Non-ossifying fibroma

Last revised by Dr Henry Knipe on 15 Jun 2022

Non-ossifying fibromas (NOF) are benign and generally self-limiting osteoclastic giant cell-rich bone tumors typically found in the metaphyses of long bones. They are classified as osteoclastic giant cell-rich bone tumors 1,2.

The terms ‘fibrous cortical defect’ (confined to the cortex), ‘metaphyseal fibrous defect’ or ‘benign fibrous histiocytoma' are no longer recommended 1

Non-ossifying fibromas are very common in children and adolescents and are considered the most common benign tumor 1-5. The true incidence of non-ossifying fibromas is not known because most lesions are not detected due to the absence of clinical symptoms and the benign natural history of the lesions 1. Their frequency has been estimated at around 30-40% of all normal children with a peak in the second decade 1-4. They are about twice as common in boys than in girls 1,3. They are usually not seen beyond the age of 30 as they heal spontaneously after puberty and ossify gradually 2-5.

Non-ossifying fibromas have been found in association with the following 1,3:

The diagnosis of non-ossifying fibromas is mainly based on characteristic radiographic or imaging features 1. They are considered as ‘do not touch’ or ‘leave me alone lesions’ 4.

Diagnostic criteria according to the WHO classification of soft tissue and bone tumors (5thedition) 1:

The essential feature is:

  • bone tumor with typical imaging findings

The following additional criteria are desirable:

  • metaphyseal location in long bones in skeletally immature individuals
  • storiform proliferation of plump spindle cells
  • interspersed giant cells, haemosiderophages and clusters of foamy histiocytes

Non-ossifying fibromas are usually asymptomatic 1,2 and incidentally found in imaging studies 3. Larger tumors can have an impact on the biomechanical properties of the affected bone and cause pain 1.

Non-ossifying fibromas might cause a pathological fracture if large.

Non-ossifying fibromas are benign spindle cell tumors of bone containing osteoclast-like giant cells 1-3.

They are biologically active and can grow initially become more polycyclic and regress after puberty filling up gradually with bone from the diaphyseal side 3.

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 (fibrous cortical defects/non-ossifying fibromas).

The etiology of sporadic non-ossifying fibromas is not known 1.

They might occur in the background of germline mutations resulting in the RAS-MAPK pathway 5 or postzygotic mutations in neurofibromatosis type 1 or Jaffe-Campanacci syndrome 1.

Most non-ossifying fibromas are found in the metaphysis of long bones of the lower extremities such as the distal femur and the distal lateral and proximal medial tibia 1,2,6. They can also occur at multiple locations 1,3.

Non-ossifying fibromas are far less common in the mandible or the flat pelvic bones in the mandible 7 and might then represent a giant cell tumor of bone with regressive changes 1.

Macroscopically non-ossifying fibromas have been described as usually well-circumscribed reddish-brown tumors with yellowish areas and sclerotic borders. They have a firm cut surface and can display cystic areas and hemorrhagic or necrotic changes, especially in the setting of pathologic fracture 1.

Histologically non-ossifying fibromas are characterized by the following 1,2,6:

  • arrangement of spindle-shaped cells in a storiform pattern
  • diffusely scattered osteoclast-like giant cells
  • hemosiderin deposition
  • foamy macrophages
  • focal cystic changes and reactive bone formation

Non-ossifying fibromas show mutations in KRAS and FGFR1 genes in > 80% of cases 1,2,5.

Characteristic radiographic appearances of non-ossifying fibromas include 3,4:

  • polycyclic, multiloculated, lucent lesion
  • eccentrically located in the metaphysis near the physis
  • long axis parallel to the axis of the bone
  • thin sclerotic rim
  • with increasing age, they seem to migrate away from the physis 6

The natural course of non-ossifying fibromas can be radiographically classified by the Ritschl stages 3,4,8 of which the duration seems to be variable 3:

  • stage A:
    • eccentric bone lesion near the growth plate
    • small oval with smooth outlines and no sclerotic border
  • stage B:
    • variable distance from the growth plate
    • progressively polycyclic shape
    • thin sclerotic border, thin sometimes protruding cortex
    • no periosteal reaction
  • stage C: progressively sclerotic with ossification beginning at the diaphysis
  • stage D: homogeneous sclerosis or disappearing lesion

MRI appearances of non-ossifying fibromas are variable and depend on when along with the development and healing phase the lesion is imaged.

Initially, the lesion has a 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 9,10. They might show septa of variable signal intensity.

Contrast enhancement is also variable 9,11.

  • T1: low signal intensity
  • T2:
    • initially heterogeneously high or intermediate signal with a low signal peripheral rim
    • later low signal intensity
  • T1 C+ (Gd): variable

Appearance on bone scan depends on the phase of the lesion:

  • non-ossifying fibromas are usually not associated with increased activity
  • mild to moderately increased activity can be seen during non-ossifying fibroma "healing", corresponding to mild hyperemia and osteoblastic activity
  • note that extensive uptake or hyperemia should prompt consideration of a superimposed fracture or alternative diagnosis 12

The radiological report should include a description of the following 13:

  • location and size
  • tumor margins and transition zone
  • relation to the growth plate
  • degree of sclerosis

On CT and MRI the lesion should be categorized as Bone-RADS 1 lesion 13.

Non-ossifying fibromas are considered as “leave alone” or “do not touch” lesions.

Due to their benign self-limiting natural history, they do not require biopsy or follow-up in the setting of characteristic imaging features 1.

As long as biomechanical stability is not affected they do not require treatment. If they are 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. Recurrence is rare and malignant transformations have not been reported 1.

Non-ossifying fibromas were first described by the American bone pathologists Henry Louis Jaffe and Louis Lichtenstein in 1942 3,14.

Non-ossifying fibromas have been also known as fibroxanthoma in the past a term considered synonymous with 'benign fibrous histiocytoma' (see above) and rarely used nowadays 15.

General imaging differential considerations include:

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Cases and figures

  • Figure 1: distribution
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  • Case 1
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  • Case 2: on MRI T1
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  • Case 3
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  • Case 4: with a pathological fracture
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  • Case 5
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  • Case 6: on CT
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  • Case 7
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  • Case 8
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  • Case 9
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  • Case 10: with fracture
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  • Case 11
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  • Case 12: healing
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  • Case 13
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  • Case 14
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  • Case 15
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  • Case 16: non-ossifying fibroma
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