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Soft Tissue Tumors

Adipose tissue

Hibernoma


Reviewer: Vijay Shankar, M.D. (see Reviewers page)
Revised: 9 April 2014, last major update August 2012
Copyright: (c) 2003-2012, PathologyOutlines.com, Inc.

General
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● Lipoma containing prominent brown adipocytes that resembles normal brown fat as classic lipoma resembles white fat

Epidemiology
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● Rare (2% of lipomas)
● Mean age 26-38 years; 60% male

Sites
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● Most commonly in axilla, back, mediastinum, shoulder, thigh (Am J Surg Pathol 2001;25:809); also abdominal cavity, arm, breast, paraglottic space, parasacral region, retroperitoneum, spermatic cord

Clinical features
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● May produce steroid hormones

Case reports
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● 3 month old (Arch Argent Pediatr 2011;109:126)
● 12 year old girl with symptomatic tumor (J Pediatr Hematol Oncol 2008;30:900)
● 25 year old man with mediastinal tumor (J Clin Pathol 2004;57:993)
● 35 year old woman with vulvar tumor (Indian J Pathol Microbiol 2011;54:817)
● 39 year old man with 9 cm thigh mass (Case of the Week #245)
● 52 year old woman with mediastinal tumor (Interact Cardiovasc Thorac Surg 2011;12:845)
● 2 hibernomas with p53 overexpression (Arch Pathol Lab Med 2002;126:975)

Treatment and prognosis
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● Excision
● May recur if incompletely excised

Clinical images
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Supraclavicular tumor

Gross description
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● Mean 9 cm, red/brown cut surface, soft, lobulated, well delineated or encapsulated
● 10% infiltrate adjacent striated muscle
● Brown color may be due to vascularity or mitochondria
● Resembles brown fat in some hibernating animals

Gross images
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Left to right: tumors of thigh, neck, axilla, vulva

Other images: Gluteal tumor

Micro description
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● Organoid arrangement of uniform large cells resembling brown fat with coarsely granular to multivacuolated cytoplasm that is eosinophilic or pale
● Vacuoles are small and stain for neutral fat
● Nucleus is small, central with no/rare atypia
● Often mixtures of white fat
● May have loose basophilic matrix, features of spindle cell lipoma (if in neck/scalp), only scattered hibernoma cells
Subtypes: classic, lipoma-like, myxoid (Ann Diagn Pathol 2006;10:104), spindle cell (CD34+)

Micro images
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Lobulated pattern




Granular to multivacuolated cytoplasm


Mixed with white fat


Vascular changes


Left to right: neck, pleural, mediastinal and vulvar tumors


Multivacuolated tumor cells are S100+


CD31+

Cytology description
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● Small, round, brown fat-like cells with uniform, small cytoplasmic vacuoles and regular, small, round nuclei
● Delicate branching capillaries
● Variable mature fat cells (Cancer 2001;93:206)

Positive stains
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● S100 (85%), Oil Red O and Sudan Black, CD31 (Arch Pathol Lab Med 2006;130:480)
● Also uncoupling protein 1 / UCP-1

Negative stains
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● CD34 (usually), p53 (usually, but see case report above)

Electron microscopy description
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● Resembles brown fat
● Each tumor cell is invested by a basal lamina
● Inverse relationship exists between lipid droplet size and the number of mitochondria per unit of cytoplasm
● Pleomorphic mitochondria with dense matrices or large round mitochondria with transverse lamellar cristae
● Undulating plasmalemmal invaginations, micropinocytotic vesicles, periodic short plasmalemmal densities
● Conspicuous lack of cytoplasmic membrane systems (Hum Pathol 1983;14:677)

Electron microscopy images
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Pleomorphic mitochondria with dense matrices

Molecular/cytogenetics description
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● 11q13-21 rearrangements (also seen in lipomas and liposarcomas)

Differential diagnosis
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Liposarcoma-well differentiated: tumors are deep, have atypia and specific translocations
Lipoma (classic): lipocytes are not multivacuolated
Residual brown fat around cervical or axillary lymph nodes: seen in children, not a distinct mass

Additional references
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Stanford University

End of Soft Tissue Tumors > Adipose tissue > Hibernoma


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