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PEST PLANTS - Secondary (Metastatic) Bone Disease

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Definition

• These five carcinomas account for >80% of all bone metastases
• Nearly 50% of all cancer patients will metastasise to the skeleton

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Pathogenesis - Batson's Vertebral Venous Plexus

• Venous drainage from the breast, lung, thyroid, kidney, and prostate drains into this valveless plexus
• The plexus has intimate connections with vertebral bodies, pelvis, skull, and proximal limb girdles
• The valveless nature allows retrograde flow under raised intra-abdominal/thoracic pressure
• This explains the axial and proximal limb predilection of bone metastases

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Clinical Presentation

• Patient older than 50 years with a single destructive bone lesion, even without a known primary - metastatic disease is the primary consideration until proven otherwise
• Pain preceding fracture, history of prior malignancy, fracture with minimal/no trauma (pathological fracture)

• Localised bone pain (dull ache to severe, exacerbated by weight-bearing)
• Referred pain (e.g., hip metastasis presenting as knee pain)
• Pathological fracture
• Hypercalcaemia (fatigue, confusion, nausea, constipation)
• Spinal cord compression (with vertebral metastases)

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Common Sites of Skeletal Metastasis

1. Spine (most common - vertebral bodies)
2. Pelvis
3. Ribs
4. Skull
5. Proximal femur (most common site of pathological fracture)
6. Proximal humerus

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Radiological Features

• Latent (inactive): Well-defined margins, rim of reactive bone
• Active: Less well-defined, may expand/thin cortex
• Aggressive: Poorly marginated, cortical destruction, permeative pattern

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Workup / Investigations

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Mirels' Scoring System (Impending Pathological Fracture)

• ≤7 points - Fracture risk 4% → Observe, radiotherapy
• 8 points - Fracture risk 15% → Borderline; consider prophylactic fixation
• ≥9 points - Fracture risk ≥33% → Prophylactic fixation indicated

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Differential Diagnosis of Lytic Bone Lesion in Patient >40 Years

1. Metastatic carcinoma (most common)
2. Multiple myeloma
3. Lymphoma of bone
4. Infection (chronic osteomyelitis)
5. Primary bone tumours (osteosarcoma, chondrosarcoma, Ewing's, UPS) - must exclude

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Treatment

General Principles

• Aim is palliative (not curative) in most cases - control pain, prevent/treat fractures, maintain function
• Prophylactic fixation is superior to treating completed pathological fractures (less blood loss, shorter surgery, faster rehabilitation, less morbidity, lower cost)

Surgical Management

• Intramedullary nailing - for long bone metastases (femur, humerus) - provides immediate stability and pain relief; fixation should span the entire bone
• Endoprosthetic replacement - for periarticular lesions (e.g., proximal femur - hemiarthroplasty/THR)
• Spinal stabilisation - for vertebral collapse with neurological compromise
• Post-operative radiotherapy is given to the surgical field to destroy residual tumour and prevent implant loosening

Adjuvant/Systemic

• Radiotherapy - effective for pain control (single fraction 8 Gy is as effective as multi-fraction); can be used as primary treatment if no fracture risk
• Bisphosphonates (zoledronic acid) - reduce skeletal related events, bone pain; indicated for breast, prostate, myeloma
• Denosumab - RANK-L inhibitor; alternative to bisphosphonates; reduces osteoclastic bone destruction
• Chemotherapy/Hormonal therapy - per oncology team depending on primary
• Analgesics - WHO ladder; NSAIDs particularly effective for bone pain

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Histology

• Appearance of epithelial cells in a fibrous stroma, often arranged in a glandular pattern
• Mixed inflammatory infiltrate
• When metastatic cancer is confirmed but no primary identified after full workup: "Carcinoma of Unknown Primary (CUP)"

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Prognosis

• Overall poor - presence of bone metastases indicates systemic disease
• Pathological fractures are a negative predictor of survival
• Thyroid and breast primaries have relatively better prognosis (longer survival with bone metastases) - surgery more likely to be worthwhile
• Lung primary has worst prognosis

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Summary Points (for exam quick recall)

1. Five PEST plants = Breast, Lung, Thyroid, Kidney, Prostate (>80% of bone mets)
2. Batson's plexus is the main route of spread
3. Spine > Pelvis > Ribs > Proximal femur - common sites; proximal femur = most common pathological fracture site
4. Prostate = osteoblastic; Kidney/Thyroid = osteolytic; Breast/Lung = mixed
5. Mirels ≥9 = prophylactic fixation; span entire bone with nail
6. Always biopsy before treating - don't miss a primary bone tumour
7. Patient >50 with lytic lesion + no known primary = assume metastasis, investigate accordingly
8. Prophylactic fixation > treating completed fracture in terms of outcomes and cost

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