SYSTEMATIC ANALYSIS OF ERRORS IN TARGET LOCALIZATION AND TREATMENT DELIVERY IN STEREOTAXIC RADIOSURGERY (SRS)

Purpose: To systematically analyze the spatial uncertainties associated with each step of the stereotactic radiosurgery (SRS) procedure and the overall spatial accuracy in the treatment delivery. Methods and Materials: A special test device has been designed and fabricated to permit accurate simulation, localization and treatment portal verification of a SRS target. This device with simulated targets dispersed in 3-D space can be rigidly attached to the BRW CT or angio frame for localization, to the BRW phantom base for target coordinate determination, and to the floor stand for portal verification. The setup allows evaluation of the accuracy of each procedure separately as well as the overall accuracy in the delivery of SRS treatment. Results: Biplanar film localizations reveal a systematic error in the phantom base pointer of the order of 0.1 mm that is applied as a correction to the measured target coordinates. Targets localized by planar film angiography had a mean positional error of 0.38 +/- 0.1 mm compared to 1.0 +/- 0.7 mm for digital angiographic localization. The positional accuracy associated with CT localization was superior when a 2 mm vs. 4 mm slice thickness was used (0.9 +/- 0.3 mm vs. 1.6 +/- 0.5 mm error). Cumulative mean errors, including inaccuracies associated with treatment setup, were 1.0 +/- 0.4 mm for radiographic localization, 1.2 +/- 0.5 mm for CT with a 512 X 512 matrix and 2 mm slice thickness, and 2.0 +/- 0.6 mm for C7: at a 4 mm slice thickness (256 X 256 matrix). Larger errors would be expected in the clinical setting. Conclusion: Spatial errors in stereotactic radiosurgery are best estimated using a systematic approach to isolate independent contributing factors. The accuracy in target localization determines the overall accuracy of SRS procedure, provided the mechanical accuracy of the treatment apparatus is assured. Biplanar treatment portal verification with a fiducial localization frame is an accurate method of verifying the target coordinates before delivering treatment.