Non-ossifying fibromas (NOF) are the most common of non-neoplastic fibrous bone lesions and are a larger version (>3 cm) of a fibrous cortical defect; both are encompassed by the term fibroxanthoma or metaphyseal fibrous defect.
NOFs are very common in children and adolescents, and the most common fibrous, bony lesion in this age group 6, with a peak incidence at 10-15 years old. The prevalence is estimated 30-40% of all normal children. It is twice as common in boys than in girls 8. They are usually not seen beyond the age of 30, as they spontaneously heal, being gradually filled in by bone.
The majority of NOFs are asymptomatic. Larger lesions may be painful and potentially weaken the bone enough to predispose to pathological fracture.
Rarely fibroxanthomas can cause hypophosphataemic vitamin D-resistant rickets 7.
An 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 multinucleated 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/NOF).
Plain radiograph / CT
NOFs typically have a sclerotic rim. They often appear multiloculated. They are located eccentrically in the metaphysis, adjacent to the physis. As the patient ages, they seem to migrate away from the growth plate.
They have no associated periosteal reaction, cortical breach or associated soft tissue mass.
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 on all sequences 3.
Contrast enhancement is also variable 1.
Appearance on bone scan again depends on the phase of the lesion. In general they are negative; however, mild hyperaemia and moderate bone uptake are present during healing; this is because of osteoblastic activity. If extensive uptake or hyperaemia is present however, then an alternative diagnosis or a superimposed fracture should be considered 4.
Treatment and prognosis
Non-ossifying fibroma is one of the skeletal “Don’t touch” lesions.
Most NOFs require no treatment and no biopsy. If large (involving more than 50% of the diameter of the parent bone) then prophylactic curettage and bone grafting may be prudent to avoid a pathological fracture 1.
General imaging differential considerations include:
- fibrous cortical defect
aneurysmal bone cyst
- eccentric lytic metaphyseal lesion
- MR differentiates ABC by fluid-fluid levels
- chondromyxoid fibroma
- fibrous dysplasia
- desmoplastic fibroma
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- 5. Iagaru A, Henderson R. PET/CT follow-up in nonossifying fibroma. AJR Am J Roentgenol. 2006;187 (3): 830-2. doi:10.2214/AJR.05.0264 - Pubmed citation
- 6. Hod N, Levi Y, Fire G et-al. Scintigraphic characteristics of non-ossifying fibroma in military recruits undergoing bone scintigraphy for suspected stress fractures and lower limb pains. Nucl Med Commun. 2007;28 (1): 25-33. doi:10.1097/MNM.0b013e328012e3de - Pubmed citation
- 7. François S, Lefort G, Poli-Merol ML et-al. Vitamin-resistant rickets cured by removal of a bone tumor. Review of the literature. Rev Chir Orthop Reparatrice Appar Mot. 1998;83 (4): 387-92. Pubmed citation
- 8. Grove J, Robbins C. Nonossifying fibroma: a literature review and case report. PDF