Non-Ossifying Fibroma (NOF)

December 04, 2024

Last updated

Overview

Non-ossifying fibroma (NOF) is a benign bone lesion, most common in children and young adults. These lesions are eccentric, radiolucent, and ovoid with scalloped margins and a sclerotic border. Typically asymptomatic, they are incidentally discovered on imaging. Malignant transformation does not occur. Management is usually observation, with surgery reserved for large or symptomatic lesions or fractures.

Orthopedic surgeons often refer to the Ritschl staging system when evaluating NOFs. Stage B is the largest and most bubbly stage with the highest risk of fracture. While this classification is helpful to be aware of, it is typically not explicitly mentioned in reports. The focus in radiology remains identifying a lesion as an NOF, then identifying features that indicate increased risk or presence of fracture.

Demographics

  • Age: Average 14 years.
  • Gender: Male > Female (2:1).
  • Common locations: Metaphyseal region of long bones—distal femur, proximal tibia, distal tibia, and fibula.

Presentation

Most are asymptomatic. Large lesions can cause pain or pathologic fractures, particularly in active adolescents.

Radiographic Features

  • Appearance:
    • Eccentric, ovoid, radiolucent lesion.
    • Narrow transition zone, scalloped margins, sclerotic border.
    • Cortex may thin or expand; larger lesions may span the bone’s width.
    • Periosteal reaction indicates presence of fracture.
non-ossifying fibroma xrays

MRI Features

  • Signal depends on stage:
    • Early: Low-signal rim (sclerotic border) on T1 and T2; high/intermediate T2 centrally.
    • Late: Low signal on all sequences as lesion ossifies.
    • Contrast enhancement is variable.

Treatment

  • Observation: Most cases.
  • Surgery: For large, symptomatic lesions or pathologic fractures.

Reporting Checklist

  1. Location: Bone and segment (e.g., metaphysis of the distal femur).
  2. Size: Highlight large lesions to alert clinicians to fracture risk. (More than half the width of the bone as a general rule)
  3. Margins: Assess for scalloping and sclerotic borders.
  4. Transition zone: Confirm narrow zone.
  5. Cortex: Note thinning, expansion, or breach.
  6. Periosteal reaction: Indicates fracture.
  7. Impression: Summarize findings succinctly.

Sample Report

FINDINGS: Eccentric, ovoid, radiolucent lesion with scalloped margins and sclerotic border in the distal femoral metaphysis. Narrow transition zone, no periosteal reaction or cortical breach.

IMPRESSION: Non-ossifying fibroma in the distal femoral metaphysis.

^^^That's the descriptive version for early trainees who need to show their work for their attendings. If it's classic and you are confident about it, just call it what it is. 

Commentary

NOFs are the poster children of benign bone lesions. They’re easy to recognize and harmless most of the time. But when they get big, particularly during the bubbly (Stage B) phase, they can fracture and cause trouble.

For radiologists, the job is simple: Call it what it is, note the size, and flag anything taking up most of the bone width. The orthopods will worry about staging. You just keep it straightforward and move on to the next case.