Tracking moving organs in real time.

In an ideal radiotherapy procedure, the treatment system would continuously adapt the radiation beam delivery to changes in the tumor position. The development of such a tracking capability has been underway for more than 10 years, beginning with the CyberKnife image-guided radiosurgery system. In that time, much has been learned about the nature of tumor motion and the technical issues that it presents to a practical real-time tracking system. In this article, I will review the basic concepts behind existing and proposed radiotherapy beam-tracking systems, show clinical evidence of the types of movement that are encountered in real-time tracking situations, describe the corresponding technical problems and solutions, and discuss the unresolved issues in making real-time tumor tracking a practical response to tumor motion.