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Non-ossifying fibroma

Dr MT Niknejad and Dr Frank Gaillard et al.

Non-ossifying fibromas (NOF) are the most common of benign fibrous bone lesions ²and is essentially a larger version (>2-3cm) of a fibrous cortical defect, both are encompassed by the term fibroxanthoma or metaphyseal cortical defect⁴.

Epidemiology

They are common in children and adolescents (peak incidence 10-15 year of age) and are usually not seen beyond the age of 30, as they spontaneously heal being gradually filled in by bone.

Clinical presentation

The majority of NOF are probably asymptomatic. Larger lesions may be painful and may weaken the bone enough to predispose to pathological fracture.

They are sometimes associated with Jaffe-Campanacci syndrome : multiple non ossifying fibromas with several other associations including multiple cafe au lait spots in skin.

Pathology

A NOF macroscopically appears as a fleshy, fibrous, yellow or tan-brown lesion with variable areas of haemorrhage ⁴.

Microscopic examination reveals the lesions to be highly cellular containing spindle-shaped cells on a background of stromal tissue in a prominent storiform pattern. Foamy histiocytes, and multi-nucleated giant cells are also seen. Mitotic figures and / or cellular dysplasia should not be seen ⁴.

The irony of non-ossifying fibromas appearing to ossify has not been lost on generations of radiologists, although strictly speaking the lesion itself does not ossify, rather being filled in by normal bone from the periphery ⁴. Indeed it is believed that many bone islands are healed fibroxanthomas (FCD or NOF).

Radiographic features

Plain film and CT

NOF are typically sharply demarcated, asymmetrical, cortically based lucencies with a thin sclerotic rim. They often appear multiloculated. They are located in the metaphysis, just distal to the physis. As the patient ages they appear to migrate away from the growth plate.

They have no periosteal reaction, no Codman triangle, no cortical breach and no associated soft tissue mass.

MRI

MR appearances are variable, and depend on when along the development and healing phase the lesion is imaged.

Initially the lesion has high or intermediate T2 signal, with a peripheral low signal rim corresponding to the sclerotic border. As it matures and begins to ossify the signal becomes low signal on all sequences ³.

Contrast enhancement is also variable ¹.

Bone scan

Appearance again depends on phase of lesion. In general they are negative, however mild hyperaemia and moderate bone uptake are present during healing. If extensive uptake or hyperaemia are present then an alternative diagnosis or superimposed fracture should be considered ⁴.

Non-ossifying fibroma is one of the skeletal “Don’t touch” lesions.

Treatment and prognosis

Most NOFs require no treatment and no biopsy. If large (involving more that 50% of the diameter of the parent bone) then prophylactic curettage and bone grafting maybe prudent to avoid a pathological fracture ¹.