The morphologic spectrum of hibernoma - A clinicopathologic study of 170 cases

Hibernoma, an uncommon tumor of brown fat, has been described only in a few case reports and small series. The authors reviewed 170 cases of hibernoma and evaluated the morphologic features and the behavior of this tumor. The records from the Soft Tissue Registry of the Armed Forces Institute of Pat hology from 1970 were searched for cases coded as "hibernoma." Clinical inf ormation and available slides from 170 hibernomas were reviewed. Immunohist ochemical staining for S-100 and CD34 was performed on select cases. Follow -up information was obtained from the patients' medical records, the patien ts' physicians, and the patients themselves. Of 170 patients with hibernoma , 99 were men and 71 were women. The tumor occurred most commonly in adults , with a mean age of 38.0 years (age range, 2-75 years). Nine tumors occurr ed in pediatric patients. The most common anatomic locations included the t high (n = 50), shoulder(n = 20), back(n = 17), neck (n = 16), chest(n = 11) , arm (n = 11), and abdominal cavity/retroperitoneum (n = 10). The average duration of the tumor was 30.6 months. Tumor size ranged from 1 to 24 cm wi th an average dimension of 9.3 cm. All tumors were composed partly or princ ipally of coarsely multivacuolated fat cells with small, central;nuclei and no atypia. Four morphologic variants of hibernoma were identified: typical , myxoid, spindle cell, and lipoma-like. "Typical" hibernoma(n = 140) inclu ded eosinophilic cell, pale cell, and mixed cell types based on the tinctor ial quality of the hibernoma cells. The myxoid variant (n = 14) contained a loose basophilic matrix. Spindle cell hibernoma (n = 4) had features of sp indle cell lipoma and hibernoma; all occurred in the neck or scalp. The lip oma-like variant (n = 12) contained only scattered hibernoma cells. Immunoh istochemically, 17 of 20 cases (85%) were positive for S-100 protein. Only one hibernoma of 20, a spindle cell variant, was positive for CD34, whereas other hibernoma variants were negative. Follow-up was obtained for 66 case s (39%) over a mean period of 7.7 years (range, 6 months-28 years). None of the patients with follow-up had a recurrence or metastasis, including eigh t with intramuscular tumors. No patient died of disease. Hibernoma is a tum or found most often in adults and most commonly in the thigh, with several morphologic variants. It is a benign tumor that does not recur with complet e excision. Hibernomas should not be confused with atypical lipomas or well -differentiated liposarcoma.