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

Purpose: To systematically analyze the spatial uncertainties associate d with each step of the stereotactic radiosurgery (SRS) procedure and the overall spatial accuracy in the treatment delivery. Methods and Ma terials: A special test device has been designed and fabricated to per mit accurate simulation, localization and treatment portal verificatio n 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 locali zation, to the BRW phantom base for target coordinate determination, a nd to the floor stand for portal verification. The setup allows evalua tion of the accuracy of each procedure separately as well as the overa ll accuracy in the delivery of SRS treatment. Results: Biplanar film l ocalizations 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 ta rget coordinates. Targets localized by planar film angiography had a m ean 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 thicknes s 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 wit h 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 ste reotactic radiosurgery are best estimated using a systematic approach to isolate independent contributing factors. The accuracy in target lo calization determines the overall accuracy of SRS procedure, provided the mechanical accuracy of the treatment apparatus is assured. Biplana r treatment portal verification with a fiducial localization frame is an accurate method of verifying the target coordinates before deliveri ng treatment.