Hibernoma

Clinical History

This patient noticed a swelling on the medial aspect of left knee after blunt trauma. On examination, a soft lobulated painless swelling was noted on the medial aspect of left knee. Plain radiographs of the knee and all laboratory tests were normal.

Imaging Findings

The patient noticed a swelling on the medial aspect of left knee after blunt trauma. On examination, a soft lobulated painless swelling was noted on the medial aspect of left knee. Plain radiographs of the knee and all laboratory tests were normal.
An ultrasound (Fig. 1) showed well-defined homogeneous hyperechoic lesion within the subcutaneous fat.
MRI of the left knee included axial(a) and sagittal(b) T1-W; coronal(c) T2-W; axial STIR(d) and T1-W post Gadolinium fat saturated (e) images (Fig. 2). All these images show a well-defined homogeneous lobulated mass in the medial subcutaneous tissue of the knee measuring approximately 4 x 5x 1.5 cm. On T2-W, the lesion displays a hyperintense signal intensity (SI) greater than that of subcutaneous fat and on T1-W, the lesion also shows hyperintense signal in-between the intensities of subcutaneous fat and muscle. On the STIR sequence, the lesion displays a similar signal intensity to muscle. Following intravenous contrast, the post-Gadolinium DTPA fat saturated T1-W image displays no significant enhancement.
Histology revealed a lipoma with hibernoma-like features.

Discussion

Hibernoma is a rare benign tumour of brown fat most commonly found in the peri-scapular region and to lesser extent in the thigh, neck and chest wall soft tissue. There is a wide range for age at presentation but the majority present in their third and fourth decades with a slight male prevalence. Pure hibernomas are rare, most hibernomas contain a mixture of white and brown (hibernomatous) adipose tissue. Lesions usually present with a slow growing painless mass. They are generally well-circumscribed, encapsulated lesions that range in size from 1 – 30cm with a mean of 9.3cm. Unlike usual lipomas that are composed of large fat cells with a single cytoplasmic fat vacuole and a peripheral nucleus, hibernomas vary in their histological appearance depending on the relative amount of multivacuolated, mitochondria-rich, brown fat cells, associated small capillary proliferation and stromal background (Fig.3). Six histological variants are described (Mietinnen et al., 2002) that are not of prognostic value although they may show different MR signals. The most characteristic variant of hibernoma (eosinophilic variant) contains large numbers of multivacuolated cells with abundant granular, eosinophilic cytoplasm and a small central nucleus. Some cases are composed entirely of pale cells - the pale variant, and others have a mixture of pale and eosinophilic cells, the mixed variant. Most commonly, hibernomas contain small clusters of brown fat amidst ordinary white fat cells - the lipoma-like variant. Myxoid and spindle cell variants are rare. Mitoses are not seen and cytological atypia is most unusual.
The MR imaging characteristics of hibernoma reflect the variable histology and depend on the relative amounts and distribution of brown and white fat, capillary proliferation and stromal background.
The MR images demonstrate well-defined, lobulated lesions. The lipoma-like hibernomas have a more homogeneous appearance with similar or slightly reduced signal intensity (SI) on T1-WSE compared with subcutaneous fat and similar or slightly increased signal on T2-WSE. Fine internal low intensity fibrous strands are common and easily depicted against the surrounding fat. On STIR, lesions display an intermediate signal intensity higher than that of subcutaneous fat and similar to muscle. Mixed lesions (pale and eosinophilic brown fat cells) display a more inhomogeneous appearance with smaller amounts of slightly altered fatty tissue and more non-specific linear or mass-like components displaying low SI on T1-W and high SI on T2-W and STIR.
Hibernomatous tissue enhances avidly but the overall enhancement of the lesion will depend on the amount of hibernomatous tissue within the lesion. In lipoma-like hibernomas the relative amount of hibernomatous tissue is small, therefore enhancement may be negligible or absent as in this case.

Differential Diagnosis List

Final Diagnosis

Hibernoma

URL:	https://www.eurorad.org/case/2463
DOI:	10.1594/EURORAD/CASE.2463
ISSN:	1563-4086