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Age / Sex : /
Age/Sex : 55/F
C.C.: Incidentally detected abnormality on health examination program
1) What is your impression?
Courtesy : Ja-Young Choi, MD Department of Radiology, Seoul National University Hospital
Diagnosis: Hibernoma
Discussion
Findings:
- PET shows intense uptake in right posterior shoulder.
- On noncontrast CT, the mass shows slightly lower attenuation than muscles and higher than subcutaneous fat, of which CT attenuation number is measured -32 HU.
- The coronal T1 & T2-weighted images reveal well-defined intermuscular mass between infraspinatus and teres minor muscles, which is slightly hypointense to subcutaneous fat and hyperintense to adjacent muscles with internal tortuous signal void tubular structures. On the coronal T2-weighted images with fat suppression, the signal of the mass is higher than that of subcutaneous fat. The fat-suppressed Gd-enhanced T1-weighted images demonstrate heterogeneous enhancement of the mass.
- USG reveals diffuse hyperechoic mass with hypervascularity.
Discussion:
Hibernoma is a rare, benign, adipose-tissue tumor of adults that is composed partly of multivacuolate adipocytes (MVAs), resembling those seen in brown fat, mixed in variable proportions with univacuolate adipocytes (white fat) similar to those seen in typical lipomas and normal adult adipose tissue. The brown color of the tumor has been attributed to its vascularity and the presence of cells with eosinophilic granular cytoplasm rich in mitochondria. Whereas white fat is important in energy metabolism, heat insulation, and mechanical cushioning, brown fat is believed to be important in non-shivering thermogenesis in hibernating animals and the newborn human, gradually diminishing in volume over the first few decades of life to comprise less than 0.1% the total body weight by age 70 years. Although earlier papers describe most cases in areas of known residual brown fat (neck, shoulder, back and mediastinum), the largest reported series showed 30% in the thigh. Hibernomas have no malignant potential and do not recur if complete excision has been performed, irrespective of location.
On radiographs, the lesion may be visible as a radiolucent mass or swelling without osseous involvement. Large feeding vessels on the surface of a hibernoma have been shown at Doppler ultrasound examination.
In the majority of cases, noncontrast CT demonstrates a fat-containing lesion, although it is important to note that some pathology-proven hibernomas do not show visible fat at CT. Hibernomas usually enhance inhomogeneously; a dominant feeding vessel and/or large intratumoral vessel often becomes visible. Angiographic evaluation shows increased vascularity of the lesion, with early washout via the venous system.
MR imaging has shown some distinguishing features between lipoma-like and non lipoma- like hibernomas. Lipoma-like hibernomas are usually isointense with subcutaneous fat on T1-weighting, are either homogeneous or slightly inhomogeneous and may contain thin tortuous vascular structures. Non-lipoma like hibernomas are predominantly slightly hypointense to subcutaneous fat on T1-weighting, often display marked or moderate inhomogeneity with prominent hypointense nodular and linear foci, and enhancement is typical. On T2-weighted sequences with fat suppression, the signal of the mass is equal to or higher than that of subcutaneous fat. Intense uptake in hibernoma at positron emission tomography (PET) scanning suggests high metabolic activity.
References:
1. Lee JC, Gupta A, Saifuddin A, Flanagan A, Skinner JA, Briggs TW, Cannon SR. Hibernoma: MRI features in eight consecutive cases. Clin Radiol. 2006; 61:1029-34.
2. Ritchie DA, Aniq H, Davies AM, Mangham DC, Helliwell TR. Hibernoma--correlation of histopathology and magnetic-resonance-imaging features in 10 cases. Skeletal Radiol. 2006; 35:579-89.
3. Hardes J, Scheil-Bertram S, Hartwig E, Gebert C, Gosheger G, Schulte M. Sonographic findings of hibernoma. A report of two cases. J Clin Ultrasound 2005; 33:298-301.
4. Chatterton BE, Mensforth D, Coventry BJ, Cohen P, Hibernoma: intense uptake seen on Tc-99m tetrofosmin and FDG positron emission tomographic scanning. Clin Nucl Med 2002; 27:369–370
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