DAILY DOSIMETRIC QUALITY-CONTROL OF THE MM50 RACETRACK MICROTRON USING AN ELECTRONIC PORTAL IMAGING DEVICE

The MM50 Racetrack Microtron, suited for advanced three-dimensional co nformal radiotherapy techniques, is a complex machine in various respe cts. Therefore, for a number of gantry angles, daily quality control o f the absolute output and fluence profiles of the scanned beams are ma ndatory. For the applied photon beams, a fast method for these daily c hecks, based on dosimetric measurements with the Philips SRI-100 Elect ronic Portal Imaging Device (EPID), has been developed and tested. Ope n beams are checked for four different gantry angles; for gantry angle 0, a wedged field is checked as well. Performing and analyzing the me asurements takes about 10 min. The applied EPID has favourable charact eristics for dosimetric quality control measurements: absolute output measurements reproduce within 0.5% (1 SD) and the reproducibility of r elative (2D) beam profile measurements is 0.2% (1 SD). The day-to-day sensitivity stability over a period of one month is 0.6% (1 SD). Measu red grey scale values are within 0.2% linear with the applied dose. Th e 2D fluence profile of the 25 MV photon beam of the MM50 is very stab le in time: during a period of 5 months a maximum fluctuation of 2.2% has been observed. Once, a deviation in the cGy/MU-value of 6% was det ected. There is no interlock in the MM50-system that would have preven ted patient treatment with this strongly deviating output. Based on th e results of this study and on clinical requirements regarding accepta bility of deviations of beam characteristics, a protocol has been deve loped including action levels for additional investigations and, if ne cessary, adjustment of the beam characteristics.