Synonyms

• FD
• Fibrous dysplasia of Bone

Category

Bone

Behaviour

Benign

Gender

Both equally

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

• Monostotic FD: clinical review + plain X-ray at 6 months, then annual for 2 years; discharge if stable
• Polyostotic FD: Annual clinical review + plain X-ray of affected sites; Bone scan at baseline to map extent
• McCune-Albright Syndrome: Annual multidisciplinary review (endocrinology, orthopaedics, ophthalmology)
• No Malignant transformation surveillance required unless rapid growth or sarcomatous features suspected
• Bisphosphonate therapy: monitor renal function and calcium/phosphate at each review
• Discharge from sarcoma service if stable, no fracture risk, and confirmed Benign FD diagnosis