How do you differentiate and treat a sebaceous cyst (benign, closed capsule containing a cheesy, semi-solid material) versus a lipoma (benign tumor composed of fat tissue) in a patient?

Differentiating Sebaceous Cysts from Lipomas

Ultrasound is the diagnostic test of choice to differentiate these lesions, with lipomas appearing intensely hyperechoic (94.1% sensitivity, 99.7% specificity) while sebaceous cysts are typically hypoechoic with a visible epidermal punctum (skin pore) in approximately 50% of cases. 1, 2, 3

Clinical Differentiation

Physical Examination Features

Sebaceous Cysts:

• Presence of a visible central punctum (opening to skin surface) - this is pathognomonic when present 3
• Firm, mobile, dome-shaped nodules typically on trunk, face, or scalp 3
• May have cheesy, foul-smelling discharge if ruptured 3
• Physical exam alone has limitations - clinical accuracy is only 72.7% 4

Lipomas:

• Soft, doughy, mobile masses in subcutaneous tissue 1, 5
• No central punctum 1
• Typically painless unless compressing adjacent structures 2
• Physical exam correctly identifies only 85% of lipomas, making imaging essential 1

Red Flags Requiring Urgent Evaluation

• Size >5 cm 2
• Rapid growth 2
• Deep-seated location (below fascia) 2
• Pain 2
• Fixed to underlying structures 2

Any of these features mandate MRI and consideration of sarcoma center referral to exclude atypical lipomatous tumor or liposarcoma. 2, 6

Diagnostic Imaging Algorithm

First-Line: High-Frequency Ultrasound

For Lipomas:

• Intensely hyperechoic appearance compared to surrounding tissue 1, 2
• Well-circumscribed borders 1, 2
• Minimal to no internal vascularity on Doppler 1, 2
• No acoustic shadowing 1
• May contain thin, curved echogenic lines 1
• Use 18-20 MHz probes for optimal visualization 3

For Sebaceous Cysts:

• Hypoechoic appearance in 91.5% of cases 3
• Oval-shaped morphology in 96.6% 3
• Homogeneous "pseudotestis" appearance in 50.8% 3
• Visible epidermal punctum (skin pore) - highly specific when identified 3

When to Advance to MRI

• Ultrasound shows atypical features (nodularity, thick septations) 2, 6
• Mass is deep-seated or >5 cm 2
• Diagnostic uncertainty between benign lipoma and atypical lipomatous tumor 2, 6
• MRI can differentiate benign lipomas from atypical lipomatous tumors in 69% of cases 2, 6

Role of Plain Radiographs

• Avoid routine radiographs - they identify intrinsic fat in only 11% of soft tissue masses and are generally unrewarding 1, 2

Management Approach

Sebaceous Cysts

Observation:

• Acceptable for asymptomatic, small cysts 4
• Malignancy risk is essentially zero (0% in 543 cases studied) 4

Surgical Excision:

• Indicated for symptomatic cysts (infected, inflamed, cosmetically bothersome) 4
• Complete excision with intact capsule to prevent recurrence 4
• Routine histopathology is NOT necessary for clinically typical sebaceous cysts without red flags 4

Lipomas

Observation:

• Appropriate for asymptomatic lipomas <5 cm with typical imaging features 2
• No routine follow-up imaging needed if classic features present 1

Surgical Excision:

• Indicated for symptomatic lipomas, rapidly growing lesions, or atypical imaging features 2
• Complete en-bloc excision with negative margins (R0) is the standard technique 2, 6
• Use lidocaine with epinephrine (maximum 7 mg/kg) for standard infiltrative anesthesia 2
• Tumescent anesthesia allows higher lidocaine doses (up to 55 mg/kg) for larger lipomas 2
• Routine histopathology is NOT necessary for clinically and sonographically typical lipomas <5 cm without red flags 4

Mandatory Sarcoma Center Referral

• Deep-seated masses (below fascia) 2, 6
• Any mass >5 cm diameter 2, 6
• Retroperitoneal or intra-abdominal location 2
• Atypical MRI features (nodularity, thick septations) 2, 6
• Percutaneous core biopsy for MDM-2 amplification testing is mandatory before surgery when atypical lipomatous tumor is suspected 2, 6

Common Pitfalls to Avoid

• Do not rely on physical examination alone - clinical diagnosis has 72.7% accuracy for sebaceous cysts and 85% for lipomas 1, 4
• Do not skip imaging for deep or large lesions - these have higher risk of being atypical lipomatous tumors 2, 6
• Do not perform inadequate excision of suspected atypical lipomatous tumors - this leads to high local recurrence rates and potential dedifferentiation with each recurrence 2, 6
• Do not send all benign-appearing lesions for routine histopathology - this increases costs without clinical benefit when lesions are <5 cm, superficial, and lack red flags 4