Abstract: Objective To compare CyberKnife doses based on lung volume determined with the patient in the supine or prone position in order to maximize efficacy of treating metastatic tumors in the spinous process of thoracic vertebra while minimizing damage to normal tissues. Method A total of 9 patients with metastatic tumors in the spinous process of thoracic vertebra were treated using CyberKnife dosing plans based on the supine or prone position， and outcomes were compared between the two approaches. Results Compared to the treatment based on the prone lung volume，treatment based on the supine lung volume required an additional 14，862-36，337 MU，lung V₅ was 5.20-7.90 Gy higher，and lung V₂₀ was 2.61-5.73 Gy higher. In addition， the treatment based on supine lung volume delivered -2.21-2.67 Gy more to the spine volume，-3.93-7.85 Gy more to the skin volume，and 0.28-6.39 Gy more to the esophagus. Conclusion Planning Cyberknife treatment based on the lung volume determined with the patient in the prone position may allow lower total radiation doses to be used and reduce exposure of adjacent healthy tissue. 

Key words:  Lung neoplasms, Supine position, Prone position, Treatment planning, Lung volume