Xsight Lung Tracking System: A Fiducial-Less Method for Respiratory Motion Tracking

Abstract

The CyberKnife® Robotic Radiosurgery System (Accuray Incorporated, Sunnyvale, CA) can treat targets that move with respiration using the Synchrony® Respiratory Tracking System (Accuray Incorporated, Sunnyvale, CA). Alignment of each treatment beam with the moving target is maintained in real time by moving the beam dynamically with the target. The Synchrony system requires fiducials that are placed in or near the tumor to target the lesion and track it as it moves with respiration. The Xsightℳ (Accuray Incorporated, Sunnyvale, CA) Lung Tracking System, which recently became available for the CyberKnife system, is a direct soft tissue tracking method for respiratory motion tracking of lung lesions that eliminates invasive fiducial implantation procedures, thereby decreasing the time to treatment and eliminating the risk of pneumothorax and other fiducial placement complications. This chapter presents the concepts, methods, and some experimental results of the Xsight Lung Tracking System, which is fully integrated with the Synchrony Respiratory Tracking System. Observation and analysis of clinical image data for patients previously treated with the CyberKnife indicates that many reasonably large tumors (larger than 15 mm) located in the peripheral and apex lung regions are visible in orthogonal X-ray images acquired by the CyberKnife system. Direct tumor tracking can be performed for such visible tumors by registration of the tumor region in digitally reconstructed radiographs (DRRs), generated from the planning CT image, to the corresponding region in the treatment X-ray images. Image processing is used to enhance the visibility of the lung tumor in the DRRs and X-ray images. Experiments with an anthropomorphic motion phantom and retrospective analysis of clinical image data obtained from patients who underwent CyberKnife treatment for lung lesions using implanted fiducial markers show that the accuracy of Xsight Lung tracking is better than 1.5 mm.