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Soft Tissue Tumors

Intramuscular 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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● Lipoma that infiltrates within muscle belly

● Called intermuscular lipoma if between muscle fascicles

Terminology

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● Also called invasive or infiltrative lipoma

● Not a WHO diagnosis

Sites

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● Intramuscular: affects thigh, trunk, head and neck of middle-aged adults

● Intermuscular: affects abdominal wall of middle-aged adults; doesn’t recur if completely excised

Clinical

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● Benign, but 20% recur due to incomplete excision

● Infiltration may be due to muscular degeneration and endomysial fatty growth secondary to atrophy (BMC Musculoskelet Disord 2004;18:20)

Case reports

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● 37 year old woman with giant infiltrative tumor of face (AJNR Am J Neuroradiol 2003;24:283)

● 58 year old woman with tumor of pectoralis major muscle simulating a breast mass (Ann Acad Med Singapore 2005;34:275)

● Well circumscribed tumor of sternocleidomastoid muscle (Auris Nasus Larynx 2004;31:283)

Clinical images

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Tumor of face     

Gross description

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● Intramuscular - poorly circumscribed, extends between and around skeletal muscle fibers

Gross images

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Yellow adipose tissue within skeletal muscle                           

Irregular lobulated tumor without                  Deep tumor of calf

encapsulation

Other images: yellow adipose tissue within skeletal muscle

Micro description

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● Mature lipocytes that infiltrate skeletal muscle

● No atypia, no mitotic figures

Micro images

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Prominent muscular atrophy                          Adipocytes and atrophic muscle

Adipocytes and atrophic muscle

Adipocytes and atrophic muscle                  

Other images: intramuscular fat tissue covered by fibrous capsule

Differential Diagnoses

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● Intramuscular hemangioma - adolescents and young adults, usually head and neck, prominent vasculature, variable adipose tissue

● Well differentiated liposarcoma - definite atypia present

End of Soft Tissue Tumors > Intramuscular lipoma

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