How can fibrous dysplasia be distinguished from Paget's disease?

Differentiating Fibrous Dysplasia from Paget's Disease

The key to distinguishing fibrous dysplasia from Paget's disease lies in patient age, radiographic appearance, and biochemical markers: fibrous dysplasia presents in younger patients (typically <30 years) with a "ground-glass" appearance on imaging and normal alkaline phosphatase, while Paget's disease occurs after age 40 with coarsened trabecular patterns, bone enlargement, and markedly elevated alkaline phosphatase 1, 2.

Age at Presentation

• Fibrous dysplasia: Manifests in childhood to young adulthood, typically before age 30. Symptoms often begin in childhood, with skeletal deformities noted as early as age 3 3, 4.
• Paget's disease: Rarely presents before age 40, with frequency increasing with advancing age. Most common in patients over 55 years 1.

This age distinction is the most reliable initial differentiating feature in clinical practice.

Radiographic Features

Fibrous Dysplasia

• "Ground-glass" appearance on CT and plain radiographs—the single most distinguishing radiographic feature 2
• Typically unilateral or asymmetric involvement 2, 5
• Commonly affects ribs, tibia, femur, and craniofacial bones 5
• Paranasal sinus involvement is frequent 2
• May show cyst-like changes 2
• Bone expansion without the coarse trabecular pattern seen in Paget's 2, 6

Paget's Disease

• Coarsened, thickened trabeculae with a "cotton wool" appearance in the skull 1
• Bone enlargement and deformity, particularly in weight-bearing bones 1
• Markedly thickened cortices 2
• Typically symmetric or polyostotic in a more organized pattern 2
• Preferentially affects pelvis, vertebrae, skull, femur, and tibia 1
• Bowing deformities of long bones, beginning proximally and advancing distally 1

Biochemical Markers

Fibrous Dysplasia

• Normal or mildly elevated alkaline phosphatase in most cases 3, 4
• Bone-specific markers (osteocalcin, bone-specific ALP) may be normal or slightly elevated 4
• This is critical: the absence of markedly elevated ALP helps exclude Paget's disease

Paget's Disease

• Markedly elevated serum total alkaline phosphatase—often the initial finding leading to diagnosis 1
• Elevated bone formation markers (P1NP, bone-specific ALP) 1
• Elevated bone resorption markers (CTX, NTX) 1
• The degree of ALP elevation correlates with disease extent and activity 1

Nuclear Medicine Imaging

Both conditions show increased uptake on bone scintigraphy, but patterns differ:

• Fibrous dysplasia: Multiple focal areas of uptake, often unilateral, involving ribs, long bones, and craniofacial bones 5
• Paget's disease: Intense uptake in affected bones with characteristic distribution (pelvis, spine, skull, long bones). The Endocrine Society recommends radionuclide bone scan to determine disease extent in confirmed Paget's cases 1

Histopathology

When imaging and biochemical findings are equivocal, biopsy is essential 4:

• Fibrous dysplasia: Normal bone and marrow replaced with fibro-osseous tissue, immature bone islands within fibrous stroma 3, 7
• Paget's disease: Chaotic lamellar bone interspersed with woven bone, very large osteoclasts, mosaic pattern of cement lines 1

Clinical Context

Fibrous Dysplasia

• May be monostotic (70%) or polyostotic (30%) 3
• Associated with café-au-lait spots and precocious puberty in McCune-Albright syndrome 3
• Caused by somatic GNAS mutation occurring during embryogenesis 3, 7

Paget's Disease

• Family history in 10-20% of cases with autosomal dominant pattern 1
• Sequestosome 1 gene mutations increase susceptibility 1
• Associated complications include hearing loss, spinal stenosis, high-output heart failure, and increased fracture risk 1

Common Pitfalls

1. Don't rely solely on bone scan: Both conditions show increased uptake; use in conjunction with plain radiographs and biochemical markers 5
2. Don't assume polyostotic disease is Paget's: Polyostotic fibrous dysplasia exists and can mimic Paget's distribution 3, 5
3. Don't skip ALP measurement: This simple test is highly discriminatory—normal ALP essentially excludes active Paget's disease 1, 4
4. Consider age first: A patient under 30 with sclerotic bone lesions is fibrous dysplasia until proven otherwise; over 40 favors Paget's 1, 4

Diagnostic Algorithm

1. Assess patient age: <30 years → fibrous dysplasia likely; >40 years → Paget's disease likely
2. Obtain plain radiographs of affected areas 1: Look for ground-glass (FD) vs. coarsened trabeculae (Paget's)
3. Measure serum total alkaline phosphatase 1: Normal/mildly elevated → FD; markedly elevated → Paget's
4. CT imaging if skull/facial involvement: Ground-glass appearance, asymmetry, sinus involvement favor FD 2
5. Bone biopsy if diagnosis remains uncertain after above workup 4