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  • A common benign tumor of mature fat
    • may be subcutaneous, extramuscular, or intramuscular 
  • Epidemiology
    • demographics
      • slightly more common in men
      • affects predominantly patients between 40-60 years old 
      • develops in sedentary individuals
    • location
      • superficial/subcutaneous location is common
        • superficial lesions are common in the upper back, thighs, buttocks, shoulders and arms
        • deep lesions are affixed to surrounding muscle, in the thighs, shoulders and arms
      • ~5-10% of patients with a known superficial lipoma, will have multiple lesions
  • Prognosis
    • size typically plateaus after initial growth
  • Common variants of lipoma include
    • spindle cell lipoma
      • common in male patients ages 45-65 years
    • pleomorphic lipoma
      • common in middle aged patients
      • may be confused with liposarcomas
    • angiolipoma
      • unique in that it is painful when palpated
      • often present with small nodules in the upper extremity
    • intramuscular lipoma
      • often symptomatic and require marginal resection
    • hibernoma 
      • tumor of brown fat
      • affects younger patients (20-40 years old) 
  • Symptoms
    • usually a painless mass that has been present for a long time
      • exception is the angiolipoma, which is painful when palpated
  • Physical exam
    • palpable, mobile, painless lesion
  • Radiographs
    • may show a radiolucent lesion in the soft tissues
    • may see mineralization, which should raise concern for synovial cell sarcoma
      • may see calcifications or presence of bone within the lesion
  • CT scan
    • well demarcated lesion
    • lesion looks akin to subcutaneous fat 
  • MRI 
    • well demarcated lesion
    • homogenous, signal intensity matches adipose tissue on all image sequencing
    • shows well demarcated lesion with same characteristics as mature fat  
      • high signal intensity on T1 weighted images   
      • high signal intensity on T2 weighted images, entirely suppressed by STIR or fat saturated sequences
      • low signal intensity on STIR image 
  • Biopsy often not necessary as diagnosis can be made by imaging (MRI)
  • Gross appearance
    • Lipomas are soft, lobular, may be encapsulated and whitish/yellowish in color
    • Hibernoma are reddish brown because of rich vascular supply in addition to high numbers of mitochondria 
  • Histology in general shows bland acellular stroma with neoplastic cells that lack cellular atypia.
  • Histology varies by variant
    • spindle cell lipoma
      • mixture of mature fat cells and spindle cells
      • mucoid matrix with varying number of birefringent collagen fibers
    • pleomorphic lipoma
      • lipocytes, spindle cells, and scattered atypical giant cells
    • angiolipoma
      • mature fat cells with nests of small arborizing vessels
    • intramuscular lipoma
      • pathology shows lipoblasts and muscle infiltration 
  • Nonoperative
    • observation only
      • indications
        •  lesion is painless and MRI is determinate for a benign fatty lesion
  • Operative
    • marginal resection (may be intralesional) 
      • indications 
        • symptomatic lesions
        • mass is rapidly growing 
        • tumors located deep to the fascia or in the retroperitoneum
          • deep or retroperitoneal lipomas show a higher likelyhood to be/become atypical lipomatous tumors 
          • in the retroperitoneum, referred to as well-differentiated liposarcoma
          • in the extremities, referred to as atypical lipomas 
        • spindle cell/pleomorphic lipomas are treated by marginal resection
  • Local recurrence
    • uncommon (< 5%) 

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