A method for determining the alignment accuracy of the treatment table axis at an isocentric irradiation facility

At an isocentric irradiation facility, the rotation axis of the treatment t able has to be accurately aligned in vertical orientation to the isocentre, which is usually marked by three perpendicular laser planes. In particular , high precision radiotherapy techniques, such as radiosurgery or intensity modulated radiotherapy, require a higher alignment accuracy of the table a xis than routinely specified by the manufacturers. A simple and efficient m ethod is presented to measure the direction and the size of the displacemen t of the table axis from the isocentre as marked by the lasers. In addition , the inclination of the table axis against the vertical direction can be d etermined. The measured displacement and inclination provide the required d ata to correct for possible misalignments of the treatment table axis and t o maintain its alignment. Measurements were performed over a period of two years for a treatment table located at the German heavy ion therapy facilit y. The mean radial distance between the table axis and the isocentre was fo und to be 0.25 +/- 0.25 mm. The mean inclination of the table axis in the X Z- and YZ-planes was measured to be -0.03 +/- 0.02 degrees and -0.04 +/- 0. 01 degrees, respectively. The measurements demonstrate the good alignment o f the treatment table over the analysed time period. The described method c an be applied to any isocentric irradiation facility, especially including isocentric linear accelerators used for radiosurgery or other high precisio n irradiation techniques.