Image localization for frameless stereotactic radiotherapy

Purpose: Infrared light-emitting diodes (IRLEDs) have been used for optic-g uided stereotactic radiotherapy localization at the University of Florida s ince 1995, The current paradigm requires stereotactic head ring placement f or the patient's first fraction. The stereotactic coordinates and treatment plan are determined relative to this head ring. The IRLEDs are attached to the patient via a maxillary bite plate, and the position of the IRLEDs rel ative to linac isocenter is saved to file. These positions are then recalle d for each subsequent treatment to position the patient for fractionated th erapy, The purpose of this article was to report a method of predicting the desired IRLED locations without need for the invasive head ring. Methods and Materials: To achieve the goal of frameless optic-guided radiot herapy, a method is required for direct localization of the IRLED positions from a CT scan. Because it is difficult to localize the exact point of lig ht emission from a CT scan of an IRLED, a new bite plate was designed that contains eight aluminum fiducial markers along with the six IRLEDs, After a calibration procedure to establish the spatial relationship of the IRLEDs to the aluminum fiducial markers, the stereotactic coordinates of the IRLED light emission points are determined by localizing the aluminum fiducial m arkers in a stereotactic CT scan. Results: To test the accuracy of direct CT determination of the IRLED posit ions, phantom tests were performed. The average accuracy of isocenter local ization using the IRLED bite plate was 0.65 +/- 0.17 mm for these phantom t ests. In addition, the optic-guided system has a unique compatibility with the stereotactic head ring. Therefore, the isocentric localization capabili ty was clinically tested using the stereotactic head ring as the absolute s tandard, The ongoing clinical trial has shown the frameless system to provi de a patient localization accuracy of 1.11 +/- 0.3 mm compared with the hea d ring. Conclusion: Optic-guided radiotherapy using IRLEDs provides a mechanism thr ough which setup accuracy may be improved over conventional techniques. To date, this optic-guided therapy has been used only as a hybrid system that requires use of the stereotactic head ring for the first fraction. This has limited its use in the routine clinical setting. Computation of the desire d IRLED positions eliminates the need for the invasive head ring for the fi rst fraction. This allows application of optic-guided therapy to a larger c ohort of patients, and also facilitates the initiation of extracranial opti c-guided radiotherapy, (C) 2000 Elsevier Science Inc.