Respiratory motion tracking for robotic radiosurgery

Abstract

Tumors in the thorax and abdomen move during respiration. One way to man age respiratory motion is to move or shape the radiation beam to dynamically follow the tumor's changing position, an approach that is often referred to as real-time tracking. The Synchrony® Respiratory Tracking System, which is an integrated subsystem of the CyberKnife® Robotic Radiosurgery System (Accuray, Incorporated, Sunnyvale, CA) is a realization of real-time tracking for tumors that move with respiration. Alignment of each treatment beam with the moving target is maintained in real time by moving the beam dynamically with the target. An advantage of the Synchrony system is that patients can breathe normally during treatment while the robotic manipulator moves the linear accelerator dynamically. The primary concept in the Synchrony system is a correlation model between in ternal tumor position and external marker position. The position of external optical markers, which are attached with Velcro to a snugly fitting vest that the patient wears during treatment, are measured continuously with a stereo camera system. At the start of treatment, the internal tumor position is measured at multiple discrete time points by acquiring orthogonal X-ray images. A linear or quadratic correlation model is generated by fitting the 3D internal tumor positions at different phases of the breathing cycle to the simultane ous external marker positions. An important feature of this method is its ability to fit different models to the inhalation and exhalation breathing phases, which enables accurate tracking even when the tumor or external marker motions exhibit hysteresis. During treatment, the internal tumor position is estimated from the external marker positions using the correlation model, and this information is used to move the linear accelerator dynamically with the target. The model is checked and updated regularly during treatment by acquiring additional X-ray images. This chapter presents the concepts and methods of the Synchrony Respiratory Tracking System. Experimental mea surements and retrospective analysis of clinical data show that the accuracy of the Synchrony System is approximately 1.5 mm. © 2007 Springer-Verlag Berlin Heidelberg.