Q: Why does fat have lower attenuation on CT than water? show answer
Q: What is the difference between the terms "hypodense" and "hypoattenuating"? show answer
Patient presented with nonspecific abdominal pain. No past medical history. The imaging finding was incidental.
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There is a relatively well-marginated mass between the right gluteus maximus and gluteus medius muscles. The attenuation of the mass is between fat and muscle and there is a prominent vessel entering the mass at the lateral margin.
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Similar to the CT study, there is a well-marginated mass between the right gluteus maximus and medius muscles. There are prominent signal voids at the lateral aspect of the mass, compatible with feeding vessels. Although slightly hypointense relative to fat on both the T1 and T2-weighted sequences, it is slightly hyperintense on the T1 fat sat sequence (it's about the same intensity as the adjacent muscle).
The mass shows moderate enhancement on the postcontrast images, greater than that of muscle.
This mass shows classic features for a rare brown fat tumour called a "hibernoma", including attenuation values between that of fat and muscle, and a prominent feeding vessel. The thigh is also a common location. An imaging differential includes liposarcoma, so if the mass is biopsied, it should be biopsied in a manner that would allow a surgical excision (i.e. avoiding a needle track that would be outside of a resection margin, to avoid seeding).
A biopsy of a hibernoma would show characteristic brown fat. A cytogenic analysis that shows an 11q13 translocation helps with a diagnosis of hibernoma over liposarcoma 2.
The mass in this patient was excised without prior biopsy and was shown on histology to be a hibernoma (much to the relief of the patient).
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