Evaluation of Tumor Position and PTV Margins Using Image Guidance and Respiratory Gating

Purpose: To evaluate the margins currently used to generate the planning target volume for lung tumors and to determine whether image-guided patient setup or respiratory gating is more effective in reducing uncertainties in tumor position. Methods and Materials: Lung tumors in 7 patients were contoured on serial four-dimensional computed tomography (4DCT) data sets (4-8 4DCTs/patient; 50 total) obtained throughout the course of treatment. Simulations were performed to determine the tumor position when the patient was aligned using skin marks, image-guided setup based on vertebral bodies, fiducials implanted near the tumor, and the actual tumor volume under various scenarios of respiratory gating. Results: Because of the presence of setup uncertainties, the reduction in overall margin needed to completely encompass the tumor was observed to be larger for imaged-guided patient setup than for a simple respiratory-gated treatment. Without respiratory gating and image-guided patient setup, margins ranged from 0.9 cm to 3.1 cm to completely encompass the tumor. These were reduced to 0.7-1.7 cm when image-guided patient setup was simulated and further reduced with respiratory gating. Conclusions: Our results indicate that if respiratory motion management is used, it should be used in conjunction with image-guided patient setup in order to reduce the overall treatment margin effectively.