Imaging of Choice for Lipoma
Ultrasound is the first-line imaging modality for suspected superficial or subcutaneous lipomas, preceded by plain radiographs in most cases. 1
Initial Imaging Algorithm
Begin with plain radiographs as the fundamental first step, which can identify intrinsic fat in approximately 11% of soft-tissue masses and detect calcification or bone involvement. 1 However, radiographs are often unrewarding for small, non-mineralized, or deep-seated masses. 1
Proceed immediately to focused ultrasound for any palpable superficial mass, which demonstrates:
- Sensitivity of 86.87-94.1% and specificity of 95.95-99.7% for lipomas 1
- Sensitivity of 94% and specificity of 99% for soft-tissue lesions in the abdominal wall 2
- Highest diagnostic accuracy among all superficial soft-tissue masses 3
Characteristic Ultrasound Features of Lipoma
A typical benign lipoma demonstrates the following features on ultrasound:
- Well-circumscribed, hyperechoic or isoechoic appearance compared to surrounding fat 1, 3
- Thin, curved echogenic lines within an encapsulated mass 1, 3
- Minimal to no internal vascularity on Doppler examination 1, 3
- No acoustic shadowing 1, 3
The ultrasound examination should be focused and limited to lesions superficial to the deep fascia, as including deep lesions markedly reduces diagnostic accuracy. 2
Red Flags Requiring MRI
MRI with and without contrast is mandatory when ANY of the following criteria are present:
- Size greater than 5 cm 1, 2
- Deep (subfascial) location 1
- Rapid growth or recent change in growth rate 1
- Pain or tenderness 1
- Atypical ultrasound features including:
- Lower limb location (heightened concern for atypical lipomatous tumor) 1
- Diagnostic uncertainty or operator uncertainty 1, 2
Role of MRI
MRI is the preferred advanced imaging modality and serves critical functions:
- Can differentiate benign lipomas from atypical lipomatous tumors in up to 69% of cases 1, 4
- Achieves 100% sensitivity for detecting well-differentiated liposarcomas 4
- Achieves 100% specificity for diagnosing simple lipomas 4
- Demonstrates 100% negative predictive value (if MRI shows typical benign features, malignancy is effectively excluded) 4
MRI features that confirm benign lipoma include:
- Homogeneous high T1 and T2 signal intensity 5
- Low STIR signal comparable to normal subcutaneous fat 5
- Uniformly homogeneous signal throughout the mass 5
This specific MRI pattern was present in 25 of 28 benign lipomas and in 0 of 18 malignant lesions in one study. 5
Role of CT
CT has limited utility for lipoma characterization and cannot reliably differentiate benign lipomas from atypical lipomatous tumors. 1 However, CT may be appropriate for:
- Complex anatomic areas 1
- Assessment of mass mineralization 1
- Retroperitoneal or intra-abdominal masses 1
Benign lipomas on CT demonstrate well-delineated, homogeneous masses with negative attenuation coefficients ranging from -95 to -160 Hounsfield units. 6
Critical Pitfalls to Avoid
Ultrasound accuracy declines markedly for deep-seated lipomas compared to superficial lesions. 1 Therefore:
- All deep-seated lipomas require MRI evaluation 1
- All lipomas in the lower limb should raise concern for atypical lipomatous tumor and warrant MRI 1
Any lipomatous mass that is increasing in size requires advanced imaging to exclude atypical lipomatous tumor (well-differentiated liposarcoma), which has different surgical management requirements due to propensity for local recurrence. 1
For retroperitoneal or intra-abdominal masses, CT or MRI is preferred over ultrasound, and any imaging suggestive of soft-tissue sarcoma requires referral to a specialist sarcoma multidisciplinary team before surgical intervention. 1, 2
Management Based on Imaging
Small (<5 cm), asymptomatic, superficial lipomas with typical ultrasound features can be observed with clinical follow-up rather than imaging follow-up. 1
Complete en bloc surgical excision is indicated when the lipoma is symptomatic, rapidly growing, large (>5 cm), or showing atypical features on imaging. 1