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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)                                     

 

Other images: scapulathorax

 

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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