What is an infiltrative lipoma?

Infiltrative Lipoma: Characteristics, Diagnosis, and Management

An infiltrative lipoma is a benign, slow-growing fatty tumor that differs from conventional lipomas by its tendency to infiltrate surrounding tissues, particularly muscle fibers, making complete surgical excision challenging and increasing the risk of recurrence. 1

Clinical Features

• Presents as a deep, nontender mass within soft tissue, particularly in extremities
• Average size around 11.2 cm (range 2-22 cm) 2
• Grows slowly but progressively
• May cause functional limitations when located near joints or within muscle
• Usually asymptomatic until it reaches sufficient size to cause compression or functional impairment
• Can occasionally cause pain if pressing on nerves or blood vessels

Pathological Characteristics

• Unencapsulated fatty tumor (unlike conventional lipomas which are typically encapsulated)
• Microscopically shows monovacuolar fat tissue infiltrating between muscle fibers 3
• In advanced cases, may completely replace preexisting muscle with fatty tissue
• Composed of mature adipocytes (fat cells) that infiltrate adjacent structures
• Histologically benign despite its infiltrative growth pattern

Diagnostic Approach

Imaging

• MRI is the preferred imaging modality:
  • Shows characteristic hyperintense signal on T1-weighted images
  • Can demonstrate the extent of muscle infiltration
  • Helps differentiate from well-differentiated liposarcoma 1
• Ultrasound can be used as initial screening tool 1
• Plain radiographs may show radiolucent mass and can help in localization 2

Tissue Diagnosis

• Core needle biopsy may be necessary to differentiate from atypical lipomatous tumors (well-differentiated liposarcoma)
• MDM-2 amplification testing by fluorescence in-situ hybridization can help differentiate between lipoma and atypical lipomatous tumor 1

Management

Surgical Treatment

• Complete surgical excision with tumor-free margins is the recommended treatment 4, 2
• Marginal excision combined with wide excision in areas of deeper infiltration 4
• R0 resection (complete removal with negative margins) is ideal but may not always be feasible when:
  • Tumor is near critical neurovascular structures
  • Complete excision would cause severe functional impairment 3

Follow-up

• Long-term follow-up is essential due to potential for recurrence
• Recurrence rates vary between 3-62% depending on completeness of excision 3
• Local recurrence is more common with incomplete resection

Special Considerations

• In elderly patients or those with significant comorbidities where surgery poses high risk, radiological surveillance may be considered 1
• For large tumors or cases where clear margins are difficult to achieve, adjuvant radiotherapy may occasionally be considered 1
• Infiltrative lipomas have been reported in various anatomical locations including the face, oral cavity, and cheek, though they are most common in extremities 5, 6

Clinical Pitfalls

• Often misdiagnosed preoperatively due to their infiltrative nature
• Can be mistaken for liposarcoma due to infiltrative growth pattern
• Inadequate excision leads to high recurrence rates
• Delayed diagnosis may result in larger tumor size and more extensive infiltration, making complete excision more challenging
• Balance between complete tumor removal and preservation of function must be carefully considered

While infiltrative lipomas are benign tumors with no metastatic potential, their infiltrative growth pattern and tendency for local recurrence necessitate careful preoperative planning and complete surgical excision whenever possible to minimize recurrence risk while preserving function.