Hibernoma – two patients with a rare lipoid soft-tissue tumour

Hibernomas are rare benign soft-tissue tumours arising from brown fat tissue [1-3]. The tumour was first described in 1906 by Merkel who named them “pseudolipoma” [4]. The name hibernoma was given by Gery in 1914 because of its similarity to brown fat in hibernating animals [5]. Hibernomas count for approximately 1.6% of all benign lipomatous tumours [6]. Until now, case reports and series with up to 170 patients were published [7]. Males are more often involved than females [7]. The typical age at presentation is between 30 and 50 years. Hibernomas are slowly growing, painless tumours, either presenting as palpable soft and mobile mass or as an incidental finding at imaging [3]. Due to the hypervascularity, hibernomas may sometimes appear with locally increased skin temperature. They can reach a size of up to 20 cm so that symptoms secondary to compression of adjacent structures may developed [7].

The aetiology of hibernomas is unknown. Molecular genetics show deletions respectively reciprocal translocation of chromosome 11 which encodes the tumour suppressor gene MEN 1 [8,9]. The development of hibernomas was initially thought to be related to areas of residual brown fat such as the interscapular area, axilla, chest wall, mediastinum or retroperitoneum. However, single case reports and a clinical-pathological study with 170 cases showed the thigh to be a preferential location with about 30% of the cases [7].

At histopathology, most hibernomas present as typical variant (82%) of which three cellular subtypes are differentiated (pale staining, mixed and eosinophilic cells). Myxoid variants (9%), lipoma-like variants (7%), and spindle cell variants (2%) of hibernoma are less common [3,7].

Except intrathoracic and retroperitoneal hibernomas, the tumours are usually well diagnosed with ultrasound showing smooth borders and hyperechoic signal. Colour duplex sonography and angiography often present a hypervascular tumour, with arteriovenous shunts in some cases. Due to the increased risk of bleeding, biopsy is not recommended in these cases. But often the diagnosis is not considered prior to biopsy in different imaging modalities, as presented in the two following cases [3,10,11].

In computed tomography (CT) and magnetic resonance imaging (MRI), hibernomas are well-circumscribed tumours, commonly located in subcutaneous tissue, skeletal muscle or intermuscular fascial planes. Common sites include thigh, peri- and interscapular region, axilla, neck, chest, abdominal cavity and retroperitoneum. The scalp, breast, scrotum and perirectal region, spine and bones are less frequently affected [3,7,12]. Hibernomas show relatively low attenuation in CT between the attenuation of subcutaneous fat and skeletal muscle. Intratumoural septa and a variable degree of enhancement may be seen [1,13,14]. In MRI, hibernomas may be slightly hypointense to fatty tissue on T1- and T2-weighted images and markedly hyperintense when fat suppressed T2-weighted inversion recovery sequences are applied. Elongated and branching vessels within the tumour are typically seen as flow void phenomena in MRI. Nevertheless, lack of intratumoural vessels does not exclude hibernoma [2,3,10]. After intravenous contrast agent administration strong enhancement of intratumoural blood vessels and homogeneous or heterogeneous tumour enhancement may be observed [10]. In ¹⁸ F-fluorodeoxyglucose (FDG) positron emission tomography-computed tomography (PET/CT) high uptake is measured in hibernomas due to the metabolic activity of brown fat. Therefore, differentiation from malignant tumours is difficult [3,10,15,16]. Compared to liposarcomas, hibernomas often show higher standardized uptake values (SUVs) and change of FDG avidity in follow-up. However, these criteria are not reliable enough to make the diagnosis based on imaging alone [16,17].

Hibernomas are benign tumours without a risk of malignant transformation or metastases [7,18]. As soon as the diagnosis is confirmed, no surgical removal or other treatment is required in asymptomatic hibernoma [16]. However, histological features similar to hibernomas are described in atypical fatty tumours as well as in myxoid and well-differentiated liposarcomas. Consequently, biopsy proven diagnosis of hibernoma does not exclude hibernoma-like differentiation in myxoid or well-differentiated liposarcomas and complete excision is indicated. Incomplete resection of hibernoma carries the risk of recurrence, whereas complete resection cures the disease [7,19].

Hibernomas are benign, well-circumscribed, slowly growing and well vascularised soft-tissue tumours containing brown fat [1-3,7]. MRI and ¹⁸ F-FDG-PET may provide imaging findings suggestive of hibernoma. These findings include hypointensity to fatty tissue on T2-weighted images and hyperintense signal intensity on TIRM images. Large intratumoural blood vessels, heterogeneous contrast enhancement and strong FDG uptake are further imaging signs [2,3,10,15,16].

The differential diagnosis of lipomatous soft-tissue tumours is broad and comprises benign (e. g. lipoma, hemangioma, angiolipoma) and malignant (e. g. liposarcoma) lesions [1,14,20]. Lipomas are characterized by homogeneous adipose tissue which can show thin septa. There is no contrast enhancement within lipomas, although infections, infarctions and necrosis as well as muscle fibres, intratumoural vessels und fibrotic septa may hamper the diagnosis [21]. Dependent on fat content, haemangiomas and angiolipomas show hypo- to hyperintense signal on T1-weighted images and hyperintense signal on T2-weighted images with intrinsic vessels. Haemangiomas can also contain calcifications similar to phlebolits [10]. Imaging characteristics of liposarcomas depends on the histological differentiation. Well-differentiated liposarcomas have thickened septa (>2 mm) as well as areas of T2-hyperintensity. Enhancement can be absent or circumscribed [10,20,21]. The signal intensity on T1-weighted images is often similar to subcutaneous fat. Moreover, large vessels are lacking which helps to distinguish liposarcomas from hibernomas. Poorly differentiated liposarcomas are rarely confounded with hibernomas due to the low fat content [20,21].

However, despite the use of multimodal imaging, the radiologic diagnosis of hibernomas is difficult and differentiation from well-differentiated liposarcomas remains a challenge even after biopsy. Therefore, complete surgical resection is highly recommended. A summary table of hibernomas is shown in Table 1.