Fibrous Dysplasia
Synonyms: FD, Fibrous dysplasia of Bone
GNAS1 mutation testing is diagnostic and distinguishes from Low-grade OS or other fibro-osseous lesions
Quick Facts
Behaviour
Benign
Category
Bone
Synonyms
- FD
- Fibrous dysplasia of Bone
Category
Bone
Behaviour
Benign
Gender
M = F
Tissue of Origin
Bone
Epidemiology
- Common fibro-osseous lesion
- Peak incidence in childhood and adolescence
- Three forms: monostotic (75%), polyostotic (25%), and McCune-Albright Syndrome
Clinical Features
- Pain, deformity, pathological fracture
- Limb length discrepancy in polyostotic form
- McCune-Albright: café-au-lait spots, precocious puberty, endocrine abnormalities
- Shepherd's crook deformity of proximal femur
Location
- Proximal femur and skull/facial bones most common
- Ribs (polyostotic)
- Tibia, humerus
- Gnathic bones (jaw)
Imaging
- Ground-glass matrix ('smoky' appearance on X-ray/CT)
- Shepherd's crook deformity of proximal femur
- MRI: heterogeneous, may show cystic areas
- Bone scan: intensely hot (polyostotic lesions demonstrate full skeletal extent)
Pathology
- Curvilinear trabeculae of woven Bone ('Chinese characters') in Fibrous stroma
- No osteoblastic rimming (unlike ossifying fibroma)
- GNAS1 mutation in osteoprogenitor cells
- Cartilaginous islands in 10% (fibrocartilaginous dysplasia)
Genetics
- Activating GNAS1 mutation (Arg201His or Arg201Cys) - postzygotic somatic
- Not heritable - sporadic mosaic mutation
- Degree of mosaicism correlates with extent of disease
Treatment
- Medical: bisphosphonates (pamidronate) for pain and to reduce fracture risk
- Surgical: curettage and Bone grafting for impending/completed fractures
- Intramedullary nailing for shepherd's crook deformity
- Denosumab for refractory cases
Prognosis
- Generally Benign - does not spontaneously resolve
- Malignant transformation <1% (to osteosarcoma, fibrosarcoma, MFH)
- McCune-Albright Syndrome associated with endocrine morbidity
- Monostotic form generally stable after skeletal maturity
Key Points
- GNAS1 mutation testing is diagnostic and distinguishes from Low-grade OS or other fibro-osseous lesions
- Malignant transformation risk is very Low but increases with radiation exposure (avoid radiotherapy)
- Bisphosphonates are the mainstay of non-surgical management
- Regular screening for endocrine abnormalities in McCune-Albright
Workup - Blood Tests
- Bone profile: ALP, calcium, phosphate - ALP elevated in polyostotic disease
- PTH, 25-OH vitamin D - exclude hyperparathyroidism
- GH, IGF-1 - if McCune-Albright Syndrome suspected
- Cortisol, ACTH - Cushing association in McCune-Albright
- Thyroid function tests - hyperthyroidism in McCune-Albright
Workup - Local Imaging
- Plain radiograph
- MRI - if Malignant transformation suspected (pain, rapid growth, soft tissue mass)
- CT - cortical thinning, full extent of lesion
Workup - Biopsy
- Biopsy NOT routinely required for classic imaging appearance
- Core needle biopsy - if Malignant transformation suspected
- Histology: woven Bone trabeculae ('Chinese letters') in Fibrous stroma
- GNAS mutation (R201H/C) - present in 95%; confirms Fibrous dysplasia
Workup - Staging
- Bone scan or whole-body MRI - polyostotic disease extent
- Full skeletal survey for McCune-Albright workup
Workup - Other
- Endocrinology referral for McCune-Albright Syndrome
- Bisphosphonates (pamidronate/zoledronate) for pain and fracture risk reduction
Follow-up Summary
- 1
Monostotic FD
clinical review + plain X-ray at 6 months, then annual for 2 years; discharge if stable
- 2
Polyostotic FD
Annual clinical review + plain X-ray of affected sites; Bone scan at baseline to map extent
- 3
McCune-Albright Syndrome
Annual multidisciplinary review (endocrinology, orthopaedics, ophthalmology)
- 4
No Malignant transformation surveillance required unless rapid growth or sarcomatous features suspected
- 5
Bisphosphonate therapy
monitor renal function and calcium/phosphate at each review
- 6
Discharge from sarcoma service if stable, no fracture risk, and confirmed Benign FD diagnosis
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