Brown fat is a form of fetal adipose tissue that may persist in adults. A lipoma formed of brown fat is referred to as a hibernoma.
Usual locations include:
(1) interscapular region
(2) chest wall
(3) axilla
(4) mediastinum
(5) retroperitoneum
(6) thigh
It may be associated with multiple endocrine neoplasia type I.
Grossly it is a benign, well-circumscribed, lipoma-like mass.
Ina routine H&E stain it consists of adipocytes with an eosinophilic cytoplasm with numerous microvacuoles.
Immunohistochemical markers for brown fat:
(1) S100 positive
(2) CD68 negative
(3) UCP-1 (uncoupling protein I) positive (associated with mitochondrial proliferation)
Genetics: mutation in 11q (in region of MEN1 and AIP)
The diagnosis requires exclusion of other tumors.
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Specialty: Hematology Oncology
