Solid Fat Mass Under the Skin: Lipoma
A solid fat mass under the skin is most commonly a lipoma, which is a benign tumor composed of mature fat cells (adipocytes) that typically presents as a soft, rubbery, mobile lump in the subcutaneous tissue. 1, 2, 3
Clinical Characteristics of Lipomas
- Location: Most commonly found on the neck, shoulders, back, abdomen, arms, or thighs 4
- Appearance: Soft, doughy, mobile masses that are usually painless 3
- Size: Typically small (less than 2 cm in diameter), but can grow larger 2
- Growth pattern: Slow-growing 2, 4
- Age of onset: Usually first appear between 40-60 years of age 3
Types and Variants
- Standard lipoma: Encapsulated, composed of mature fat cells
- Giant lipoma: Larger than 10 cm in diameter (approximately 1% of all lipomas) 4
- Variants: May include fibrolipoma, angiolipoma, spindle cell lipoma, and others 3
- Atypical lipomatous tumor (ALT): Also known as well-differentiated liposarcoma (WDL), has potential for local recurrence 1
Diagnostic Approach
Initial Evaluation
- Ultrasound: Recommended first-line imaging modality for evaluating soft tissue masses 5
- High accuracy for superficial soft-tissue masses (sensitivity 94.1%, specificity 99.7%)
- Particularly useful for differentiating lipomas from other lesions
When Further Imaging Is Needed
- MRI: Indicated when ultrasound results are inconclusive or for deep-seated masses 5
- CT scan: Generally not preferred for extremity soft tissue masses unless specific concerns exist 5
Concerning Features Requiring Further Evaluation
- Mass size greater than 5 cm
- Deep-seated location
- Increasing size over time
- Pain associated with the mass 5
Differentiation from Malignancy
It's crucial to differentiate lipomas from liposarcomas or atypical lipomatous tumors:
- Lipoma vs. ALT/WDL: Molecular demonstration by fluorescence in-situ hybridization of MDM-2 cell cycle oncogene amplification can differentiate between these entities 1
- Imaging features: MRI with expert review can differentiate between large, deep lipomas and ALT/WDL in up to 69% of cases 1
- Biopsy: Core needle biopsy should be considered if malignancy is suspected after initial imaging 5
Management
- Observation: Most lipomas can be left alone if asymptomatic 3
- Surgical excision: Indicated for:
- Rapidly growing lipomas
- Painful lipomas
- Cosmetic concerns
- Need to rule out malignancy 3
Follow-up
- Regular clinical monitoring every 6-12 months is recommended if managed conservatively 5
- Patient education on when to return sooner (growth, increased pain, changes in appearance) 5
Pitfalls and Caveats
- Relying solely on physical examination for diagnosis is not recommended, as only 85% of lipomas are correctly identified by physical examination alone 5
- Giant lipomas (>10 cm) require careful evaluation to rule out malignancy before surgical treatment 4
- Some lipomas may undergo changes over time, including calcification or rarely malignant transformation, particularly when present for a long time 2