This mass shows classic features for a rare brown fat tumor 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).