Correspondence

Cardiac hibernoma: a case report

DOI: 10.1111/j.1365-2559.2012.04264.x

� 2012 Blackwell Publishing Limited.

Sir: Brown fat was first observed in 1551 by KonradGessner1 in the interscapular area of an alpine marmot.Since then it has been described in several dormantanimals, but its role in the acclimatization and adap-tation to cold became clear only 50 years ago.2 Thispeculiar type of mitochondria-rich adipose tissue istypical of hibernating animals, but can also be detectedin humans, both as normal and tumoral tissue. Thelatter, unsurprisingly, has been defined as hibernoma.

Hibernomas are rare tumours, and most of theavailable data on this topic come from a large clinico-pathological study by Furlong et al.3 Four main histo-logical subtypes were identified, namely typical (82%),myxoid (9%), lipoma-like (7%) and spindle (2%), withtypical hibernoma cells, either pale or granular, rangingfrom scattered to at least a 50% coverage. There was aslight male predominance for the typical and lipoma-like variants; the median age for all variants was in thethird decade of life (37.1 years), with higher and lowervalues observed, respectively, in the typical (38 years)and myxoid forms (32 years). The most commonanatomical location was represented by the thigh(30%), followed by shoulder (11%), back (10%) and

A B

C D

Figure 1. Radiological and morphological features of cardiac hibernoma. A, The computerized tomography scan showed a low attenuation mass

involving the right atrium and the interatrial septum; B, gross inspection revealed a yellow–brown tumour; C, at histology the lesion was

characterized by a mixture of finely granular ⁄ vacuolated cells, mature adipocytes and cardiac muscle cells (haematoxylin and eosin, ·20); D, nuclear

and cytoplasmic S-100 immunostaining highlighted the finely granular ⁄ vacuolated cells (S-100, ·40).

Histopathology 2012

neck (10%). To the best of our knowledge, a uniquecardiac hibernoma has been reported to date.4

A 51-year-old woman, with no clinical history,presented at our Institution for an acute haemoptysis.A computerized tomography (CT) scan revealed a 1-cmnodule of the lower left pulmonary lobe; in addition, itdisclosed a 4-cm lesion involving almost completely theinteratrial septum and extending into the right atriumnear the superior cava vein (Figure 1A). A MagneticNuclear Resonance (MNR) excluded any relationshipwith the superior cava vein. The patient underwentsurgery. At gross examination the lesion presented as ayellow–brown 3.5-cm tumour with well-defined mar-gins (Figure 1B). At histology the lesion was composedof a mixture of large, finely vacuolated cells (30%)admixed with mature adipocytes (60%) and cardiacmuscle cells (10%) (Figure 1C). Finely vacuolated cellswere characterized by centrally located nuclei, stainedby S-100 (Figure 1D). These findings, as well asnegativity for murine double minute-2 (MDM-2) andCD68, were consistent with a diagnosis of lipoma-likehibernoma. Cardiac muscle cells, small vessels (CD34+)and macrophages (CD68+) were frequently observedintermingled with brown fat.

The seminal paper by Heaton5 described a widedistribution of brown fat in humans; the highestamounts were found in the interscapular, mediastinal,perinephric and neck sites. It was also observed thatthese deposits decreased with advancing age, and thatthe more peripheral areas were the first to lose brownfat as opposed to the more deeply situated deposits.Accordingly, it was thought that hibernoma, thetumoral counterpart of brown adipose tissue, was morefrequent in the mediastinum or neck of young people.Furlong et al.3 partly confirmed this hypothesis [27 of170 (15%) of their cases involved the neck or thechest], but also observed that hibernoma most fre-quently involved the thigh (30% of cases). Herein wereport a case of hibernoma which probably originatesfrom the interatrial septum and further involves theright atrium. Among the fatty lesions of the interatrialseptum, cardiac lipoma was excluded as it usuallyentraps few (if any) cardiac cells and is devoid of brownfat cells.6 The distinction between cardiac hibernomaand lipomatous hypertrophy of the interatrial septum(LHIS) is more difficult. The latter shares the histolog-ical aspect of hibernoma, but presents as a thickeningrather than an overt mass which, by contrast, is thehallmark of hibernoma and the key feature for adifferential diagnosis. The overlapping histology andthe common site of origin suggest a possible relation-ship between hibernoma and LHIS. In keeping with this

hypothesis, it has been observed that the interatrialarea, which normally contains extracardiac fat tissue,6

can also include normal brown adipose tissue.7 Thislatter finding seems to be in keeping with the hypoth-esis of residual brown fat as the origin of the lesion wedescribe herein. This is also supported by a peculiarmorphological aspect of the present case: intermingledwith the hibernoma cells we observed a network ofsmall capillaries, similar to those observed in normalbrown tissue, where the rich net of small capillariesplays a pivotal role in warming the blood duringexposure to cold.

In conclusion, we suggest that hibernoma is includedamong the rare primitive tumours of the heart. Thistumour, which probably arises from residual brown fat,should be kept in mind in differential diagnoses withother benign tumours of this site; namely, either thosewith a granular ⁄ eosinophilic appearance, i.e. rhabd-omyoma and granular cell tumours, or those with alipomatous ⁄ vascular appearance, i.e. lipoma, LHIS andangiolipoma.

Luca Di Tommaso1

Giuseppe Chiesa2

Vincenzo Arena3

Giovanni Guanella2

Carlo Galli4

Massimo Roncalli1

1Department of Pathology, IRCCS Istituto Clinico

Humanitas, Rozzano, Milan, Italy and University

of Milan, Milan, Italy 2Department of Thoracic Surgery,

Cliniche Gavazzeni, Bergamo, Italy 3Department

of Cardiosurgery, Cliniche Gavazzeni, Bergamo, Italy4Department of Pathology, Lodi Hospital, Lodi, Italy

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spectrum of hibernoma A clinicopathologic study of 170 cases.

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very rare cardiac hibernoma in the right atrium: a case report.

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J. Anat. 1972; 1; 35–39.

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atria and interatrial septum. Hum. Pathol. 2006; 37; 1245–1251.

7. Paidisetty S, Blodgett TM. Brown fat: atypical locations and

appearances encountered in PET ⁄ CT. Nuclear Med. Mol. Imaging

2009; 193; 359–366.

2 Correspondence

Histopathology