Answer: Hibernoma

Histology: On gross examination, the tumor was lobulated, demarcated, and measured 8 cm in its greatest dimension. Its cut surface was soft, spongy and tan. Microscopically, the tumor was comprised of large numbers of brown fat cells with round centrally placed nuclei, prominent nucleoli and abundant and finely vacuolated (i.e. granular) cytoplasm. Interspersed among these granular cells were occasional cells with coarsely vacuolated cytoplasm representing a transition toward white fat.

Discussion: Hibernoma is an uncommon benign slow growing soft-tissue tumor consisting of brown fat. The term hibernoma relates to its microscopic similarity to brown fat in hibernating animals. Brown fat is believed that the brown adipose tissue has a role in thermoregulation. Hibernomas usually arise from vestiges where brown fat persists beyond fetal life. The soft tissues of the neck, axilla and back are the most commonly involved sites. Hibernomas usually occur in the 3rd and 4th decades. Most tend to come to attention as painless enlarging masses, but they can present with symptoms related to compression of adjacent structures.

Hibernomas are typically fatty vascular lesions that are grossly similar to lipomas. They are well defined, encapsulated and mobile mass. Their color varies from tan to red brown, depending on the amount of intracellular lipid. Microscopically, they are composed of multivacuolated adipocytes and brown fat cells having granular eosinophilic cytoplasm. The background stroma may be myxoid. Provided one is familiar with the histologic appearance of brown fat, there should be little difficulty in recognizing hibernoma and differentiating it from other lipomatous and granular tumors.

The treatment consists in complete surgical resection and local recurrence does not occur. There are no reports of metastases or malignant transformation.