Locating and targeting moving tumors with radiation beams.

3D knowledge of the tumor position during abdominal and thoracic radiotherapy is an important component of motion management in radiation therapy. A wide variety of real-time position monitoring systems are available or under development. These are based on a diversity of modalities including radiofrequency, radioisotopes, ultrasound and MRI in addition to the optical, kilovoltage and megavoltage imaging systems available on conventional accelerators. These systems are also providing new insights into the magnitude and complexity of target and normal tissue motion during a course of therapy, and are driving the development of real-time targeting systems. Real-time targeting devices to align the tumor and the radiation beam have built upon technologies of robots, multileaf collimators, and couch-based and gimbaled positioning systems. The integration and widespread dissemination of systems that locate and target moving tumors are ongoing developments in the early 21st century, and future systems are likely to include the functionality of targeting temporally changing tumors and normal tissue physiology as well as anatomy.