Tumors of adipose tissue

1. Lipoma

Definition

• Benign fatty tumor of adipose tissue.
• Most common soft tissue tumor of adulthood.

Location

• Can arise in any location; mainly superficially located.
• Beneath the skin, upper back, abdominal region, shoulders, neck.

Features

• Single or multiple, encapsulated, often grows to large size.
• Composed of mature fatty tissue.
• May be mixed with other tissues:
  • Angiolipoma = lipoma + blood vessels.
  • Angiomyolipoma = lipoma + smooth muscle + vessels (classically in kidneys, associated with tuberous sclerosis).
  • Fibrolipoma, myolipoma.

Pathogenesis

• Chromosomal aberrations of 12q + 6p.
• HMGA2 gene rearrangements common.

Clinical

• Painless, soft, mobile subcutaneous mass.
• Slow-growing.
• Treatment: surgical excision if symptomatic / cosmetic.

2. Liposarcoma

Definition

• Malignant mesenchymal tumor of adipocytes.
• One of the most common adult soft tissue sarcomas.

Location

• Lower extremities, retroperitoneum, trunk, head + neck region.

Age

• Adults 40–60 yr.

Subtypes

Subtype	Features
Well-differentiated / atypical lipomatous tumor	Most common; low-grade; mimics lipoma but with atypia + lipoblasts
Myxoid liposarcoma	Young adults; t(12;16) FUS-DDIT3 fusion; “chicken-wire” vasculature
Dedifferentiated	From well-diff; aggressive
Pleomorphic	Highly atypical, worst prognosis

Morphology

• Gross: well-circumscribed mass (often > 10 cm in retroperitoneum); fat necrosis in large lesions.
• Histology: lipoblasts = multi-vacuolated cells with scalloped atypical nuclei indented by lipid droplets (diagnostic).

Clinical Features

• Painless lumps or painful swelling.
• Tx: surgical removal ± radiation/chemo.
• Prognosis depends on subtype: well-differentiated = good; pleomorphic = poor.

3. Comparison Table

Feature	Lipoma	Liposarcoma
Behavior	Benign	Malignant
Age	Any age, adults	40–60 yr
Site	Superficial (skin, back, abdomen)	Lower extremities, retroperitoneum
Gross	Encapsulated, mature fat	Large mass, fat necrosis
Histology	Mature adipocytes	Lipoblasts (multi-vacuolated, scalloped nuclei)
Genetics	12q, 6p (HMGA2)	t(12;16) FUS-DDIT3 (myxoid)

💡 High-yield: Lipoma = #1 soft tissue tumor of adulthood; benign, encapsulated, mature fat; 12q/6p rearrangement (HMGA2); angiomyolipoma in kidneys = tuberous sclerosis. Liposarcoma = malignant; 40–60 yr, lower extremities + retroperitoneum; subtypes: well-differentiated (#1), myxoid (t(12;16) FUS-DDIT3, chicken-wire vasculature), dedifferentiated, pleomorphic (worst); diagnostic cell = lipoblast (multi-vacuolated with scalloped nucleus).