I have a small, firm subcutaneous mass that has been present for years with very slow growth; what is the likely diagnosis and what are the recommended evaluation and management steps?

Subcutaneous Mass Evaluation and Management

Most Likely Diagnosis

A small, firm subcutaneous mass present for years with very slow growth is most likely a benign lipoma, which accounts for the overwhelming majority (96%) of superficial soft tissue masses. 1, 2

Initial Diagnostic Approach

Step 1: Plain Radiographs First

• Obtain plain radiographs as the initial imaging study, which can identify intrinsic fat in approximately 11% of soft tissue masses and detect calcification or bone involvement 2, 3
• Radiographs identify positive findings in 62% of cases and are essential before proceeding to other modalities 3

Step 2: Ultrasound for Superficial Masses

• Ultrasound is the first-line imaging modality for suspected subcutaneous lipomas, with sensitivity of 86.87-94.1% and specificity of 95.95-99.7% 2, 3
• Characteristic ultrasound features of benign lipomas include:
  • Well-circumscribed, hyperechoic or isoechoic appearance compared to surrounding fat 2
  • Thin, curved echogenic lines within an encapsulated mass 2
  • Minimal to no internal vascularity on Doppler examination 4, 2
  • No acoustic shadowing 2

Critical Red Flags Requiring Advanced Imaging

You must obtain MRI with and without contrast if any of the following features are present: 1, 2, 3

• Size larger than 5 cm
• Deep location (subfascial)
• Rapid growth or recent change in growth rate
• Pain or tenderness
• Firm consistency with irregular margins
• Atypical ultrasound features (thick septations, nodularity, or soft tissue components)

When to Suspect Atypical Lipomatous Tumor (ALT)

Any lipomatous mass that is increasing in size, deep-seated, or located in the lower limb raises concern for atypical lipomatous tumor (well-differentiated liposarcoma), which requires different surgical management due to its propensity for local recurrence 1, 2

MRI Characteristics Suggesting ALT vs Benign Lipoma

• MRI can distinguish benign lipomas from ALT in up to 69% of cases based on: 2
  • Nodularity within the mass
  • Thick septations (>2mm)
  • Soft tissue stranding
  • Relative size and deep location
• Masses that lack isointense signal to subcutaneous fat on MRI may represent sarcoma and require biopsy 5

Tissue Diagnosis Algorithm

When Biopsy is Required

Obtain core needle biopsy under image guidance if: 2, 3

• MRI shows concerning features (nodularity, thick septations, stranding)
• Diagnostic uncertainty persists after imaging
• Any mass with firm consistency, irregular margins, size >5 cm, or rapid growth

Biopsy Technique

• Multiple core samples should be obtained to maximize diagnostic yield 3
• Request MDM-2 amplification analysis by fluorescence in-situ hybridization, which is the defining diagnostic test to differentiate lipoma from ALT 2, 3
• For small lesions (<2 cm or <5 cm depending on source), excisional biopsy may be more practical 2, 3
• Plan the biopsy tract for potential excision if malignancy is found 4

Management Based on Diagnosis

For Typical Benign Lipoma

Observation is appropriate for small (<5 cm), asymptomatic, superficial lipomas with typical ultrasound features, with clinical follow-up rather than imaging follow-up being sufficient 2

Indications for Surgical Excision

Complete en bloc surgical excision is indicated when the lipoma is: 2

• Symptomatic
• Rapidly growing
• Large (>5 cm)
• Showing atypical features on imaging

For Confirmed ALT

• If MDM-2 positive (confirming ALT), refer to sarcoma specialist for marginal en bloc resection 2
• ALT requires en bloc resection even if margins are R1, whereas simple lipoma excision requires different margins 2

Critical Pitfalls to Avoid

Physical examination alone correctly identifies only about 85% of soft tissue tumors, making imaging mandatory 3

Ultrasound accuracy drops precipitously for deep lipomas compared to superficial ones—all deep-seated lipomas or those in the lower limb should raise concern for atypical lipomatous tumors 1, 2

Avascularity does not guarantee benignity, as some malignant lesions can be relatively avascular, though this is uncommon 4

Any retroperitoneal or intra-abdominal mass with imaging suggestive of soft tissue sarcoma should be referred to a specialist sarcoma multidisciplinary team before surgical treatment 1, 2

Do not assume all slow-growing masses are benign—giant lipomas (>10 cm) represent only 1% of lipomas but can undergo malignant transformation, calcification, or other changes, particularly in subcutaneous tissues of the thigh, buttock, or retroperitoneum 6, 7