Chondromyxoid fibroma
Key points
- Least common benign cartilage tumour
- Eccentric metaphyseal lytic lesion with sclerotic rim in a young adult - classic description
- Unlike other chondroid tumours, rarely calcifies on XR ("least chondroid" chondroid tumour)
- Proximal tibia is the single most common site
- Frequently misdiagnosed as chondrosarcoma histologically due to pseudomalignant atypia
Epidemiology & risk factors
- Age: 2nd-3rd decades (peak 10-30 years)
- M:F = 1.5:1
- Presentation: chronic pain (most common), local swelling, or incidental
Pathophysiology
Lobules of spindle/stellate cells in a myxoid stroma separated by cellular bands. The myxoid component accounts for the markedly high T2 signal on MRI. Nuclear atypia can mimic low-grade chondrosarcoma histologically ("pseudomalignant" trap).
Imaging / investigations
XR
- Lobulated, eccentric, lytic metaphyseal lesion (often abutting the physis)
- Sclerotic scalloped rim with septated/bubbly internal appearance (pseudotrabeculation)
- Cortical expansion and thinning - may appear to "bite" into the cortex
- Calcification rare (key differentiator from other chondroid tumours)
- No periosteal reaction unless pathological fracture
CT
- Confirms cortical thinning/breach extent
- Subtle punctate calcification in ~25% (not visible on XR)
- Mild contrast enhancement
MRI
- T1: hypointense to isointense
- T2/STIR: markedly hyperintense (myxoid content)
- Post-contrast: peripheral nodular enhancement with septal enhancement; central non-enhancement (myxoid/cystic degeneration)
Nuclear medicine
- Bone scan: intense radiotracer uptake (hot spot)
Differentials
| Lesion | Key distinguishing features |
|---|---|
| Aneurysmal bone cyst | Fluid-fluid levels on MRI; rapid expansion |
| Non-ossifying fibroma | Usually asymptomatic; migrates away from physis; less expansile |
| Chondrosarcoma | Older age (>40); central; definite chondroid matrix (popcorn calcification); aggressive features |
| Giant cell tumour | No sclerotic rim; extends to subchondral surface; closed physes |
| Enchondroma | Central location; rings and arcs calcification |
Management
- Surgical curettage with bone grafting
- Recurrence ~25% with simple curettage; lower with wide local excision
- Malignant transformation: extremely rare
- Rare cause of secondary ABC - if fluid-fluid levels are seen within a lesion with a thick scalloped sclerotic rim, consider CMF with secondary ABC
Exam pearls
- Classic exam look: eccentric, lytic, metaphyseal, sclerotic rim, young adult, proximal tibia
- The "least chondroid" chondroid tumour - no calcification on XR despite being a cartilage tumour
- Radiology-pathology discordance: if radiology says benign (sclerotic rim) but pathology says low-grade malignancy, suggest review for CMF
Common pitfalls
- Do not dismiss as chondrosarcoma based on histology alone - CMF has pseudomalignant atypia
- Fluid-fluid levels do not exclude CMF - it can develop secondary ABC
