Evaluation of tumor position and PTV margins using image guidance and respiratory gating.

Abstract

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. 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. 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. 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.