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Fibrous Dysplasia

Summary

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  • Benign bone disorder characterised by replacement of normal bone with fibro-osseous tissue
  • Presents with bone pain, deformity, or pathological fractures
  • Imaging shows expansile lesions with ground-glass appearance on radiographs and CT

Pathophysiology

  • Caused by activating mutations in the GNAS1 gene
  • Results in abnormal proliferation and differentiation of bone marrow stromal cells
  • Leads to replacement of normal bone with fibrous tissue and immature woven bone
  • Can affect single (monostotic) or multiple (polyostotic) bones

Demographics

  • Accounts for 5-7% of benign bone tumours
  • Typically presents in children and young adults
  • No significant gender predilection
  • Monostotic form more common (70-80% of cases) than polyostotic form

Diagnosis

  • Often asymptomatic and discovered incidentally on imaging
  • Clinical presentation may include:
    • Bone pain
    • Pathological fractures
    • Deformity (especially in craniofacial involvement)
  • Laboratory findings usually normal, but may show elevated alkaline phosphatase
  • Biopsy may be necessary for definitive diagnosis in atypical cases

Imaging

  • Radiographs:
    • Expansile lesions with ground-glass appearance
    • Thinning of cortex without periosteal reaction
    • "Shepherd's crook" deformity in proximal femur
  • CT:
    • Better delineation of lesion extent and cortical involvement
    • Homogeneous ground-glass attenuation
  • MRI:
    • T1: low to intermediate signal intensity
    • T2: variable signal intensity (low to high)
    • Enhancement with gadolinium contrast
  • Bone scintigraphy:
    • Increased uptake in affected areas
    • Useful for detecting polyostotic involvement

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  • A 50-year-old patient complained of a hard lump behind there left ear.
  • CT showed a heterogeneously expanded left occipital bone.
  • MRI showed the lesion to have mixed signal intensity on diffusion-weighted and T2-weighted imaging and patchy enhancement.

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  • Fibrous dysplasia involving multiple bone of the skull in the context of McCune-Albright syndrome.

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  • The expansile lesion in the sphenoid bone has a ground-glass texture on CT.
  • On MRI, the lesion was hypointense on all sequences.

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  • 45-year-old patient presented with a longstanding skull deformity.
  • CT showed an expansile lesion centered on the left temporal bone with the ground glass texture that is classic for fibrous dysplasia.

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  • A 50-year-old patient presented with a painless lump over the back of the head.
  • Imaging showed a hazy well-demarcated low-density lesion with heterogenous enhancement consistent with fibrous dysplasia.

Treatment

  • Asymptomatic lesions: observation and monitoring
  • Symptomatic lesions:
    • Bisphosphonates to reduce pain and improve bone density
    • Surgical intervention for:
    • Pathological fractures
    • Severe deformity
    • Impending fractures
    • Radiation therapy for craniofacial lesions (controversial)
  • Regular follow-up to monitor for progression and malignant transformation (rare, <1% of cases)

Differential diagnosis

Differential Diagnosis Differentiating Feature
Ossifying fibroma Well-defined borders, capsule present
Osteosarcoma Aggressive periosteal reaction, cortical destruction
Paget's disease Affects entire bone, thickened cortex
Aneurysmal bone cyst Fluid-fluid levels on MRI, expansile lytic lesion
Giant cell tumour Occurs in epiphysis, soap bubble appearance
Enchondroma Occurs in hands/feet, stippled calcifications
Osteoblastoma Expansile lytic lesion in posterior elements; vascular enhancement; no ground-glass matrix
Eosinophilic granuloma Bevelled edge appearance on CT; permeative bone destruction; no ground-glass matrix
Metastatic disease Multiple lesions; destructive or permeative pattern; irregular margins; no ground-glass matrix
Osteoid osteoma Small, round lucency with central nidus