Hibernoma is a rare, benign tumor originating from fetal brown adipose tissue. Although it is usually found in the interscapular region, it has also been reported in sites where brown fat is less common. Diagnosis of hibernoma is based on histological examination of an excised specimen. Since it is difficult to differentiate preoperatively between hibernoma and other soft-tissue tumors, such as lipoma or, more importantly, liposarcoma, total surgical excision is the treatment of choice. This paper offers what would appear to be the first reported case of pleural hibernoma treated by videothoracoscopic surgery.
Hibernoma is a rare, benign tumor, originating from fetal brown fat. It was first described in 1906 by Merkel as a benign tumor composed of brown adipose tissue; its name was suggested by Gery in 1914, in view of a certain similarity to the multivacuolated cells of animal hibernating glands. The most common location is the subcutaneous tissue of the back, and particularly the interscapular area . Other locations include scalp, neck, larynx, armpit, breast, pericardium, abdominal wall, buttocks, thigh, sperm cord and popliteal fossa; it has also been reported in intracranial, intrathoracic, submandibular, thoracic epidural, lumbar intradural and retroperitoneal locations [2–4]. Peak incidence tends to be between the ages of 30 and 50, and it is marginally more common in females .
Since hibernoma is considered a benign tumor, and there have been no reports of malignant transformation, complete surgical excision remains the treatment of choice; it is curative, and no recurrence has been reported following careful removal of the whole tumor .
A 53-year-old male was admitted to our General and Thoracic Surgery Department with a left pleural mass. There was no significant past medical or surgical history and only mild erratic pain in the left thorax, of some 6 months’ standing.
Chest X-ray showed a nodule measuring roughly 3 cm, in the left hemithorax, between the posterior portion of the sixth costal arch and the scapula; the nodule was peripherally located, and displayed a well-defined inner border and a poorly-defined outer border. Computed axial tomography disclosed an apparently extra-pleural structure of around 3 cm in diameter, with an attenuation coefficient of −110 Hounsfield units (Fig. 1A) .
Thoracoscopic surgery was performed using three 12 mm-diameter entry ports; the parietal pleura mass was completely excised (Fig. 1B), and a Pleurecath® pleural catheter was implanted, which was removed after 3 days. Grossly, the surgical specimen was soft, ovoid in shape, encapsulated, with a maximum diameter of 3.5 cm; the cut surface was yellowish, with a central, whitish, star-shaped scar (Fig. 2) . The intraoperative diagnosis was pleural oncocytic tumor, but definitive histological examination showed a well-circumscribed and encapsulated, tumor. Microscopically, there were three types of cells: (a) large, multivacuolated cells with eosinophilic granular cytoplasm; (b) small, rounded cells with granular cytoplasm; and (c) large, univacuolated cells with peripheral nuclei, resembling mature adipocytes. There was no evidence of mitosis or cellular atypia. The final diagnosis was hibernoma.
Postoperative progress was complication-free, and the patient was discharged from hospital on the fourth postoperative day. Two years later, he remains free of symptoms, with no signs of recurrence.
Hibernoma is a benign soft-tissue tumor deriving from vestigial remnants of so-called brown fat. This highly-specialized form of adipose tissue was first described in hibernating animals, but has subsequently been reported in over 50 animal species, including non-hibernating animals such as rats, rabbits, monkeys, as well as man. Usually the amount of brown adipose tissue declines after birth, thus accounting for only 1% of total fatty tissue in adults .
Hibernoma is a rare tumor. Histological examination reveals a well-circumscribed, encapsulated, lobulated and homogeneous tumor, reddish-brown in color and with a rich vascular network. Microscopically, the tumor displays a lobular arrangement of rounded or polygonal cells classifiable in three types: (a) large, multivacuolated cells with eosinophilic granular cytoplasm; (b) small, rounded cells with granular cytoplasm; and (c) large, univacuolated cells with peripheral nuclei, resembling mature adipocytes. There is no evidence of mitosis or cellular atypia. A rich capillary network is generally found. Electron microscopy discloses multivacuolated cells containing abundant, large, pleomorphic mitochondria, with clearly-visible cristae and dense matrix; Golgi apparatus and endoplasmic reticulum are rarely seen .
Few intrathoracic cases have been reported, and even fewer involving pleural locations; it is more frequently found in mediastinum and pericardium [4,8].
In one of the largest published series, comprising 32 cases with a mean age of 26 years, Enzinger and Weiss  report the most common location to be interscapular subcutaneous tissue. Where the tumor is more deep-seated or concealed, the mean patient age tends to be higher, as was the case here; this is probably because greater difficulty in locating the tumor, coupled with the absence of symptoms, means that it goes unnoticed over a much longer period.
Some authors  argue that intrathoracic lesions with fatty content can be identified by computed axial tomography or magnetic resonance imaging, on the basis of the attenuation coefficient, and thus recommend a ‘watch-and-wait’ approach with regular check-ups. However, no studies to date have reported an effective system for distinguishing between a hibernoma and a liposarcoma or other soft-tissue tumor with fatty content; the authors therefore feel that complete surgical excision is the only reliable approach.
One histological analysis has been performed, there is little likelihood of confusion with other tumors; nevertheless, differential diagnosis is advisable, in order to rule out a number of tumors , in including: lipoma, which displays far fewer vascular structures than hibernoma; adult rhabdomyoma, in which eosinophilic cells, though also present, are larger and rich in glycogen, and also display characteristic banding; chondroid lipoma, recognizable by the presence of chondroid material and by cell pleomorphism not found in hibernoma; granular cell tumor, distinguishable by the complete absence of lipid vacuoles in cytoplasm; and finally, renal carcinoma metastasis which raise doubts but is immunohistologically different.
Given that hibernoma is a benign tumor, which does not recur following surgical excision, it is felt that excision should be effected with as little aggression as possible for the patient. Videothoracoscopic excision is advisable for intrathoracic hibernoma, where tumor size makes it feasible, as in the case reported here. This approach ensures rapid patient recovery, less postoperative pain and a reliable histological diagnosis.