Tumors of fat, muscle, cartilage, and bone

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Chapter 21

Tumors of fat, muscle, cartilage, and bone

Tammie Ferringer

Lipoma

Key features

• Well-circumscribed tumor with a thin capsule

• Mature lipocytes

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’s 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’s disease), sometimes associated with weakness and mental disturbances. Lipomas may also be a component of Gardner’s syndrome, Bannayan–Zonana syndrome, Cowden’s syndrome, and Proteus syndrome.

Fig 21-1 Lipoma

Fig 21-2 Lipoma

Angiolipoma

Key features

• Lipoma with proliferations 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”:

• Eccrine 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 containing multinucleate floret cells with 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.” More information on liposarcoma and other soft tissue neoplasms is available in the on-line component of this text.

Fig 21-8 Pleomorphic lipoma

Fig 21-9 Pleomorphic liposarcoma

Angiomyolipoma

Key features

• Well-circumscribed tumor

• Smooth muscle radiating in a pinwheel fashion from the walls of muscular vessels

• Mature fat

Cutaneous angiomyolipomas are rare, solitary, painless subcutaneous nodules most commonly on the extremity of middle-aged men. Unlike renal angiomyolipomas, cutaneous lesions are not associated with tuberous sclerosis. Some consider these lesions to be angioleiomyomas with admixed mature fat.

Fig 21-10 Angiomyolipoma

Fig 21-11 Angiomyolipoma

Hibernoma

Key features

• Well-circumscribed tumor

• Multivacuolated “mulberry cells”

Hibernomas are rare tumors that demonstrate differentiation toward brown (fetal) fat. While brown fat occurs anywhere in a fetus, hibernomas in adults most frequently arise in the subcutis of the shoulder girdle, posterior neck, and axilla. The gross brown color is a result of the prominent vascularity and many mitochondria within the tumor cells.

Mulberry cells with central nuclei predominate in most hibernomas. Smaller cells with granular cytoplasm and univacuolated cells may sometimes be seen.

Differential Diagnosis

Lipoblasts of liposarcoma have hyperchromatic scalloped nuclei. The tumors are large, poorly circumscribed, and have an arborizing vascular pattern.

Fig 21-12 Hibernoma

Fig 21-13 Nevus lipomatosis superficialis

Nevus lipomatosis superficialis of Hoffmann and Zurhelle

Key features

• Mature adipocytes replacing much of the dermis

Nevus lipomatosis superficialis presents as multiple soft yellow-tan papules or nodules on the hip or buttock that coalesce into plaques or form a linear arrangement. Rarely, there is disseminated body involvement with extensively folded skin (“Michelin tire baby syndrome”).

Large acrochordons with a broad base may have a similar appearance.

Differential Diagnosis

The histologic differential diagnosis includes Goltz syndrome (focal dermal hypoplasia), an X-linked dominant syndrome with a Blaschkoid distribution. Long-standing dermal nevi may also undergo extensive fatty degeneration but retain focal residual nevus.

Fig 21-14 Piloleiomyoma

Leiomyoma

Leiomyomas are benign smooth muscle tumors. Smooth muscle is normally found in the skin as arrector pili muscles, in the walls of blood vessels, breast, and genital skin (periareolar or dartos muscle). Smooth muscle tumors can arise from each of these sources and are known as piloleiomyoma, angioleiomyoma, and leiomyoma of the genital skin, respectively.

Smooth muscle cells are characterized by long, thin cigar-shaped nuclei with blunt ends. A paranuclear vacuole, representing a glycogen “snack” for the muscle, is well demonstrated in cross-section.

Piloleiomyoma

Key features

• Circumscribed but non-encapsulated dermal nodule

• Interlacing bundles of smooth muscle fibers resemble arrector pili muscle “on steroids”

• Mitotic activity is negligible

• Nuclear density matches that of normal smooth muscle

Piloleiomyomas, derived from the arrector pili muscle, typically present as multiple firm, red-brown lesions in the third decade. They may be solitary, grouped, or in a linear pattern. Manipulation and exposure to cold result in pain.

Multiple lesions have been associated with papillary renal cell carcinoma and uterine leiomyomas (fibroids) in females. This constellation of findings, known as Reed’s syndrome, is inherited in an autosomal-dominant pattern and is due to a mutation in the fumarate hydratase gene.

Differential Diagnosis

Smooth muscle hamartomas consist of smaller bundles that are sparsely distributed in a broad hyperpigmented patch or plaque-like lesion. Terminal hair may be present, and these lesions overlap with Becker’s nevi.

The muscle bundles of piloleiomyoma can be distinguished from collagen bundles by the blunt, cigar-shaped nuclei with adjacent vacuole rather than the short tapered nuclei of the fibroblasts in collagen. The cytoplasm of smooth muscle cells is more conspicuous than in fibroblasts. A trichrome stain will stain muscle red and collagen green-blue. Immunoperoxidase markers for smooth muscle (smooth muscle actin and desmin) can also be used.

Fig 21-15 Piloleiomyoma

Fig 21-16 Angioleiomyoma

Angioleiomyoma

Key features

• Subcutaneous nodule of smooth muscle containing round and slit-like vascular spaces

Angioleiomyomas are usually tender subcutaneous nodules on the lower extremities of middle-aged adults. More than half of lesions are spontaneously painful. Each vessel has several layers of smooth muscle that merge with the intervascular smooth muscle fascicles.

Fig 21-17 Angioleiomyoma

Fig 21-18 Leiomyosarcoma

Leiomyosarcoma

Key features

• High cellularity

• Mitoses and nuclear atypia

Superficial or dermal leiomyosarcomas are neoplasms that arise from the pili or genital smooth muscle. They occur predominantly in middle-aged men on the extensor extremities. Lesions recur in 30% of cases but metastasis is extremely rare.

Subcutaneous leiomyosarcomas, presumably arising from vascular smooth muscle, are larger, and have a greater tendency for metastasis to lung, other soft-tissue sites, and the liver, in one-third of cases.

Differential Diagnosis

Although a grenz zone is often present, dermal leiomyosarcomas may be slammed up against the epidermis and resemble other spindle cell neoplasms. Immunohistochemistry can aid in the microscopic differential diagnosis.

• Spindled squamous cell carcinoma (keratin+)

• Leiomyosarcoma (desmin+)

• Atypical fibroxanthoma (diagnosis of exclusion)

• Melanoma (S100+)