-B High-power photomicrograph showing multivacuolated fat cells with central nuclei and rare nucleoli (hematoxylin and eosin; original magnification, ×40).

Discussion

A hibernoma is a benign soft-tissue tumor that resembles brown fat. Its name is derived from its morphologic similarity to the brown fat found in hibernating animals¹. The lesion is most commonly seen in patients who are in the second through fifth decades of life^2,3, but it can occur at any age. The largest study to date reports a slight male predominance².

Hibernoma typically presents as a large, painless soft-tissue mass that is slow growing^2,4. It is more commonly seen in and around the thigh, shoulder, neck, back, axilla, thoracic region, arm, and retroperitoneum². Other sites of involvement have been reported, including the scalp, breast, spine, and spermatic cord².

While computed tomography and magnetic resonance imaging are commonly used to evaluate soft-tissue masses in musculoskeletal diseases, large series describing specific imaging findings of hibernomas are lacking. Reported findings on computed tomography have described large, well-circumscribed lesions with a subtle difference in density between the lesion and the subcutaneous fat^5-7, as was seen in our patient (Figs. 1-A and 1-B). On T1-weighted magnetic resonance images, hibernomas usually appear as isointense to hypointense to fat (Fig. 3-A). On T2-weighted images, they are typically seen to be isointense compared with the subcutaneous fat, whereas T2-weighted fat-saturated images may demonstrate foci of high signal in the lesion. Mild heterogeneity may be noticed on many sequences. Most computed tomographic and magnetic resonance images of hibernomas demonstrate foci of contrast enhancement (Fig 3-B)^8-11, but this is not always seen¹². These lesions tend to be large and wrap around adjacent structures but without invasion of adjacent muscles or bone. Angiography demonstrates a highly vascular tumor with large, irregular vessels^9,13. FDG PET scan reveals the high metabolic activity of the tumor¹⁴. No evidence of bone erosion, calcification, or periosteal reaction has been reported with hibernomas.

Fig. 3-A	Fig. 3-B	For larger view, click on image
Fig. 3-A T1-weighted coronal magnetic resonance image of a hibernoma in the thigh of another patient (a thirty-five-year-old woman), demonstrating the subtle signal difference between the hibernoma and the subcutaneous fat. The blue arrow points to the junction of the lesion and the subcutaneous fat. Fig. 3-B T1-weighted fat-saturated magnetic resonance image with contrast, showing a few foci of contrast enhancement (black arrowhead) and the similarity between the lesion and the subcutaneous fat. The blue arrows points to the junction of the lesion and the subcutaneous fat.

On gross examination, hibernomas are generally well-circumscribed, yellow-to-brown tumors with lobular arrangements separated by fibrovascular septa. Rare areas of hemorrhage may be noted, but there is no necrosis. The tumors consist of pale to eosinophilic brown fat-like cells which are large, polygonal cells containing numerous, small lipid vacuoles with granular cytoplasm and a small round nucleus (Figs. 2-A and 2-B)^2-4,15. Variable amounts of large, mature adipocytes that are seen in normal white fat and lipomas are present in these tumors as well^2-4,15. No cellular atypia, lipoblasts, mitotic figures, or necrosis is present². A tenuous capillary network often surrounds the brown, fatlike cells^15,16. The prominent vascularity and abundance of mitochondria gives the tumor its brown color¹⁷. Hibernomatous-like areas can be seen in lipomas, although the exact prevalence of this is unknown⁸.

While the typical histologic pattern is rarely mistaken, the large size, unusual sites, and some of the magnetic resonance imaging characteristics may complicate the diagnosis. Hibernoma should be considered as part of a differential diagnosis that includes lipoma variants, atypical lipoma, and low-grade liposarcoma, when magnetic resonance imaging demonstrates a large, lipid-containing mass. Contrast enhancement can help distinguish hibernomas from lipomas; however, contrast is not always given when assessing a possible lipoma and not all hibernomas show contrast enhancement. Careful evaluation of imaging (computed tomography or magnetic resonance imaging) often reveals only a subtle difference in signal of a hibernoma compared with the signal of the subcutaneous fat. A definitive diagnosis can only be made by thorough histologic examination. The lipoma-like variant of hibernoma may be difficult to distinguish histologically from a liposarcoma because the multivacuolated cells can be mistaken for lipoblasts and the myxoid variant of hibernoma may be mistaken histologically for myxoid liposarcoma².

Since these tumors are benign, treatment with complete excision is usually adequate^4,15, although local recurrence has been reported after intralesional surgery^3,18. Excessive bleeding during operative treatment of these tumors has been described³. Although one case report¹⁹ described some areas of the lesion resembling well-differentiated liposarcoma, to our knowledge, malignant transformation potential in hibernomas has not been reported.

*In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from Stryker-Howmedica-Osteonics. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated. Each author certifies that the subject represented in this research has given informed consent for this project, that his or her institution has approved or waived approval for the human protocol for this investigation, and that all investigations were conducted in conformity with ethical principles of research.

References

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Revised June 5, 2007