What is the appropriate workup for a patient presenting with a lipoma (benign tumor composed of fat tissue)?

Workup for Lipoma

Begin with plain radiographs followed by ultrasound as first-line imaging for suspected superficial lipomas, reserving MRI for lesions >5 cm, deep-seated masses, rapidly growing tumors, or those with atypical ultrasound features. 1

Initial Clinical Assessment

Evaluate for specific red flags that mandate advanced workup:

• Size >5 cm in any dimension 1, 2
• Deep location (subfascial or intramuscular) 1
• Rapid growth or recent size change 1, 2
• Pain or tenderness 1, 2
• Location in lower limb or retroperitoneum (higher risk for atypical lipomatous tumor) 1

Imaging Algorithm

Step 1: Plain Radiographs

• Obtain radiographs first to identify intrinsic fat (detectable in ~11% of soft tissue masses) and assess for calcification or bone involvement 1, 3
• Radiographs may be unrewarding for small, deep-seated, or non-mineralized masses 1, 3

Step 2: Ultrasound (First-Line for Superficial Lesions)

• Ultrasound has excellent diagnostic accuracy: sensitivity 86.87-94.1% and specificity 95.95-99.7% 1, 2
• Typical benign features include:
  • Well-circumscribed, hyperechoic or isoechoic appearance compared to surrounding fat 1, 3
  • Thin, curved echogenic lines within encapsulated mass 1, 3
  • Minimal to no internal vascularity on Doppler 1, 3
  • No acoustic shadowing 3

Step 3: MRI (For High-Risk Features)

Obtain MRI when any of the following are present:

• Size >5 cm 1
• Deep-seated location 1
• Atypical ultrasound features (nodularity, thick septations, stranding) 4, 1
• Rapid growth 1
• Diagnostic uncertainty on ultrasound 1

MRI can differentiate benign lipomas from atypical lipomatous tumors (ALT/well-differentiated liposarcoma) in up to 69% of cases based on nodularity, septations, stranding, and relative size 4, 1

Biopsy Indications

Proceed to percutaneous core needle biopsy with MDM-2 amplification analysis when:

• MRI shows concerning features (nodularity, thick septations, stranding) 4, 1
• MRI is indeterminate and cannot reliably exclude ALT 4, 1
• Deep lipomas in lower limb (heightened ALT concern) 1

MDM-2 amplification by fluorescence in-situ hybridization is the defining diagnostic test to differentiate lipoma from ALT and will alter surgical approach 4, 1

Critical Pitfalls to Avoid

• Never rely on physical examination alone - it correctly identifies only 85% of lipomas 2, 3
• Ultrasound is considerably less accurate for deep lipomas - all deep-seated lipomas require MRI evaluation 1
• Do not use CT for tissue characterization - CT is insufficient to differentiate benign lipomas from ALT, though useful for retroperitoneal masses 1
• Plan biopsy tracts carefully - ensure the tract can be safely removed during definitive surgery if malignancy is found 2

Special Anatomic Considerations

Retroperitoneal or Intra-abdominal Masses

• CT or MRI is preferred over ultrasound 1
• Any retroperitoneal mass with imaging suggestive of soft tissue sarcoma requires referral to specialist sarcoma multidisciplinary team (MDT) before surgical treatment 1

Lower Limb Deep Lipomas

• Heightened concern for ALT - proceed directly to MRI evaluation 1
• If MDM-2 positive (confirming ALT), refer to sarcoma specialist for en bloc resection 1

Management Based on Workup Results

Small (<5 cm), asymptomatic, superficial lipomas with typical ultrasound features:

• Observation with clinical follow-up (imaging follow-up not required) 1

Symptomatic, rapidly growing, large (>5 cm), or atypical features:

• Complete en bloc surgical excision 1, 2

Confirmed ALT (MDM-2 positive):

• Refer to sarcoma MDT for marginal en bloc resection preserving neurovascular structures 4, 1
• Adjuvant radiotherapy may be considered for larger tumors or difficult-to-achieve clear margins 4