Description

• Lipomas are the most common soft tissue tumor (benign or malignant).
• One of several distinct benign lipomatous tumors. In its most conventional (simple) form, lipomas are composed of mature adipose tissue, typically enveloped by a well-defined thin fibrous capsule.
• Not distinguishable histologically from normal adipose tissue
• Found both superficially and deeply
• Superficial lipomas are found above the waist involving the neck, shoulders, and trunk, followed by proximal limbs.
• Deeper lipomas are uncommon and found subfascially in limbs, head and neck, trunk, abdomen, and mediastinum.
• Several nonconventional forms of lipomas (less commonly encountered) have been described, which may present in multiple locations, contain varying amounts of both adipose and nonadipose tissue, and/or have more atypical cytologic features.
• These nonconventional variants include chondroid lipoma, myolipoma, lipoblastoma, angiolipoma, spindle cell lipoma/pleomorphic lipoma, intramuscular/intermuscular lipoma, lipoma of tendon sheath and joint, lipoma arborescens, lipomatosis of nerve, diffuse lipomatosis, multiple symmetric lipomatosis, adiposis dolorosa, and hibernoma.
• Typically slow growing, superficial (subcutaneous), and asymptomatic; less commonly presents as a painful (angiolipoma) or deep-seated lesion
• May present as an infiltrative lesion (cellular angiolipoma), diffuse lipomatosis, multiple symmetric lipomatosis (Madelung disease), and adiposis dolorosa (Dercum disease)
• More often found in the upper trunk, especially the shoulders, back, neck, and head; but can be found anywhere in the body
• Lipomas have been reported in anatomic locations as varied as cardiac, intrathoracic, endobronchial, retroperitoneal, breast, calf, thigh, scapular, intraosseous, fingers, palmar, toe, epidural, spinal, intra-articular (knee), parapharyngeal, nasopharyngeal, adrenal, inguinal, bladder, scrotal, ovarian, intracranial, intraneural, and GI tract (most often in the ileum).
• Lipomas can cause respiratory distress due to bronchial obstruction of major airways, GI obstruction or bleeding, neuropathic symptoms from compression in locations such as the forearm or ankle, or cord compression if in dural/medullary components of the spinal cord.
• Must be differentiated from other tumors, particularly liposarcomas because treatment protocols differ

Epidemiology

• Lipomas can occur at any age but most commonly occur in middle-aged adults, typically in the 40- to 60-year-old age group, and more likely in men.
• Hibernomas typically present in the 3rd decade.
• Conventional lipomas and most variants are rare in children; however, lipoblastoma and diffuse lipomatosis typically present in children <3 years of age.
• No clear gender predilection in conventional lipomas and most variants
• Females reportedly more commonly diagnosed with chondroid lipoma, myolipoma, and adiposis dolorosa
• Males also reportedly more commonly diagnosed with spindle cell/pleomorphic lipoma, intramuscular/intermuscular lipoma, lipoblastoma, and multiple symmetric lipomatosis
• Approximately 5% of patients have multiple lipomas.

Incidence
Has been reported as 2.1/1,000 individuals per year; however, incidence is difficult to accurately estimate.

Prevalence
Has been reported as present in ~1% the population; however, as with incidence, prevalence is difficult to accurately estimate and is most likely underestimated.

Etiology and Pathophysiology

• The etiology and pathogenesis of lipomas is unclear.
• A pathogenetic link between soft tissue trauma and the formation of lipomas has been suggested.
• Two potential explanations to correlate soft tissue trauma and adipose tissue tumor growth have been proposed:
  • Formation of so-called posttraumatic pseudolipomas by prolapsing adipose tissue through fascia resulting from direct impact
  • Lipoma formation as a result of preadipocyte differentiation and proliferation mediated by cytokine release following soft tissue trauma and hematoma formation

Genetics

• Approximately 2/3 of lipomas demonstrate chromosomal aberrations.
• The most common chromosomal aberrations are rearrangements of the chromatin remodeling gene HMGA2 on chromosome 12q15 (67%).
• Less common chromosomal aberrations include anomalies of the 13q12–22 region (15%) and rearrangements of the 6p21 region (5%).
• Frequent association of chromosomal aberrations in the long arm of chromosomes 13 and 16 with spindle cell/pleomorphic lipomas
• Consistent chromosomal aberrations identified in the long arm of chromosome 11 with hibernomas
• Various case reports of other chromosomal aberrations exist.

Risk Factors

• Because the pathogenesis of lipomas is unclear, risk factors are difficult to qualify.
• Possible risk factors include obesity, alcohol abuse, liver disease, glucose intolerance, and soft tissue trauma (see “Etiology and Pathophysiology”).

Commonly Associated Conditions

May appear as part of a hereditary syndrome: Dercum disease, Madelung syndrome (also known as Launois-Bensaude syndrome), Gardner syndrome, Cowden syndrome, and Bannayan-Riley-Ruvalcaba syndrome (children)