## Most Common Site of Metastasis in Prostate Cancer **Key Point:** Bone is the most common site of metastasis in prostate cancer, occurring in approximately 80–90% of patients with distant metastases. The lumbar spine and pelvis are the most frequently involved skeletal sites. ### Mechanism of Bone Tropism Prostate cancer cells preferentially metastasize to bone through several mechanisms: 1. **Batson's venous plexus** — retroperitoneal veins drain directly into the vertebral venous plexus, bypassing the lungs and allowing direct seeding to the spine and pelvis. 2. **Osteomimicry** — cancer cells express adhesion molecules (e.g., RANKL, TGF-β) that promote osteoclast activation and bone remodeling. 3. **Bone microenvironment** — high local concentration of growth factors (IGF-1, TGF-β) creates a permissive niche for prostate cancer cells. ### Pattern of Bone Metastases | Site | Frequency | Clinical Significance | | --- | --- | --- | | Lumbar spine | Most common | Risk of spinal cord compression | | Pelvis | Very common | Risk of pelvic fracture | | Femur | Common | Pathological fracture risk | | Thoracic spine | Moderate | Vertebral body involvement | | Ribs | Less common | Usually with advanced disease | **Clinical Pearl:** Bone metastases in prostate cancer are typically **osteoblastic** (sclerotic), unlike most other cancers which are osteolytic. This is why bone scan (technetium-99m MDP) and CT are more sensitive than plain radiographs for detecting early metastases. **High-Yield:** In a patient with prostate cancer presenting with back pain, spinal metastasis must be ruled out urgently to prevent spinal cord compression and paraplegia. ### Why Bone and Not Lung? Although the lungs receive the entire cardiac output, prostate cancer cells preferentially lodge in bone because: - Batson's plexus provides a direct anatomical route. - The bone microenvironment is uniquely permissive for prostate cancer growth. - Lung metastases occur late and are less common than bone metastases. [cite:Harrison 21e Ch 93]
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