Infrared patient positioning for stereotactic radiosurgery of extracranial tumors

Purpose: To analyze the localization accuracy of a novel, non-invasive patient positioning system for radiosurgery of extracranial tumors. Methods and Materials: Infrared reflective markers served as landmarks to establish the orientation of a patient. Imaging studies provided the three-dimensional relationship between the external markers and an internal target such as a tumor. An infrared camera system detected the location of the markers for positioning the patient for treatment. Any uncertainty in the location of a marker contributed to uncertainty in localizing the target. Sources of uncertainty included imaging studies, marker repositioning, and infrared detection. Theoretical analysis was used to predict errors in targeting and to study methods for minimizing errors. A computer simulation program modeled marker uncertainty by assigning random offsets within a specified error sphere to the marker location. For CT imaging, the maximum uncertainty in marker and target locations was assumed to be 0.75 mm. Marker repositioning contributed up to 5 mm of uncertainty and the infrared detection system contributed up to 0.3 mm. The use of redundant markers was studied to reduce targeting error. For groups of 4-6 markers, estimates of the target location were calculated from sets of 3 markers selected from the group. The final target location was derived by averaging the estimates. Results: The average uncertainty and 95% confidence level were determined for different numbers of markers. The analysis was done with a target distance of 16m. Number of markers Average Uncertainty(mm) 95% Confidence Level (mm) 3 2.3 4.6 4 1.6 2.9 5 1.5 2.8 6 1.4 2.6 Conclusions: The use of the minimum set of 3 markers was insufficient to achieve accurate targeting of extracranial tumors. Redundant markers improved the overall accuracy of targeting.