steroid cell tumors, fibrothecomas, or any benign tumor with abundant adipose tissue Malignant: Well-differentiated liposarcomas, adrenal cortical carcinomas, clear cell RCCs, or any malignant tumor with abundant adipose tissue Prosthetic groups Tumors with high content of mitochondria or (protoporphyrin cytochromes necessary for redox reactions ring + metal atom) Heme and Benign: Renal oncocytoma, hepatic cytochromes: adenoma, hibernoma
(also carotenes), protoporphyrin + oncocytic tumors in other organs: salivary iron gland (oncocytoma, Warthin tumor), ACTH-producing pituitary adenoma, Hurthle cell adenoma, some parathyroid adenomas?
Tumors composed of brown fat are called hibernomas, a term that originated due to the similarity to brown fat in hibernating animals .
Since these tumors are hypothesized to arise from tiny remnants of fetal brown fat, it is logical that common sites for hibernomas are areas known to contain brown fat in adults [1, 3].
Pure hibernomas, containing brown fat only, are less common than those comprising a combination of white and brown fat.
Given the spectrum of histological features, it is of little surprise that the imaging characteristics are so variable, and on the basis of imaging alone they can be confused with more aggressive liposarcomas; several case reports of hibernomas mimicking liposarcomas have been reported .
can occur in any place where the brown fat remains like our case of a 47 years old man with a large soft tissue mass in the posterior triangle of the neck which was slowly and painlessly growing, while the patient was asymptomatic.
manifest radiographically as radiolucent masses with an absence of calcification, osseous, or chondroid matrix and lacking associated aggressive osseous pathology, (14) as illustrated by our first case.
arising in extramammary sites are usually radiolucent in contrast to the adjacent muscle; however, in fatty breast tissue, this mass is relatively radiodense.
The differential diagnosis includes other primary soft tissue neoplasms, such as rhabdomyoma, hibernoma
, clear cell sarcoma of soft tissue, perivascular epithelioid cell neoplasm (PEComa), paraganglioma, and granular cell tumor.
A granular cell tumor was considered, but such tumors are positive for S100. The patient's age is compatible with a rhabdomyoma, but these lesions present in the head and neck, and have actin-positive cells. "Mulberry cells" of hibernomas
may appear similar to the cells seen in our tissue biopsy, but hibernomas
have S100-positive cytoplasmic rimming, clear lipid droplets, and present less commonly in the retroperitoneum.