Purpose: A simple method that verifies the coincidence of the isocenter with the center of the target volume in radiosurgery treatment conditions is described. The accuracy is compared to that of accepted computerized 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 tungsten rod coincident with the beam axis and is projected onto the x-ray portal verification films. Prior to irradiation, the coordinates of the intersection of the beams axes, which is in a known spatial relationship with the isocenter, are read directly on portal x-ray films and their coincidence with the coordinates set during patient positioning, is checked.
Results: The mean displacement in AP, Lat, and Vert coordinates respectively, over 84 patients, between the coordinates calculated by the computerized procedure employing fiducial markers and the coordinates calculated by using the rulers was 0.3 +/- 0.4 mm.
Conclusions: From the results obtained with the two methods we can conclude that 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 can be also documented.