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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.

Cylindrical ionization chambers are used for the determination of absorbed dose in beams of heavy charged particles, where the effective point of measurement, P eff (the point in depth to which the measured dose refers), is a priori not known. A measurement of P eff for a Farmer-type chamber in a carbon ion beam is presented. It is based on a comparison of relative depth dose curves measured with a cylindrical chamber and a plane-parallel Markus chamber. Both measurements were compared against another high-precision relative depth dose measurement using large-area plane-parallel chambers. For P eff , a value of 72±7% of the inner radius of the chamber is obtained. The relative depth dose curve for the cylindrical chamber is calculated using an averaging of the depth dose values over the curved inner surface of the active volume while taking account of the different depths of points on the inner surface. Within the measurement uncertainty of 0.2 mm the measurements agree well with the calculated Bragg curve for the Farmer chamber. The result for P eff is in correspondence with the value suggested in a new code of practice by the IAEA for protons and ions, and somewhat less than that suggested by Palmans for protons. The measurements show that cylindrical chambers are in general well suited for depth dose measurements in fields of heavy charged particles if the correct P eff is used.

The method to measure absorbed dose to water in a field of carbon ions as applied for the heavy ion therapy project at the Heavy Ion Research Laboratory in Darmstadt (GSI), Germany, is described in detail. Thimble ionization chambers with a water absorbed calibration factor are applied. The dose obtained with this method was compared with that obtained at the heavy ion therapy facility HIMAC at the National Institute of Radiological Sciences in Chiba, Japan, using the Japanese code of practice. The agreement found was better than 1%. The combined uncertainty of the determination of absorbed dose to water was estimated to amount to 5%.