Performance and characteristics of an IR localizing system for radiation therapy.

We report the development of a new system for interactive patient posture, position and respiratory control during radiation therapy treatment. The system consists of an infrared camera, retro-reflective markers and dedicated software that makes it practical to use in the clinic. The system is designed to be used with multiple retro-reflective markers to monitor not only position, but also the posture of the patient in real time. Specific features of the system include: 1. The system reports an absolute misalignment at several points on a patients, and also provides feedbacks on any necessary adjustments in terms of site specific set-up parameters, such as focus to surface distance (PIN), superior and inferior alignment, chest-wall angle, etc. 2. The system is based on the set of predefined templates containing number and position of control points and feedback parameters developed for different treatment sites. 3. A "virtual portal vision" procedure is developed to project organ contours in the beams-eye-view (BEV) based on the marker locations obtained in real time and compare them with digitally reconstructed radiographs (DRRs) from CT simulation. Assuming good correlation between external markers and internal anatomy, the system offers the possibility of mimicking a verification procedure without taking port-films, which can potentially reduce the setup time. In this paper, we concentrate on system properties and performance, while initial applications on a number of clinical sites is ongoing. Accuracy and precision of this system are evaluated in the context of breast/chest treatments using rigid phantoms. The system has an intrinsic uncertainty of +/- 1 mm; and when two systems in different rooms (CT and treatment room) are used for correlating positional information, the uncertainty is less than 2 mm.