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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: benign, slow ...

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