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Soft Tissue Tumors
Hibernoma
Author: Nat Pernick, M.D., PathologyOutlines.com, Inc.
Reviewer: David Lucas, M.D., University of Michigan Health Systems (January 2009)
Revised: 26 June 2009, last major update June 2009
Definition
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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 thigh, shoulder, back, neck, axilla, mediastinum (AJSP 2001;25:809)
Clinical
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● May produce steroid hormones
Radiologic images
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MRI of axillary tumor
Case reports
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● 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)
● 42 year old woman with elbow tumor (Surgical Rounds, Nov 2007)
● 45 year old woman with neck tumor (Internet Journal of Surgery 2007;7(2))
● 2 hibernomas with p53 overexpression (Archives 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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Elbow tumor Neck tumor Thigh tumor
Richly vascularized tumor Light brown tumor
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
Granular to multivacuolated cytoplasm
Mixed with white fat Central indented nuclei Various images
Vascular changes Neck tumor Pleural tumor
Multivacuolated tumor cells are S100+ Oil Red O stain
CD31 (Fig 4)
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 (Archives 2006;130:480)
Negative stains
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● CD34 (usually), p53 (usually, but see case report above)
Electron microscopy
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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 Path 1983;14:677)
Electron microscopy images
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Pleomorphic mitochondria
Molecular / cytogenetics
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● 11q13-21 rearrangements (also seen in lipomas and liposarcomas)
Differential Diagnosis
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● Liposarcoma - tumors are deep, have atypia and specific translocations
● Classic lipoma - 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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End of Soft Tissue Tumors > Hibernoma
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