Non-ossifying fibromas (NOF) are the most common type of non-neoplastic fibrous bone lesion 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 at 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 hypophosphatemic vitamin D-resistant rickets 7.
NOF appear macroscopically as a fleshy, fibrous, yellow or tan-brown lesion with variable areas of hemorrhage 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 are typically a multiloculated lucent lesion with a sclerotic rim. They are located eccentrically in the metaphysis, adjacent to the physis. As the patient ages, they seem to migrate away from the physis 9.
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 depends on the phase of the lesion:
- NOF usually not associated with increased activity
- mild to moderately increased activity can be seen during NOF "healing", corresponding to mild hyperemia and osteoblastic activity
- note that extensive uptake or hyperemia should prompt consideration of alternative diagnosis or a superimposed fracture 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
- 1. Jee WH, Choe BY, Kang HS et-al. Nonossifying fibroma: characteristics at MR imaging with pathologic correlation. Radiology. 1998;209 (1): 197-202. Radiology (abstract) - Pubmed citation
- 2. Hetts SW, Hilchey SD, Wilson R et-al. Case 110: Nonossifying fibroma. Radiology. 2007;243 (1): 288-92. doi:10.1148/radiol.2431040427 - Pubmed citation
- 3. Stacy GS, Dixon LB. Pitfalls in MR image interpretation prompting referrals to an orthopedic oncology clinic. Radiographics. 27 (3): 805-26. doi:10.1148/rg.273065031 - Pubmed citation
- 4. Betsy M, Kupersmith LM, Springfield DS. Metaphyseal fibrous defects. J Am Acad Orthop Surg. 12 (2): 89-95. J Am Acad Orthop Surg (full text) - Pubmed citation
- 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
- 9. Goldin A, Muzykewicz DA, Dwek J, Mubarak SJ. The aetiology of the non-ossifying fibroma of the distal femur and its relationship to the surrounding soft tissues. (2017) Journal of children's orthopaedics. 11 (5): 373-379. doi:10.1302/1863-2548.11.170068 - Pubmed
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The differential diagnosis for bone tumors is dependent on the age of the patient, with a very different set of differentials for the pediatric patient.
- bone-forming tumors
- cartilage-forming tumors
- chondromyxoid fibroma
- juxtacortical chondroma
- fibrous bone lesions
- bone marrow tumors
- other bone tumors or tumor-like lesions
- aneurysmal bone cyst
- benign fibrous histiocytoma
- giant cell tumor of bone
- Gorham massive osteolysis
- haemophilic pseudotumor
- intradiploic epidermoid cyst
- intraosseous lipoma
- musculoskeletal angiosarcoma
- musculoskeletal hemangiopericytoma
- primary intraosseous hemangioma
- post-traumatic cystic bone lesion
- simple bone cyst
- impending fracture risk