Diagnostic Studies. The lesion commonly develops in the metaphysis of the distal femur or distal tibia and is eccentrically located, usually within or adjacent to the cortex. Radiographs reveal a well-marginated radiolucent zone, with distinct trabeculation producing a multilocular appearance. In addition, nonossifying fibromas usually cause benign cortical thinning, or erosion. The radiographic pattern is usually diagnostic, and further staging studies are seldom indicated.

Histologic features include a combination of dense collagen arranged in a storiform pattern, a scattering of small, multinucleated giant cells, hemosiderin, and lipid-filled histiocytes. The tissue does not contain the degree of hemorrhage and necrosis or the number of giant cells seen in the giant cell tumor of bone. Although the tissue may resemble the lining tissue of an aneurysmal bone cyst, the lesion does not have a large, central, blood-filled cavity.

Treatment. Reassurance and watchful waiting is usually sufficient except with fracture when closed or open reduction and immobilization with bone grafting is necessary.

DESMOPLASTIC FIBROMA

Desmoplastic fibroma (desmoid tumor) is a rare intraosseous fibroma that typically develops as an aggressive stage 3 tumor. It occurs primarily in young adults but may occur at any age. The long bones—particularly the tibia and the fibula—are the most common sites, although it may occur throughout the skeleton. Its behavior corresponds to that of its soft tissue counterpart, aggressive fibromatosis (see Plate 6-23).

Diagnostic Studies. Radiographs show a centrally located metaphyseal or diaphyseal lesion, poorly or incompletely contained by a thin margin of reactive bone, which frequently has a trabeculated appearance. It may remain within the bone for some time, surrounded by a thin cortical shell, but eventually it extends through the cortex into the soft tissues. The radiographic hallmark is a loculated intraosseous lesion that stimulates very little bony reaction. The radiographic differential diagnosis includes giant cell tumor of bone (see Plate 6-13) and fibrosarcoma of bone (see Plate 6-18). The most characteristic feature is the contrast between the relatively benign findings of the radiographic studies and the tumor’s fairly aggressive clinical behavior.

The lesion is composed of dense, white, fibrous tissue with a rubbery consistency and is easily removed with curettage. The histologic features of dense, irregularly arranged bundles of collagen with an occasional spindle cell closely resemble fibromatosis. Mitoses, vascularity, and necrosis are unusual microscopic findings. The histologic differential diagnosis usually involves low-grade fibrosarcoma of bone.

Treatment. Excision with a wide margin results in local control of the lesion. Recurrence after curettage is frequent, with invasion of the soft tissue.