Metastatic Bone Disease

Overview & Primaries - Cancer's Bony Outposts

• Definition: Secondary tumor deposits in bone from a distant primary malignancy. Most common bone tumors in adults.
• Common Adult Primaries (Mnemonic 📌 PB KTL):
  • Prostate cancer
  • Breast cancer
  • Kidney (Renal Cell Ca)
  • Thyroid cancer
  • Lung cancer
  • Myeloma (primary marrow cancer, presents with bone lesions).

⭐ Most common primary cancer metastasizing to bone in adult males is Prostate cancer; in adult females is Breast cancer.

• Commonest Primary in Children: Neuroblastoma.
• Skeletal Predilection: Axial skeleton (vertebrae, pelvis, ribs, skull) & proximal long bones; due to abundant red marrow (hematogenous spread).

Pathophysiology & Patterns - How Tumors Invade

• Mechanisms of Tumor Spread:
  • Hematogenous: Most common; Batson's plexus of veins (valveless) facilitates vertebral metastases.
  • Lymphatic spread.
  • Direct extension from adjacent tumor.
• Patterns of Bone Involvement:
  • Osteolytic (bone destruction): Lung, Kidney, Thyroid, Multiple Myeloma. Mediated by ↑osteoclast activity (e.g., RANKL, PTHrP).
  • Osteoblastic (bone formation): Prostate, Carcinoid. Mediated by ↑osteoblast activity (e.g., Endothelin-1).
  • Mixed patterns: Breast, GI tumors.
• Key Molecular Players: RANKL, OPG (Osteoprotegerin), PTHrP (Parathyroid Hormone-related Protein), DKK-1 (Dickkopf-1), Endothelin-1.
• Commonest Skeletal Sites: Vertebrae (>50%), Pelvis, Ribs, Femur (proximal), Humerus (proximal). (📌 "Very Painful Ribs Feel Horrible")

⭐ Osteolytic lesions are predominantly mediated by osteoclast activation via the RANK-RANKL pathway, often stimulated by PTHrP.

Diagnosis & Staging - Spotting the Spread

• Presentation: Bone pain (often nocturnal, mechanical, worse with activity), pathological fractures, hypercalcemia (nausea, confusion), spinal cord compression (neurological deficits, bladder/bowel dysfunction).
• Labs: Sr. Calcium, Alkaline Phosphatase (Alk Phos), PSA (for prostate cancer), other relevant tumor markers.
• Imaging:
  • X-ray: Initial; may show lytic (lucent), blastic (sclerotic), or mixed lesions; periosteal reaction.
  • Bone Scan (Tc-99m MDP): Detects ↑osteoblastic activity / bone turnover (hot spots).

  ⭐ Bone scan (Tc-99m MDP) is highly sensitive for osteoblastic metastases but can be falsely negative in purely lytic lesions (e.g., multiple myeloma, renal cell carcinoma); skeletal survey or PET-CT preferred then.

  • MRI: Best for marrow infiltration, soft tissue extension, spinal cord assessment (especially for compression).
  • PET-CT: Whole-body staging, assessing metabolic activity, and treatment response.
• Biopsy Indications: Unknown primary tumor, solitary bone lesion (to rule out primary bone tumor), or if diagnosis is unclear after imaging.
• Mirels' Scoring: For impending pathological fracture risk in long bones (Site, Pain, Lesion type [lytic/blastic/mixed], Size). Score >8 often indicates need for prophylactic fixation.
• Diagnostic Flow:

Treatment & Complications - Fighting Back & Fallout

Goals: Pain relief, function, stability, local control, prevent fractures. Multidisciplinary.

Systemic Therapy:

• Chemo, Hormonal, Targeted, Immunotherapy.
• Bone-modifying: Zoledronic acid (4mg IV q3-4wks), Denosumab (120mg SC q4wks).

Local Therapy:

• Radiotherapy (RT): EBRT for pain (8Gy x1, 20Gy x5, 30Gy x10), MSCC.
• Surgery: Prophylactic fixation (Mirels' score), fracture stabilization, vertebroplasty.

Key Complications & Management:

• Pathological fractures: Surgical fixation.
• Spinal Cord Compression (MSCC): 🚨EMERGENCY!

• Hypercalcemia: IV fluids, bisphosphonates.

⭐ For suspected MSCC, immediate high-dose corticosteroids (e.g., Dexamethasone) are crucial pending urgent MRI and definitive treatment.

High‑Yield Points - ⚡ Biggest Takeaways

• Metastatic disease is the most common malignant bone tumor overall.
• Common primaries: Prostate (blastic), Breast (mixed/lytic), Lung (lytic), Kidney (lytic), Thyroid (lytic) (PBLKT).
• Axial skeleton (vertebrae, pelvis, ribs) is most frequently affected.
• Clinical features: Severe bone pain (worse at night), pathological fractures, hypercalcemia.
• Diagnosis: X-ray (lytic/blastic lesions), bone scan (sensitive for screening), biopsy (definitive).
• Management is primarily palliative, focusing on pain relief and fracture prevention (radiotherapy, bisphosphonates).