Hibernoma of the left axilla; a case report and review of MR imaging.

We present a case of a hibernoma arising from the left axilla in a 24-year-old woman. The imaging characteristics on both contrast-enhanced CT (attenuation similar to surrounding skeletal muscle) and T1-weighted MR imaging (signal intensity intermediate between those of skeletal muscle and subcutaneous fat) were inconsistent with a fatty mass. The diagnosis of hibernoma was only made after marginal excisional biopsy, which confirmed the presence of brown fat on histologic analysis. We briefly present the imaging findings in our case and compare the imaging characteristics of previously reported hibernomas. Although hibernomas classically contain brown fat, imaging characteristics on T1-weighted images typically demonstrate a mass that is hypointense to adjacent subcutaneous fat. Knowledge of its MR imaging characteristics may help to suggest hibernoma in the preoperative diagnosis, although other tumors should still be considered in the differential diagnosis