Age / Sex : 6 / M

Discussion

Answer:  McCune-Albright syndrome

(or polyostotic fibrous dysplasia)

Findings:

In Pelvic AP, there is expansile osteolytic lesions at right femoral neck, intertrochanteric region with pathologic fracture at the neck. Some portion of the expansile lesion shows somewhat ground glass density. Another osteolytic lesion is also suspected in right acetabulum. In plain radiographs of both lower extremity, multiple osteolytic lesions are suspected in both distal femora, and right fibula shaft.  Except the lesion of right fibular shaft, well defined thick peripheral sclerotic rims are seen around these osteolytic lesions, representing “rind sign”. Multiple sclerotic lesions are also seen at right femoral shaft and right proximal fibular shaft eccentrically.

In images of MRI of right hip joint, the expansile lesions in right femur and right acetabulum show heterogeneous low signal in T1 weighted image and heterogeneous mixed signal in T2 weighted image with intense enhancement.

All these osteolytic and sclerotic lesions show increased MDP uptake in bone scan.

Differential Diagnosis:

 Polyostotic fibrous dysplasia only, Mazabraud syndrome

Discussion:

Fibrous dysplasia (FD) is a non-neoplastic tumour-like congenital process, manifested as a localised defect in osteoblastic differentiation and maturation, with the replacement of normal bone with large fibrous stroma and islands of immature woven bone. Fibrous dysplasia has a varied radiographic appearance. If asymptomatic, it does not require treatment. 

• Subtype
  
  
  
  1. monostotic: single bone
  
  
  2. polyostotic: multiple bones
  
  
  3. craniofacial fibrous dysplasia: skull and facial bones alone
  
  
  4. cherubism: mandibleand maxilla alone (not true fibrous dysplasia)
  
  
  
  


• Epidemiology:
  
  
  
  1. predominantly in children and young adults
  
  
  2. ~75% of patients presenting before the age of 30 years (highest incidence between 3 and 15 years).
  
  
  3. no recognized gender predilection 
  
  
  
  


• Pathology

Fibrous dysplasia is due to developmental dysplasia and focal arrest in normal osteoblastic activity secondary to a non-hereditary mutation which results in the presence of all of the components of normal bone with a lack of normal differentiation into their mature structures. Macroscopically, lesions are intramedullary and well circumscribed with abnormal whitish-grey colour. Microscopically it manifests as large fibrous matrix with scattered curvilinear irregularly shaped trabeculae of immature, inadequately mineralized bone.

• Radiographic features
  
  
  
  1. Plain radiograph
     
     
     
     • ground-glass matrix
     
     
     • may be completely lucent (cystic) or sclerotic
     
     
     • well circumscribed lesions
     
     
     • no periosteal reaction
     
     
     • rind sign: lesion is surrounded by a layer of thick, sclerotic reactive bone (i.e. a rind) and is suggestive of fibrous dysplasia
     
     
     • fusiform enlargement of ribs
     
     
     • protrusio acetabuli
     
     
     • premature fusion of growth plates leading to short stature
     
     
     • bowing deformities
     
     
     • shepherd crook deformityof the femoral neck
     
     
     • discrepant limb length
     
     
     • Looser zones
     
     
     
     
  
  
  2. CT
     
     
     
     • ground-glass opacities: 56% ⁴
     
     
     • homogeneously sclerotic: 23%
     
     
     • cystic: 21%
     
     
     • well-defined borders
     
     
     • expansion of the bone, with intact overlying bone
     
     
     • endosteal scalloping may be seen 
     
     
     
     
  
  
  3. MRI
  
  
  
  

MRI is not particularly useful in differentiating fibrous dysplasia from other entities as there is marked variability in the appearance of the bone lesions, and they can often resemble a tumour or more aggressive lesions. 

T1: heterogeneous signal, usually intermediate

T2: heterogeneous signal, usually low, but may have regions of higher signal

T1 C+ (Gd): heterogeneous contrast enhancement 

1. Nuclear Medicine

increased tracer uptake on Tc⁹⁹ bone scans (lesions remain metabolically active into adulthood).

• Complications
  
  
  
  • pathological fractures.
  
  
  • Sarcomatous dedifferentiation(< 1%): more common in the polyostotic form and may relate to previous treatment with radiation therapy.
  
  
  
  


• Associated syndrome
  
  
  
  1. McCune-Albright syndrome
     
     
     
     • in 2-3% of cases with the polyostotic form 
     
     
     • a genetic disorder characterised by the association of:
       
       
       
       1. endocrinopathy: precocious puberty
       
       
       2. polyostotic fibrous dysplasia:more severe than in sporadic cases
       
       
       
       
     
     
     
     
  
  
  
  

• cutaneous pigmentation: coast of Maine 'cafe au lait' spots

1. Mazabraud syndrome: soft-tissue myxomas (rare); typically multiple intramuscular lesions in the vicinity of most severely affected bone

References:

 https://radiopaedia.org/articles/fibrous-dysplasia

Imaging of Bone Tumors and Tumor-Like Lesions: Techniques and Applications. Springer. ISBN:3540779825