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Findings: There is a well circumscribed eccentric lytic lesion involving the right tibial distal metadiaphyseal region. There are sclerotic margins and what appears to be small septations in the lesion. There is a periosteal reaction along the medial and posterior margins of this lytic lesion.

Diagnosis:Fibroxanthoma of the right distal tibia

Discussion: Fibroxanthomas are benign fibrous lesions and can be grouped into two categories, fibrous cortical defects and non- ossifying fibromas, depending on size and location within bone from which the lesion arises. The two groups are histologically identical. These lesions are not neoplasms but are considered developmental defects. They are composed of fibrous tissue, xanthoma cells, and giant cells. They do not produce bone and are therefore differentiated from lesions such as fibrous dysplasia, ossifying fibroma, and low-grade intra-osseous osteosarcoma.

Fibrous cortical defects arise on the cortical surface of bone. It is estimated that 30 to 40% of children develop this lesion at one time or another. Fibrous cortical defects most commonly occur between the ages of four to eight years and has a two-to- one male to female ratio. They are usually asymptomatic and require no treatment.

Non-ossifying fibromas arise eccentrically within the medullary cavity and may be multiple. They usually occur between the ages of two and twenty years with a slight male preponderance. These lesions are less common than fibrous cortical defects. They are usually asymptomatic although larger lesions may cause pain and may lead to pathologic fractures. Large lesions may require careful observation and occasionally prophylactic curettage and bone grafting.

Fibrous cortical defects appear as round or oval geographic radiolucencies eroding the cortical surface on plain radiography. They usually have sharp margins and are bounded by a rim of reactive bone. They are found in the metaphyses of long bones, usually in the lower extremities and knees. They range in size from 1 to 4 cm. in diameter. Non-ossifying fibromas are seen lying within the medullary cavity. They also appear as geographic lucencies with sharp margins and a sclerotic rim. They have a multiloculated appearance as well. They can erode the endosteal surface of the overlying cortex causing thinning or expansion of the cortex. These lesions range in size from 1 to 7 cm. in diameter and are usually located near the physeal plate.

References:
Moser RP, Editor: The Radiologic Clinics of North America.
Imaging of Bone and Soft Tissue Tumors, 1993, Saunders Company,
Philadelphia.

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