Diagnostic Studies. Radiographs may also show calcification in areas of necrosis and metaplastic bone or cartilage inside lipomas. MRI reveals a homogenous fatty tumor that can be quite large.

Treatment/Prognosis. Marginal excision is successful for lipoma, and recurrences are rare. However, recurrences are higher (approximately 25%) in atypical lipomas with MDM2 amplification by fluorescence in-situ hybridization or immunohistochemistry. Lipomas and even atypical lipomas do not metastasize.

NEUROFIBROMA

A neurofibroma is a benign tumor of neural origin. Although it may affect almost any tissue type, it is most commonly found in skin and subcutaneous tissue and is associated with nerve fibers. Whether they are solitary or multiple, neurofibromas can occur in persons of any age. Multiple neurofibromas are a primary manifestation of neurofibromatosis. Neurofibromas also occur in conjunction with scoliosis, congenital pseudarthrosis of the tibia, and gigantism of a limb. (For a discussion of these conditions, see Section 4, Congenital and Developmental Disorders.)

Long-standing neurofibromas, particularly in patients with multiple neurofibromas (neurofibromatosis), may undergo malignant transformation.

Diagnostic Studies. Neurofibroma is frequently seen as a fusiform mass in continuity with a major nerve best demonstrated by MRI. These lesions originate from a nerve, and therefore resection will usually require at least cutting nerve bundles. In contrast, schwannomas are benign tumors of the nerve sheath and can frequently be excised without demonstrable damage to the adjacent nerve. Histologic examination of a neurofibroma reveals a loose, spindle-cell stroma containing wavy eosinophilic fibrillar material and occasional Verocay bodies, which are composed of amorphous eosinophilic material that is surrounded by spindle-shaped or oval cells.

Treatment. Neurofibromas can be excised if painful or they can be followed, since they are benign. If a neurofibroma is excised, the nerve, or at least the nerve fascicle of origin, will need to be excised. If a neurofibroma is rapidly enlarging, a biopsy should be performed to rule out malignant degeneration to a malignant peripheral nerve sheath tumor (MPNST). MPNST occurs primarily in patients with neurofibromatosis.

MYOSITIS OSSIFICANS

Myositis ossificans is a non-neoplastic reparative or reactive ossification of soft tissue that usually occurs after blunt trauma, particularly in conjunction with head trauma. It can occur in adolescents as a painless, enlarging mass in the upper arm, thigh, or buttocks. This clinical presentation may erroneously suggest an extraosseous osteosarcoma or soft tissue sarcoma.

Diagnostic Studies. Radiographs reveal a round mass with a distinct margin of mature ossification and a radiolucent center of immature osteoid and primitive mesenchymal tissue (the reverse of that seen in a malignant tumor). Differential diagnosis includes juxtacortical osteosarcoma; however, the peripheral maturation in myositis ossificans is so characteristic that documentation with CT can resolve fears of malignancy even when suggested by the histologic appearance of the central immature area.

Treatment/Prognosis. Because recurrence is high, early excision of myositis ossificans should not generally be attempted except for diagnostic purposes when diagnostic studies are inconclusive.