Chapter 10: Disorders of Fat

NEOPLASMS OF THE SUBCUTANEOUS FAT

Lipoma

• Most common soft tissue tumor, usually present in adults over the age of 30 years

• Wide anatomic distribution, with relative sparing of the head, hands, and feet

• Usually solitary, but 5% of patients have multiple lipomas, often in those with a lipomatosis or a multisystem syndrome

• Two-thirds of lipomas have chromosomal abnormalities; translocations between 12q13-15 and several other chromosomes are most common

• Clinical features: benign, asymptomatic; soft, and mobile subcutaneous nodule with a normal overlying epidermis

• Histology

  • Circumscribed nodule surrounded by a thin fibrous capsule

  • Lobules of mature adipocytes divided by fibrous septa containing capillary-sized vessels; adipocytes have eccentrically placed nuclei (Fig 10-1A,B)

  • Absent mitotic figures

  • Microscopically may be indistinguishable from the surrounding adipose tissue

• Histologic variants

  • Angiolipoma

    • – Clinical features: occurs predominantly in men; presents as a small painful nodule, often on the forearm of young adults

    • – Histology: well-circumscribed and encapsulated admixture of mature adipocytes and capillary-sized vessels which contain fibrin microthrombi; histologic presentation has a large degree of variability (Fig 10-2A,B)

  • Pleomorphic lipoma

    • – Clinical features: occurs predominantly in middle-aged men, located on posterior neck, upper back, and shoulders

    • – Histology: circumscribed; admixture of bizarre multinucleate giant cells (floret cells) and normal adipocytes

  • Spindle cell lipoma

    • – Clinical features: occurs predominantly in middle aged men, located on posterior neck, upper back, and shoulders

    • – Histology: circumscribed; admixture of bland-appearing spindle cells and normal adipocytes with variable myxocollagenous matrix stroma, containing rope-like collagen fibers (Fig 10-3)

    • – Absent mitotic activity

  • Adenolipoma

    • – Clinical features: most commonly located on the proximal limbs

    • – Histology: admixture of mature adipocytes with eccrine sweat glands

  • Intramuscular lipoma

    • – Composed of mature adipocytes, located within skeletal muscle; no cytologic atypia

    • – Infiltrating type has higher local recurrence rate than other types of lipomas and requires more extensive tissue resection

  • Fibrolipoma: lipomas with increased fibrous tissue; no cytologic atypia

  • Sclerotic lipoma

    • – Clinical features: more common in distal extremities

    • – Histology: well-circumscribed nodules, bland spindled and stellate cells, few adipocytes embedded in a collagenous to myxocollagenous stroma

  • Myxolipoma: lipoma with prominent myxoid stroma

  • Infarcted lipoma: necrotic adipocytes surrounded by multinucleate giant cells, lymphocytes, and extravasated erythrocytes

• Other types of lipomatous tumors

  • Chondroid lipoma

    • – Clinical features

      • ▴ Benign tumor, occurs predominantly in females

      • ▴ Most common on proximal extremities and limb girdle

      • ▴ Involves deep subcutaneous tissue, fascia, and skeletal muscle

    • – Histology:

      • ▴ Encapsulated tumor, containing nests and cords of lipoblasts with variable nuclei but no atypia; found within a chondroid matrix that can be hyalinized

      • ▴ Cells contain periodic acid–Schiff-positive glycogen

      • ▴ Alcian blue stains the matrix

      • ▴ These tumors are highly vascular

  • Myolipoma/lipoleiomyoma

    • – Clinical features: occurs in deep soft tissue of abdomen and retroperitoneum; yellow-white cut surface; often presents as a palpable mass

    • – Histology: admixture of mature adipocytes and smooth muscle cells, no atypia

  • Angiomyolipoma/angiolipoleiomyoma

    • – Clinical features: most commonly located in kidney and are associated with tuberous sclerosis; these tumors are positive for HMB-45

    • – Histology: convoluted thick-walled blood vessels, adipocytes, smooth muscle cells

  • Cutaneous angiomyolipoma:

    • – Clinical features: ...