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Soft Tissue Tumors
Lipoma
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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● Benign tumor composed of mature white adipocytes with uniform nuclei resembling normal white fat
● Most common mesenchymal and soft tissue tumor (100x more common than liposarcoma)
Epidemiology
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● Adults, age 40+; no gender or ethnic preference; rare in children
● Associated with obesity
● Multiple lipomas: 5%, more common in women, often familial, associated with neurofibromatosis, multiple endocrine neoplasia syndromes, Bannayan syndrome (macrocephaly, hemangiomas and lipomas, Wikipedia)
Sites
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● Usually trunk, back, shoulder, neck, proximal extremities
● Rare on hands, feet, face, lower leg, retroperitoneum
● Usually subcutaneous (liposarcomas are usually deep seated)
Clinical
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● Relatively static after initial growth period; don’t regress even with starvation
● Become hard after application of ice
Treatment and prognosis
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● Benign, but 1-4% recur
● Excision is adequate treatment
Gross description
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● Bright yellow homogeneous fat with fine fibrous capsule (superficial lesions only) and trabeculae
● May be very large (particularly if deep)
● Greasy cut surface
Gross images
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Encapsulated mass Fatty mass with surgery-related hemorrhage Cardiac
Subcutaneous Unknown site
Other images: encapsulated mass
Micro description
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● Mature white adipose tissue without atypia
● 2-5x variation in cell size (more than normal white adipose tissue), with obvious large cells up to 300 microns
● Cytoplasmic vacuoles are relatively uniform
● May have intranuclear vacuoles, thickened fibrous septa in buttocks, foot or hand
● May contain areas of fat necrosis with histiocytes, infarct or calcification; rarely contains bone or cartilage
● No mitotic figures
● Note: diagnosis of lipoma requires presence of a mass
Micro images
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Unencapsulated subcutaneous lipoma Mass has normal adult fat cells
Endobronchial lipoma
Lipoma (Virtual slides)
Retroperitoneal lipoma-various
images (Virtual slides)
Positive stains
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● Vimentin, S100, CD34 (slender spindle cells)
● Leptin, PAS (highlights capillaries), reticulin (surrounds each adipocyte)
Electron microscopy
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● Univacuolar mature adipocytes that compress peripheral nuclei (Cancer 1982;50:102), pinocytotic vessels, cells are surrounded by external lamina
Molecular / cytogenetics
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● 55%-75% of solitary lipomas with cytogenetic abnormalities have rearrangements of HMGA2/HMGIC at 12q13-15, although no associated clinicopathologic features are associated with the resulting fusion transcripts (Anticancer Res 2008;28:535)
● Marker ring or giant chromosomes are extremely rare
● Multiple lipomas usually have normal phenotype
Molecular / cytogenetics
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Schematic of HMGA2
and LPP
Differential Diagnoses
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● Normal fat - not circumscribed or encapsulated, not a mass
● Pneumatosis cystoides intestinalis in small bowel - not actually adipocytes
● Mesenchymal cells producing acid mucopolysaccharide - vacuoles contain fluid and are not clear, nuclei are not deformed
Additional references
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● eMedicine #1, #2, Atlas of Genetics and Cytogenetics
End of Soft Tissue Tumors > Lipoma
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