A SIMPLE METHOD TO VERIFY IN-VIVO THE ACCURACY OF TARGET COORDINATES IN LINEAR-ACCELERATOR RADIOSURGERY

Purpose: A simple method that verifies the coincidence of the isocente r with the center of the target volume in radiosurgery treatment condi tions is described. The accuracy is compared to that of accepted compu terized procedures employing fiducial markers. Methods and Materials: The center of the beam is identified by a cylindrical localizer, fixed to the plate of the supplemental collimator, with a 2 x 50 mm tungste n rod coincident with the beam axis and is projected onto the x-ray po rtal verification films. Prior to irradiation, the coordinates of the intersection of the beams axes, which is in a known spatial relationsh ip with the isocenter, are read directly on portal rr-ray films and th eir coincidence with the coordinates set during patient positioning, i s checked. Results: The mean displacement in AP, Lat, and Vert coordin ates respectively, over 84 patients, between the coordinates calculate d by the computerized procedure employing fiducial markers and the coo rdinates calculated by using the rulers was 0.3 +/- 0.4 mm. Conclusion s: From the results obtained with the two methods eve can conclude tha t rulers method can be used as a fast indirect control of the position of the radiation isocenter. Moreover, the dimensions of the radiation field and the correct alignment of the tertiary circular collimator c an be also documented. (C) 1998 Elsevier Science Inc.