Anatomic Pathology: Bone and Joint Pathology

• Chondromyxoid fibroma is a very rare tumor of bone representing 1% of all bone tumors. It affects young individuals, with a peak incidence in the second and third decades. It involves primarily the metaphyses of long bones, particularly distal femur and proximal tibia, and less often involves the bones of the foot and the ilium.

• Patients usually present with pain and local discomfort, which may be of some duration, sometimes associated with local swelling.

• The radiologic features are very helpful and should be correlated with the microscopic findings. Radiographic images show a sharply demarcated, purely lytic lesion located in the metaphysis of a long bone and with an orientation parallel to the long axis of the bone. Distinctly eccentric, its medullary margins are scalloped and sclerotic, suggesting a benign lesion. Its peripheral margin may be expanded by secondary aneurysmal transformation. The lesion may extend into the epiphysis or diaphysis of the bone but usually does not transgress an open growth plate.

• Chondromyxoid fibroma is characterized by pseudolobules of cellular myxoid and chondroid tissue separated by stalks of connective tissue. These stalks contain blood vessels, mononuclear cells, and scattered giant cells. The periphery of the lobules comprises a dense population of small spindle, often stellate cells in a myxoid stroma containing microcysts. Toward the center of the lobules, differentiation to immature cartilage is represented by round chondrocytes in a focally chondroid stroma. Chondrocytes often show hyperchromasia and double nucleation but no mitoses. The immaturity of the cartilage and the presence of atypia may lead to misinterpretation of the lesion as chondrosarcoma.

• The treatment of choice is en bloc excision. Chondromyxoid fibroma has an overall recurrence rate of 20%.