Huge progress has been made since the descriptions by Merkel in 1906 and Gery in 1914 [7, 8]. Brown fat is a particular form of fat found in the hibernating and non-hibernating animals [9]. Representing approximatively 1,6% of all benign lipomatous tumors, hibernomas are very slow growing tumors usually occurring between the age of 20 and 50 years [11, 12]. Average age at diagnosis is 38 years and various gender predominance have been described in the literature. Intraosseous hibernomas are found in older patient [13]. The most common location, as the majority of fat tumors, are neck, shoulder and periscapular regions which are areas with possible remaining brown fat tissue [14]. However, hibernomas can occur anywhere in the body [15]. The clinical manifestations are rare, but might occur as a painless soft tissue mass, with a locoregional irritation or neurovascular compression [16]. Conventional radiography demonstrates a radiolucent mass with no adjacent osseous abnormalities or mineralization [17]. Ultrasound describes a rather uniformly hyperechoic mass [5], with increased vascular flow [17] generally unspecific, like other soft tissue tumors. In our case, the diagnosis of a typical lipoma [5] was ruled out due to vascular hyperemia on color doppler ultrasound. On CT scan, hibernoma presents as a homogenous low-density mass of lipidic composition between the attenuation of subcutaneous fat and skeletal muscle [5]. Contrast enhanced CT scan usually shows variable degree of enhancement [16] and intra-tumoral septas [10]. Diagnosis of hibernoma cannot be excluded based on the lack of intra-tumoral vessels [10]. On MRI, the imaging modality of choice [3], hibernoma shows an hypoT1 and hyperT2 signal because of the fatty component alike our case but differs slightly from the fat signal. On a fat-suppressed sequence, attenuation is similar to fatty tissue. After Gadolinium contrast media injection, enhancement is variable but predominantly present and rather heterogeneous [5]; a common sign are large flow voids indicating fast flowing flood in intratumoral vessels. Internal septation, one of the features of hibernoma [3], was absent in our case. High level of FDG uptake has been described in brown adipose tissue [16], sometimes even higher than liposarcoma [10] but overlapping results [2] prevent clear distinction between both tumors. Four types of hibernomas have been described, based on a histological classification [15]: the typical variant, the most common one like our case, the “myxoid” variant, the “lipoma-like” variant and the “spindle-cell” variant. Differences in imaging characteristics among these different histological subtypes of hibernomas are due to the variable amounts of water and fat content. One study distinguished between “lipoma-like” and “non lipoma-like” hibernoma [18]. Non-lipomatous hibernomas are probably indistinguishable from other liposarcomas [17]. Differential diagnosis of hibernoma is very large and may include benign tumors (lipoma, angiolipoma, hemangioma, hemangiopericytoma, hemangioblastoma, xanthoma, granular cell tumor, adult rhabdomyoma, lipoblastoma…) [16, 19] and malignant tumors (liposarcoma, clear cell sarcoma,…) [5, 10, 16]. Clinical and imaging features as previously described may guide the diagnosis, but the histological examination gives the final diagnostic. In the absence of treatment, hibernoma continues to grow and leads to functional or cosmetic consequences [20]. Tumor regrowth has been reported with incomplete excision of hibernoma [19]. The standard of care of hibernoma is surgical removal with clear margins. Surgical excision is advised to rule out a possible malignant lesion with hibernoma-like differentiation which could be missed by core needle biopsy [1]. No risk factor has been described [1]. Multiple lesions in a single patient are possible [21]. After resection, the tumor generally does not recur. Since no malignant transformation or metastatic [22] spread have been described in the current literature, hibernoma can be considered as a benign tumor [1]. Prognosis for patients is excellent [20].

In conclusion, the diagnosis of fatty tumors, often challenging especially with rare tumors like hibernomas, should comprise ultrasound, MRI and large core needle biopsies. Since hibernomas are benign tumors with no tendency to recur or metastasize, surgical resection with clear margins is recommended [17].