The American Journal of Surgical Pathology 25(6): 809–814, 2001 © 2001 Lippincott Williams & Wilkins, Inc., Philadelphia

The Morphologic Spectrum of A Clinicopathologic Study of 170 Cases

Mary A. Furlong, M.D., Julie C. Fanburg–Smith, M.D., and Markku Miettinen, M.D.

Hibernoma, an uncommon tumor of brown fat, has been de- disease. Hibernoma is a tumor found most often in adults and scribed only in a few case reports and small series. The authors most commonly in the thigh, with several morphologic vari- reviewed 170 cases of hibernoma and evaluated the morpho- ants. It is a benign tumor that does not recur with complete logic features and the behavior of this tumor. The records from excision. should not be confused with atypical the Registry of the Armed Forces Institute of Pa- or well-differentiated . thology from 1970 were searched for cases coded as “hiber- Key Words: Hibernoma—Brown fat—Soft-tissue neoplasm— noma.” Clinical information and available slides from 170 hi- Benign—Adult—Child. bernomas were reviewed. Immunohistochemical staining for S-100 and CD34 was performed on select cases. Follow-up Am J Surg Pathol 25(6): 809–814, 2001. information was obtained from the patients’ medical records, the patients’ 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 Hibernoma is an unusual tumor of brown fat. Thought of 38.0 years (age range, 2–75 years). Nine tumors occurred in pediatric patients. The most common anatomic locations in- to mimic morphologically the brown fat of hibernating -animals, it has been previously called adenoma of seba ,(17 ס back (n ,(20 ס shoulder (n ,(50 ס cluded the thigh (n -and abdominal ceous gland and pseudolipoma.4,16,23,26 Since its first de ,(11 ס arm (n ,(11 ס chest (n ,(16 ס neck (n The average duration of the scription in 1906, only a few case reports and small .(10 ס cavity/retroperitoneum (n tumor was 30.6 months. Tumor size ranged from 1 to 24 cm series have described this rare, somewhat peculiar, tu- with an average dimension of 9.3 cm. All tumors were com- posed partly or principally of coarsely multivacuolated fat cells mor. We present 170 cases of hibernoma, report the with small, central nuclei and no atypia. Four morphologic spectrum of morphologic features, and confirm its be- variants of hibernoma were identified: typical, myxoid, spindle nign behavior. included (140 ס cell, and -like. “Typical” hibernoma (n eosinophilic cell, pale cell, and mixed cell types based on the tinctorial quality of the hibernoma cells. The myxoid variant (n MATERIALS AND METHODS -contained a loose basophilic matrix. Spindle cell hiber (14 ס had features of spindle cell lipoma and hiber- The records from the Soft Tissue Registry of the (4 ס noma (n noma; all occurred in the neck or scalp. The lipoma-like variant Armed Forces Institute of Pathology (AFIP) in Wash- ס (n 12) contained only scattered hibernoma cells. Immuno- ington, DC, USA, from 1970 to the present were histochemically, 17 of 20 cases (85%) were positive for S-100 protein. Only one hibernoma of 20, a spindle cell variant, was searched for cases coded as hibernoma. Clinicopatholog- positive for CD34, whereas other hibernoma variants were ic and demographic information was obtained. Available negative. Follow-up was obtained for 66 cases (39%) over a slides from 175 cases were reviewed. Of the 175 cases, mean period of 7.7 years (range, 6 months–28 years). None of four neck masses from young adults showed a dominant the patients with follow-up had a recurrence or metastasis, neck mass of reactive lymph nodes surrounded by re- including eight with intramuscular tu