What is a lipoma, specifically in an adult patient with one located in the popliteal fossa?

What is a Lipoma?

A lipoma is a benign tumor composed of mature adipocytes (fat cells) encapsulated in fibrous tissue that typically presents as a soft, painless, slow-growing mass that can occur anywhere in the body where adipose tissue exists. 1, 2

Composition and Pathology

• Lipomas consist of mature adipocytes that are relatively uniform in size and lack cytologic atypia 3
• They are the most common benign mesenchymal tumors and are definitively composed of mature fat cells, not fibrous tissue 3, 1
• Most lipomas are small (typically <2 cm diameter, weighing only a few grams), though "giant" lipomas are defined as ≥10 cm in diameter or weighing ≥1,000 grams 1, 4

Clinical Characteristics

• Lipomas present as soft, rubbery lumps that are usually painless and move easily when touched 2
• They are slow-growing and typically asymptomatic, though large lipomas can cause functional limitations including lymphedema, pain syndromes, or nerve compression 1, 4
• Physical examination alone correctly identifies only about 85% of lipomas, highlighting the importance of diagnostic imaging 3

Location and Distribution

• Lipomas can develop in any region containing adipose tissue, most commonly in the subcutaneous tissue of the trunk, neck, shoulders, back, abdomen, arms, and thighs 1, 2
• They can be classified by anatomical location as: subcutaneous, subfascial, intramuscular, subserous, submucous, intra-articular, or intraglandular 1
• In the popliteal fossa specifically, lipomas present as deep-seated masses that require careful evaluation to distinguish from more concerning lesions 5, 6

Diagnostic Approach for Popliteal Fossa Lipomas

Initial Imaging

• Ultrasound is the first-line imaging modality with 86.87-94.1% sensitivity and 95.95-99.7% specificity 5, 7
• Characteristic ultrasound features include: well-circumscribed hyperechoic or isoechoic appearance compared to surrounding fat, thin curved echogenic lines within an encapsulated mass, minimal to no internal vascularity on Doppler, and no acoustic shadowing 5, 3
• Plain radiographs are often unrewarding, identifying intrinsic fat in only 11% of soft tissue masses 5, 3

Red Flags Requiring MRI

The National Comprehensive Cancer Network recommends proceeding to MRI if any of the following are present 5:

• Size larger than 5 cm
• Deep-seated location (which applies to popliteal fossa lipomas)
• Rapid growth
• Pain or tenderness
• Atypical ultrasound features
• Diagnostic uncertainty

Critical Distinction: Lipoma vs. Atypical Lipomatous Tumor (ALT)

• MRI can differentiate benign lipomas from atypical lipomatous tumors in up to 69% of cases, but this is not completely reliable 8, 5
• MRI variables associated with ALT/well-differentiated liposarcoma include nodularity, thick septations, stranding, and relative size 8
• The defining diagnostic test is molecular demonstration of MDM-2 amplification by fluorescence in-situ hybridization on percutaneous core needle biopsy 8, 7
• This distinction is critical because ALT of the extremities has propensity for local recurrence (though dedifferentiation is extremely rare), requiring different surgical margins 8

Management Algorithm for Popliteal Fossa Lipomas

Observation Criteria

• The American Academy of Orthopaedic Surgeons recommends observation for lipomas <5 cm that are asymptomatic and show typical ultrasound features, with clinical follow-up rather than imaging follow-up 5, 7

Surgical Indications

Complete en bloc surgical excision is indicated when 5, 7:

• The lipoma is symptomatic
• Rapidly growing
• Large (>5 cm)
• Showing atypical features on imaging

Specialist Referral

• Given the deep location in the popliteal fossa, any imaging suggestive of soft tissue sarcoma requires referral to a specialist sarcoma multidisciplinary team before surgical treatment 5
• The British Sarcoma Group recommends referral to a sarcoma center for deep-seated masses or any mass >5 cm diameter 7
• Deep extremity or lower limb masses have higher suspicion for ALT and often warrant sarcoma specialist evaluation 7

Common Pitfalls to Avoid

• Do not rely on physical examination alone, as it correctly identifies only 85% of lipomas 5, 3
• Do not use CT for tissue characterization, as it cannot reliably differentiate benign lipomas from atypical lipomatous tumors 5
• Do not perform simple excision if atypical lipomatous tumor is confirmed, as this requires different surgical margins and approach 5
• Popliteal masses are not always simple cysts, and thorough evaluation is always necessary to avoid misdiagnosis 6

Special Considerations for Popliteal Location

• The popliteal fossa location makes these lipomas deep-seated by definition, which automatically places them in a higher-risk category requiring more thorough evaluation 5, 7
• Large lipomas originating in the popliteal fossa can migrate through anatomical spaces (such as the adductor canal) into adjacent compartments, complicating surgical planning 9
• Complete en bloc resection preserving adjacent neurovascular structures (which are abundant in the popliteal fossa) but with no attempt to gain wide surgical margins will afford long-term local control for confirmed lipomas 8