Tumour-like Lesions of Bone

Overview

Common Lesions:

  • Fibrous Dysplasia
  • Osteofibrous Dysplasia
  • Paget’s Disease of Bone

Fibrous Dysplasia

Epidemiology

  • Females > males (only other tumour with this ratio is GCT).
  • 75% occur in individuals under 30 years old.

Aetiology

  • Unknown cause.
  • GS alpha protein mutation: Overproduction of cAMP.
  • High expression of FGF-23.
  • Failure of bone to fully mineralize and mature into lamellar bone.
  • Immature woven bone with chondroid elements.
  • Non-hereditary.

Clinical Features

  • Usually an incidental finding.
  • May present with pain or pathologic fracture.
  • Monostotic (80%) or Polyostotic (20%).
  • Polyostotic often associated with endocrinopathy:
    • McCune-Albright Syndrome: Polyostotic fibrous dysplasia, precocious puberty, and pigmented skin lesions.
  • Rare malignant transformation (1%) to osteosarcoma, fibrosarcoma, or MFH.

Location

  • Common: Proximal femur, ribs, maxilla, tibia, pelvis, spine.

Pathology

  • No neoplastic cells.
  • Alphabet soup/Chinese letter appearance of immature woven bone.
  • Secondary ABC may occur.

Differential Diagnosis

  • Low-grade osteosarcoma or fibrosarcoma.

Imaging Features

  • Ground-glass matrix with well-demarcated lytic lesion.
  • No periosteal reaction or cortical breech.
  • Long bone deformities (e.g., shepherd’s crook deformity in femur).
  • Bone scan: High uptake.

Management

  • Asymptomatic: Non-operative.
  • Pain relief: Bisphosphonates.
  • Surgical: Fix impending fractures; intralesional curettage and cortical allografts.
  • Avoid cancellous autograft due to replacement by dysplastic bone.

Osteofibrous Dysplasia

Epidemiology

  • More common in males.
  • Occurs in children.

Aetiology

  • Unknown.
  • Associated with Trisomy 7, 8, 12, 22.
  • May be a precursor to adamantinoma.

Clinical Features

  • Predilection for the anterior tibia.
  • Causes anterior or anterolateral tibial bowing.
  • May result in pseudarthrosis of the tibia.

Pathology

  • Fibrous tissue with immature woven bone (similar to fibrous dysplasia).

Imaging Features

  • Diaphyseal eccentric, well-demarcated lytic lesions within the cortex.

Management

  • Non-operative if possible (may regress with physeal closure).
  • Surgical: Intralesional curettage and cortical allograft for bowing or pseudarthrosis.

Paget’s Disease of Bone

Definition

  • Metabolic bone disease with overactive osteoclasts and osteoblasts, leading to disorganized bone formation.

Epidemiology

  • Slight male predominance.

  • Geographic predilection (e.g., USA, UK).

  • Prevalence increases with age:

    > 40 years: 3%.

    • 80 years: 10%.

Types

  • Monostotic: 20%.
  • Polyostotic: 80%.

Cause

  • Viral (e.g., paramyxovirus).
  • Genetic predisposition.

Pathophysiology

  1. Osteoclastic overactivity: Increased size and number of osteoclasts.
  2. Compensatory osteoblast activity: Disorganized, accelerated bone formation.

Phases

  1. Lytic: Osteoclastic dominance.
  2. Active: Mixed osteoclastic and osteoblastic activity.
  3. Burnt out: Residual effects with coarse trabeculae.

Clinical Features

  • Asymptomatic in 95% (incidental finding).
  • Bone pain (rare but indicates high turnover or malignancy).
  • Fractures, arthritis, deformity, nerve compression, cardiac failure, gout.
  • Malignant transformation: Osteosarcoma, fibrosarcoma, chondrosarcoma.

Imaging Features

  • Widened bones, stress fractures.
  • Blade/flame sign in lytic phase.
  • Mixed sclerotic/lytic patterns (mosaic-like).
  • Deformities (e.g., sabre tibia).

Laboratory Tests

  • Normal calcium.
  • Elevated alkaline phosphatase and acid phosphatase.
  • Increased markers of collagen turnover.

Management

Medical

  • Bisphosphonates: Osteoclast inhibition.
  • Calcitonin (rarely used).

Surgical

  • Favor fracture fixation due to long healing times.
  • Arthroplasty, nerve decompression.
  • Treat malignant transformation.

Malignant Transformation

  • 30x risk of osteosarcoma in polystotic disease.
  • Fibrosarcoma and chondrosarcoma also possible.
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