What is a lipoma?

Lipoma: A Comprehensive Overview

A lipoma is a benign, slow-growing tumor composed of mature fat cells (adipocytes) that typically appears as a soft, mobile, painless lump under the skin. 1

Clinical Characteristics

• Composition: Made of mature adipose (fat) tissue
• Presentation: Usually presents as a solitary, soft, rubbery, mobile mass
• Growth pattern: Slow-growing and generally asymptomatic
• Size: Typically small (less than 2 cm in diameter), but can occasionally grow larger
• Location: Can occur anywhere in the body where fat is present, but most commonly found on:
  • Trunk
  • Neck
  • Shoulders
  • Back
  • Arms
  • Thighs 2, 3

Types of Lipomas

Lipomas can be classified based on their location and composition:

1. Based on location:

   • Subcutaneous (most common)
   • Subfascial
   • Intramuscular
   • Subserous
   • Submucous
   • Intra-articular
   • Intraglandular 2

2. Based on composition:

   • Encapsulated (typical)
   • Diffuse
   • Multiple lipomas
   • Mixed tissue variants:
     • Fibrolipoma (contains fibrous tissue)
     • Angiolipoma (contains blood vessels)
     • Myelolipoma (contains bone marrow elements) 2, 4

Diagnostic Features

Clinical Examination

• Yellow hue may be visible through the skin
• "Pillow sign" - indentation when pressed with closed biopsy forceps (98% specificity but only 40% sensitivity)
• Mobile when manipulated 1

Imaging Characteristics

1. Ultrasound:

   • Useful for superficial lipomas
   • Shows encapsulated mass with minimal acoustic shadowing
   • Minimal vascularity
   • Simple curved echogenic lines within the mass
   • High sensitivity (94.1%) and specificity (99.7%) for superficial lipomas 1

2. MRI:

   • Shows well-circumscribed, homogeneous, fat-containing mass
   • Useful for deeper lipomas or when malignancy is suspected 1, 4

3. CT Scan:

   • Reveals homogeneous low-attenuation mass consistent with fat density
   • Useful for deeper lipomas or when evaluating potential malignant transformation 5

4. Endoscopic Ultrasound (for gastrointestinal lipomas):

   • Shows intensely hyperechoic, well-circumscribed mass arising from the submucosal wall layer
   • Diagnostic for gastric lipomas with no further evaluation needed 1

Clinical Significance and Complications

While most lipomas are benign and asymptomatic, they can occasionally cause:

• Cosmetic concerns
• Pain if they compress nearby nerves
• Functional limitations if they grow large enough
• Obstruction (particularly in gastrointestinal lipomas)
• Bleeding due to ulceration (in gastrointestinal lipomas) 1, 2

Giant Lipomas

• Defined as lipomas larger than 10 cm in diameter or weighing more than 1,000 g
• Represent approximately 1% of all lipomas
• May cause more significant symptoms including:
  • Lymphedema
  • Pain syndromes
  • Nerve compression
  • Significant cosmetic deformity 2, 3

Management

For most lipomas, treatment is not necessary unless they:

• Cause symptoms
• Create cosmetic concerns
• Show rapid growth
• Have atypical features suggesting malignancy

When treatment is indicated, surgical excision with complete removal is the standard approach. For giant lipomas, thorough preoperative evaluation is essential to rule out malignancy (liposarcoma) 6, 3.

Differentiation from Malignancy

It's important to distinguish lipomas from atypical lipomatous tumors (well-differentiated liposarcomas):

• Atypical lipomatous tumors are much less common than benign lipomas
• They tend to be larger, deep-seated, and more commonly found in the lower limb
• MRI cannot reliably differentiate between deep lipomas and atypical lipomatous tumors
• Molecular testing for MDM-2 amplification via core needle biopsy is the defining diagnostic test 1

When to Refer to a Specialist

Consider referral to a specialist if the lipoma:

• Is larger than 5 cm
• Is deep-seated
• Is painful
• Is rapidly growing
• Has atypical imaging features 1

According to NICE guidelines, consider an urgent direct access ultrasound scan (within 2 weeks) to assess for soft tissue sarcoma in adults with an unexplained lump that is increasing in size 1.