Updated: 11/5/2016

Oncogenic Osteomalacia

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Introduction
  •  Definition
    • paraneoplastic syndrome of renal phosphate wasting
    • caused by bone tumor or soft tissue tumor 
  • Epidemiology
    • demographics
      • age bracket
        • age of onset is late childhood to early adulthood
  • Pathophysiology
    • the tumor secretes a humoral factor ("phosphatonin") that affects the proximal renal tubules
    • reduces calcitriol production in the kidney and inhibits phosphate transport
    • leads to increased renal phosphate excretion, hypophosphatemia and osteomalacia
    • types of tumors that cause oncologic osteomalacia (known as phosphaturic mesenchymal tumor)
      • benign tumors (more common)
        • phosphaturic mesenchymal tumors (mixed connective tissue variant) e.g. hemangiopericytoma (commonest cause)
        • osteoblastoma-like tumors
        • ossifying fibrous tumors
        • nonossifying fibrous tumors
      • malignant causes (rare)
        • osteosarcoma
        • fibrosarcoma
  • Genetics
    • mutations
      • phosphatonin gene is FGF23
  • Associated conditions
    • orthopaedic manifestations
      • pathological fractures of long bones and vertebrae
Presentation
  • Symptoms
    • generalized bone and muscle pain
    • fractures of long bones, ribs and vertebrae
    • proximal muscle weakness
    • fatigue
Imaging
  • Radiographs
    • findings
      • diffuse osteopenia 
      • Looser's zones (pseudofractures) 
  • Octrotide scans (radiolabeled somatostatin analog)
    • gallium-68 DOTA-octreotate PET scan  
    • indium-111 pentetreotide SPECT/CT
    • indications
      • to identify primary tumors when TIO is suspected 
      • will only identify tumors expressing somatostatin receptors
Studies
 
Serum Ca
Serum P
Alk phos
PTH
25-(OH)vit D
1,25-(OH)vit D
Urinary Ca
Osteomalacia 
low 
low
high
high
low
low
low
Osteoporosis normal normal variable normal normal  normal  normal 
Tumor induced osteomalacia low very low low low low low low
Osteopetrosis normal normal high normal normal normal normal

Treatment
  • Nonoperative
    • phosphate supplementation with 1,25-dihydroxyvitamin D
  • Operative
    • tumor removal
      • outcomes
        • resolution of hypophosphatemia and low vitamin D levels within hours of resection
 

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