Growing Lipoma Previously Imaged by CT: Next Step
Order an MRI to differentiate between a benign lipoma and an atypical lipomatous tumor (ALT), followed by percutaneous core needle biopsy with MDM-2 amplification analysis if imaging features are concerning. 1
Why MRI is the Appropriate Next Step
The 2025 UK guidelines for soft tissue sarcomas explicitly state that any lipomatous mass that is increasing in size requires advanced imaging to exclude atypical lipomatous tumor (well-differentiated liposarcoma), which has propensity for local recurrence and requires different surgical management than simple lipoma. 1
Key Clinical Reasoning
Growth is a red flag: An enlarging lipoma raises concern for ALT/well-differentiated liposarcoma, particularly if the lesion is deep-seated or located in the lower limb. 1
MRI can differentiate in up to 69% of cases: MRI with expert review can distinguish between benign lipomas and ALT based on specific features including nodularity, septations, stranding, and relative size. 1, 2
CT is insufficient for tissue characterization: While the patient had prior CT imaging, CT cannot reliably differentiate benign lipomas from ALT and is primarily useful for detecting fatty tissue density and assessing retroperitoneal masses. 1, 2
When to Proceed to Biopsy
If MRI shows concerning features or diagnostic uncertainty persists, obtain percutaneous core needle biopsy to analyze for MDM-2 amplification. 1
Rationale for Biopsy
MDM-2 amplification is the defining diagnostic test: Fluorescence in-situ hybridization demonstrating MDM-2 oncogene amplification definitively differentiates lipoma from ALT and will alter the surgical approach. 1
Multiple cores should be obtained: This maximizes diagnostic yield, and the biopsy tract should be planned so it can be safely removed during definitive surgery if needed. 3
Critical Red Flags Present in This Case
Your patient meets multiple criteria requiring urgent evaluation:
- Increasing size (patient-reported growth) 1, 2, 3
- Size >5 cm (if applicable—giant lipomas are defined as >10 cm and represent only 1% of lipomas but warrant heightened concern for malignancy) 2, 3, 4
- Deep location (if applicable—deep-seated lipomas in the lower limb are particularly concerning for ALT) 1
Common Pitfalls to Avoid
Do not rely on ultrasound for deep lipomas: Ultrasound accuracy is considerably less for deep-seated masses compared to superficial ones, and all deep lipomas should raise concern for ALT. 2, 5
Do not assume benignity based on prior imaging: The fact that growth has occurred since the prior CT necessitates re-evaluation, as ALT can be initially mischaracterized as simple lipoma. 1
Do not proceed directly to excision without tissue diagnosis: If ALT is suspected, the surgical approach differs significantly—marginal en bloc resection is appropriate for ALT (even if R1), whereas simple lipoma excision requires different margins. 1
Management Algorithm Based on MRI Findings
If MRI shows typical benign lipoma features:
- Consider observation if asymptomatic and <5 cm 2, 3
- Offer surgical excision if symptomatic, cosmetically concerning, or patient preference 3
If MRI shows concerning features (nodularity, thick septations, stranding):
- Obtain core needle biopsy with MDM-2 analysis 1
- If MDM-2 positive (confirming ALT): refer to sarcoma specialist for en bloc resection 1
- If MDM-2 negative: proceed with standard lipoma excision 1
If MRI is indeterminate:
- Proceed directly to biopsy with MDM-2 analysis to resolve diagnostic uncertainty 1
Special Consideration for Retroperitoneal Location
If this lipoma is retroperitoneal or intra-abdominal, any imaging suggestive of soft tissue sarcoma requires referral to a specialist sarcoma multidisciplinary team before surgical treatment. 1, 2