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 table 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 axis than routinely specified by the manufacturers. A simple and efficient method is presented to measure the direction and the size of the displacement 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 determined. The measured displacement and inclination provide the required data to correct for possible misalignments of the treatment table axis and to maintain its alignment. Measurements were performed over a period of two years for a treatment table located at the German heavy ion therapy facility. The mean radial distance between the table axis and the isocentre was found to be 0.25 +/- 0.25 mm. The mean inclination of the table axis in the XZ- 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 of the treatment table over the analysed time period. The described method can be applied to any isocentric irradiation facility, especially including isocentric linear accelerators used for radiosurgery or other high precision irradiation techniques.