Development of computer-assisted system for setting the appropriate margin of the organ with respiratory movement in radiation therapy planning

External beam radiation therapy is becoming more common due to the increase in cancer rate because of the aging of society and respect for quality of life. Accurate radiation therapy planning is crucial to prevent cancer recurrence and damage to normal tissues. However, the margin for respiratory movement is commonly set based on the operators' experience, and sometimes lacks reproducibility and is not quantitative. The present study was performed to develop a computer-assisted system for setting appropriate margins of organs with respiratory movement in radiation therapy planning. Frontal and lateral chest fluoroscopic images (43x43cm) were obtained during respiration using a dynamic flat-panel detector system. Computer tomography (CT) images were obtained for radiation therapy planning, and digital reconstructed radiographs (DRRs) were created. The respiratory level of CT images was determined by measuring the distance from the lung apex to the diaphragm in DRR, and then one of the fluoroscopic images in the same respiratory level was determined. The thoracic vertebrae were automatically determined as landmarks, and then image registration between DRRs and fluoroscopic images was performed by template-matching. The range of respiratory movement of the target area in the lung was then measured in fluoroscopic images. The quantified range of respiratory movement was then correlated to CT data, and the appropriate margin for respiratory movement was displayed as 3D volume data in the lung. Our system could provide accurate margins for respiratory movement based on the quantified range of movement of the target in the lung during respiration.