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Clinical findings of hibernoma of the buttock and thigh: rare involvements and extremely high uptake of FDG-PET.

BACKGROUND
Hibernoma is a rare adipose tissue tumor of the soft tissue and the term is derived from the histological similarities to the brown fat found in hibernating animals. It usually occurs in the interscapular area, back, and neck and a few cases have been reported in the buttock or thigh.


CASE REPORT
Two cases are presented, one of which had a lesion in the buttock and the other a lesion in the thigh. The lesion in the buttock extended into the pelvis. CT and MR findings suggested liposarcoma in both cases, but the uptake was extremely intense on 18F-FDG-PET in one case. This was not typical of liposarcoma and suggestive of hibernoma. Biopsy specimens revealed a proliferation of adipose cells with vacuolated granular eosinophilic cytoplasm. No cellular atypia or mitotic figures were observed in either case. A marginal excision was performed in one case and an intralesional excision in the other. There were no signs of local recurrence at the final follow-up.


CONCLUSIONS
While occurrences in the buttock or thigh are exceedingly rare, hibernoma should be included in the differential diagnosis of an adipose tissue tumor in the thigh, even though the imaging findings mimic liposarcoma. 18F-FDG-PET may be a very meaningful technique to differentiate hibernoma from liposarcoma. A correct diagnosis should be established to prevent over-surgery.