Commissioning an image-guided localization system for radiotherapy

To describe the design and commissioning of a system for the treatment of classes of tumors that require highly accurate target localization during a course of fractionated external-beam therapy. This system uses image-guided localization techniques in the linac vault to position patients being treated for cranial tumors using stereotactic radiotherapy, conformal radiotherapy, and intensity-modulated radiation therapy techniques. Design constraints included flexibility in the use of treatment-planning software, accuracy and precision of repeat localization, limits on the time and human resources needed to use the system, and ease of use.

A commercially marketed, stereotactic radiotherapy system, based on a system designed at the University of Florida, Gainesville, was adapted for use at the University of Washington Medical Center. A stereo pair of cameras in the linac vault were used to detect the position and orientation of an array of fiducial markers that are attached to a patient's biteblock. The system was modified to allow the use of either a treatment-planning system designed for stereotactic treatments, or a general, three-dimensional radiation therapy planning program. Measurements of the precision and accuracy of the target localization, dose delivery, and patient positioning were made using a number of different jigs and devices. Procedures were developed for the safe and accurate clinical use of the system.

The accuracy of the target localization is comparable to that of other treatment-planning systems. Gantry sag, which cannot be improved, was measured to be 1.7 mm, which had the effect of broadening the dose distribution, as confirmed by a comparison of measurement and calculation. The accuracy of positioning a target point in the radiation field was 1.0 +/- 0.2 mm. The calibration procedure using the room-based lasers had an accuracy of 0.76 mm, and using a floor-based radiosurgery system it was 0.73 mm. Target localization error in a phantom was 0.64 +/- 0.77 mm. Errors in positioning due to couch rotation error were reduced using the system.

The system described has proven to have acceptable accuracy and precision for the clinical goals for which it was designed. It is robust in detecting errors, and it requires only a nominal increase in setup time and effort. Future work will focus on evaluating its suitability for use in the treatment of head-and-neck cancers not contained within the cranial vault.