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Tumors of fat, muscle, cartilage, and bone


Fat

Lipoma

Key Features

  • Well-circumscribed tumor with a thin capsule

  • Mature lipocytes

  • Inconspicuous septae

Lipomas typically present as asymptomatic, mobile, soft nodules in the deep soft tissue or subcutis. Histologically, they are thinly encapsulated tumors composed of sheets of mature adipocytes that are indistinguishable from the fat cells in the subcutaneous tissue. Each adipocyte has a single vacuole and an eccentric nucleus. The thin fibrous septa, which contain sparse blood vessels, are delicate and inconspicuous. Intramuscular lipomas, commonly of the forehead, consist of mature fat cells that displace muscle, splaying the fibers.

There are several rare syndromes in which multiple lipomas occur. Hundreds of slow-growing subcutaneous and deep or visceral lipomas develop in early adulthood in the autosomal-dominant condition familial multiple lipomatosis. Benign symmetric lipomatosis (Madelung disease) has a predilection for middle-aged men with a propensity to develop multiple lesions, especially in the region of the neck in a “horse-collar” distribution. Tender, circumscribed or diffuse fatty deposits of the lower legs, abdomen, and buttocks in obese patients exemplify adiposis dolorosa (Dercum disease), sometimes associated with weakness and mental disturbances. Lipomas may also be a component of Gardner syndrome, Bannayan–Zonana syndrome, Cowden syndrome, and Proteus syndrome.

Fig. 21.1, Lipoma

Fig. 21.2, Lipoma

Angiolipoma

Key Features

  • Lipoma with proliferation of capillary-sized blood vessels

  • Erythrocytes and scattered fibrin microthrombi are commonly present in the lumens

Clinically, angiolipoma are often tender to palpation. They commonly occur in women, are multifocal, and have a predilection for the forearm.

Differential Diagnosis

The differential diagnosis of painful tumors can be remembered by the mnemonic “BANGLE”:

  • B lue rubber bleb nevus

  • A ngiolipoma

  • N euroma

  • G lomus tumor

  • L eiomyoma

  • E ccrine spiradenoma

Fig. 21.3, Angiolipoma

Fig. 21.4, Angiolipoma

Spindle cell lipoma

Key Features

  • Well-circumscribed tumor of mature fat with interspersed zones of bland spindle cells and variable amounts of collagen

  • In young lesions, the spindle cell areas are myxoid, with many mast cells

  • No lipoblasts or mitotic figures

  • CD34+ spindle cells

  • Ropey collagen bundles

The most common presentation is a solitary lesion at the base of the neck, shoulder, or upper back in an older man. These lesions are generally firmer and more fixed to surrounding tissue than the usual lipoma.

Differential Diagnosis

When spindle cell and myxoid components predominate, there may be confusion with neural or fibroblastic proliferations, especially diffuse neurofibroma.

Fig. 21.5, Spindle cell lipoma

Fig. 21.6, Spindle cell lipoma

Fibrolipoma

Key Features

  • Well-circumscribed tumor of mature lipocytes containing large bundles of mature collagen

Fig. 21.7, Fibrolipoma

Pleomorphic lipoma

Key Features

  • Well-circumscribed tumor of lipocytes with multinucleate floret cells containing overlapping nuclei arranged at the periphery like the petals of a flower

  • Hyperchromatic nuclei may be present

  • Myxoid areas and ropey collagen bundles may be present, as in spindle cell lipoma

Pleomorphic lipomas have a firm consistency and similar distribution to spindle cell lipomas on the neck and shoulder girdle of older men.

Differential Diagnosis

The sharp circumscription, superficial location, floret cells, paucity of mitotic activity, and absence of lipoblasts distinguish pleomorphic lipoma from pleomorphic liposarcoma. Lipoblasts are immature fat cells that may have an eccentric nucleus (signet-ring lipoblasts) or have a central scalloped nucleus indented by lipid vacuoles (mulberry lipoblasts). Liposarcoma typically arises in deep soft tissue, especially the retroperitoneum, but may involve the skin. Liposarcomas demonstrate an arborizing pattern of blood vessels that resembles “chicken wire.”

Fig. 21.8, Pleomorphic lipoma

Atypical lipomatous tumor (ALT)

Key Features

  • Represents well-differentiated liposarcoma

  • Lipocytes vary in size

  • Atypia noted near septae

  • Positive for MDM2 and CDK4 by immunohistochemistry (IHC) or fluorescence in situ hybridization (FISH)

Fig. 21.9, Atypical lipomatous tumor (ALT)

Pleomorphic and myxoid liposarcoma

Key Features

  • Lipocytes vary in size

  • Mulberry or signet-ring lipoblasts

  • Arborizing (chicken-wire) vascular pattern

Fig. 21.10, Liposarcoma. (A) Pleomorphic liposarcoma. (B) Myxoid liposarcoma

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