Ann Thorac Surg 2007;84:1376-1378
© 2007 The Society of Thoracic Surgeons
Case Reports
Subpleural Hibernoma
Paula A. Ugalde, MD,
François Guilbault, MD,
Rosaire Vaillancourt, MD,
Christian Couture, MD, MS*
Thoracic Surgery and Anatomic Pathology and Cytology Services, Laval Hospital, Laval University Institute of Cardiology and Pulmonology, Québec City, Québec, Canada
Accepted for publication May 7, 2007.
* Address correspondence to Dr Couture, Hôpital Laval, 2725, chemin Sainte-Foy, Québec City, Québec, G1V 4G5, Canada (Email: christian.couture{at}ssss.gouv.qc.ca).
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Abstract
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Here we report the case of a 41-year-old man with an asymptomatic, 5-cm, pleural-based mass excised by thoracoscopy. We also review the clinical, radiologic, and pathologic features of hibernomas.
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Introduction
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Hibernomas are uncommon, benign tumors of adipose tissue composed of fetal brown fat cells as first described by Merkl [1] in 1906. Althouh tumors arising from white adipose tissue are among the most common soft tissue lesions, hibernomas are among the rarest [2]. The most common site of involvement is the thigh [3]. Deep-seated hibernomas have been reported in intrathoracic sites, but only less than 10% are believed to have occurred in the thoracic or abdominal cavities. Hibernomas are slow growing, painless tumors for which total excision is advocated [4].
A 41-year-old asymptomatic man was referred for an abnormal chest roentgenogram obtained during a routine clinical examination. Except for a thoracic spine fracture and skull trauma suffered in a motor vehicle accident 14 years earlier, he had no medical or surgical history. Physical examination and laboratory studies were normal.
The chest roentgenogram showed a peripheral mass in the superior sulcus behind the right upper lobe. On computed tomography the mass was centered on the chest wall anterior to the scapula, which measured 4.3 x 2.1 cm, was rounded, homogeneous, not calcified, comprised of fat (low) density, and was not associated with neurovascular infiltration, bony destruction, erosion, or periosteal reaction. With the clinical diagnosis of a primitive intrathoracic mass, which was most likely a benign fatty tumor, and with no confirmed histologic diagnosis, a decision was made to surgically resect the mass.
On video-assisted thoracoscopic surgery using one 12-mm and two 5-mm diameter entry ports, the tumor was visualized under the parietal pleura of the superior and posterior chest wall. The base of the tumor was firmly attached to the posterior portion of the third and fourth ribs with no apparent infiltration of the chest wall and no adhesions to the visceral pleural surface of the lung. The tumor was resected extrapleurally in two separate fragments due to its friability.
The resected specimen consisted of two beige-yellow fragments of 4.3 x 3.0 x 1.6 cm and 2.5 x 1.5 x 1.2 cm with a soft consistency (Fig 1). The largest fragment was partly covered by parietal pleura. On frozen section examination, the mass was moderately cellular and vaguely encapsulated with entrapment of adjacent skeletal muscle fibers. The mass formed sheets of large, round to polygonal cells with abundant pale or eosinophilic, predominantly multi-vacuolated cytoplasm (Fig 2), which was positive for the oil red O stain for lipids. There was no necrosis or mitotic activity. Based on these findings a diagnosis of hibernoma was made. Immunohistochemistry on paraffin-embedded tissue sections confirmed the adipocytic nature of the lesion with S100 protein reactivity. The tumor cells were also weakly positive for AE1/AE3. Immunostains for desmin, cytokeratin CAM5.2, and epithelial membrane antigen (EMA) were negative. The periodic acid-Schiff reagent reaction was negative.
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Fig 2. Histologically the tumor consisted of many multi-vacuolated and few uni-vacuolated (right) fat cells with entrapped skeletal muscle cells (upper left). (Hematoxylin and eosin; x400).
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The postoperative recovery was uncomplicated, and the patient left the hospital the next morning. At 1 year follow-up he is free of symptoms with no sign of recurrence.
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Comment
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In 1906, Merkl [1] described a soft-tissue tumor of brown fat, referring to it as a pseudolipoma. It was only in 1914 that this tumor was named hibernoma due to its multi-vacuolated cells resembling the cells seen in the brown fat of hibernating animals [2]. In a large review of 170 cases from the Armed Forces Institute of Pathology [3], the median age was 38 years (range, 2 to 75 years) with males being more affected at a ratio of 1.4 to 1. At a mean follow-up of 7.7 years, no patients had recurrence, metastases, or died. The most common sites were the thigh (29%), shoulder (20%), back (17%), neck (16%), thorax (11%), upper extremity (11%), abdomen, and retroperitoneum (10%). Specific intrathoracic locations described elsewhere include the mediastinum and the pericardium [4–6], with only very few pleural-based lesions [7–9].
Hibernomas present as painless, slow-growing, progressively enlarging masses that may impinge on neighboring structures to cause symptoms. They consist of heat-generating brown adipose tissue, whereas lipomas originate from heat-insulating white adipose tissue [3, 10]. The skin overlying the tumor may be warm, probably due to the hypervascularity of hibernomas [11].
Although the likelihood of confusion with other tumors is minimal, the differential diagnosis when the tumor is subpleural includes lipoma, localized mesothelioma, neurofibroma, or solitary pleural fibrous tumor [6, 11]. Computed tomography, ultrasonography, and angiography have been used to investigate this tumor [11, 12]. Hibernomas and lipomas have similar signal characteristics on magnetic resonance imaging and computed tomography because of their fat content. However, hibernomas are more heterogeneous due to their different fibrovascular composition. Their computed tomographic density is low with no infiltration of surrounding structures [12]. Contrast enhanced-computed tomographic highlights areas of heterogeneity are helpful in distinguishing hibernomas form lipomas [13]. Angiography also differentiates the highly vascular hibernoma from hypovascular lesions, such as lipomas, fibromas, and neurofibromas; it also defines the vascular anatomy for surgical planning [11]. Due to the risk of hemorrhage, preoperative biopsy seems ill advised. Rather, complete marginal surgical resection with meticulous hemostasis and identification of the feeding vessels is the treatment of choice [14]. Intraoperative tumor fragmentation experienced in this case did not result in hemorrhage. Nonetheless, delicate dissection around the tumor should be attempted.
Histologic variants of hibernoma include typical, myxoid, and lipoma-like. The typical variant is the most frequent and includes three subtypes (1) pale cell, (2) mixed, and (3) eosinophilic cell. The lesion observed here corresponds to the typical variant, mixed subtype. The histologic differential diagnosis includes (1) rhabdomyoma a muscle-derived tumor with glycogen rich, eosinophilic, striated cells; (2) granular cell tumor, distinguishable by a strong Periodic acid Schiff reagent reactivity and lack of cytoplasmic lipids; (3) lipomas, composed of uni-vacuolated cells and much less vascularized; and (4) liposarcoma that shows lipoblasts, high cellularity, cellular atypia, and mitotic activity [3, 10]. The oil red O stain is useful to demonstrate the lipid content of hibernomas. Immunohistochemically, hibernomas are usually reactive for S100 protein and usually show no or little reactivity for keratin, CD68, epithelial membrane antigen, and carcinoembryonic antigen [3, 4]. Electron transmission microscopy discloses multi-vacuolated cells, a high concentration of mitochondria, and paucity of organelles [15]. Despite one case report of a malignant hibernoma based on muscle infiltration, no histologic criteria have been established for malignancy in this soft-tissue tumor [16].
All intrathoracic hibernomas reported in the literature are benign, solitary, and encapsulated tumors. Most are found on incidental routine chest x-ray films. Because it does not recur, and complete surgical excision is the treatment of choice, we consider videothoracoscopic resection the ideal approach as it allows excellent visualization and reliable resection with minimal invasiveness. The rarity and unusual histologic features of hibernoma can make the diagnosis difficult.
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References
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- Merkl H. Uber ein Pseudolipom der Mamma (Eigeuartiger Fetttzellentumor) Beitr Pathol Anat Alleg Pathol 1906;39:152-157.
- Gery L, Bonnel MF. Discussion du Tumeur du Creux de l’Aisselle Bull Men Soc Anat (Paris) 1914;89:110-112.
- Furlong MA, Fanburg-Smith JC, Miettinen M. The morphologic spectrum of hibernoma a clinicopathologic study of 170 cases Am J Surg Pathol 2001;25:809-814.[Medline]
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- Lowry WS, Halmos PB. Malignant tumor of brown fat in patient with Turner’s syndrome Br Med J 1967;4:720-721.[Free Full Text]
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Abstract
Introduction
