A NEW METHOD FOR OUTPUT FACTORS MEASUREMENTS/MC COMPUTATIONS FOR STEREOTACTIC AND DYNAMIC JAWS TOMOTHERAPY

Abstract

was hampered by the fact that the material properties of the walls were not exactly known before installation. Results: During ramping of the magnet the magnetic fringe field in the two nearest shelters was measured as function of the magnetic field strength of the magnet and a perfectly linear increase was found, see figure 1. A linear increase indicates that for most of the iron rebar material in the walls the coercitive field was not exceeded. So also no permanent remanent field is expected after ramping down the magnet. At 7.5 m a maximum increase of 1.5 G was measured, at 12 m, 0.6 G. This is up to 3 times the earth’s magnetic field and the clinical accelerators needed to be re-calibrated in order to operate in such external magnetic field. The Elekta accelerators are standardly equiped with electro-magnets for compensating the impact of the earth’s magnetic field. These were used to compensate also for the fringe field of the MRI accelerator. The required currents for the compensating magnets were increased by a factor of 3. The resulting radiation field flatness of the clinical accelerators was measured and was similar to the situation before ramping the magnet, i.e. the field flatness is within the clinical limits of 2%. Conclusions: The conclusion is that installation of a 1.5 T magnet in the vicinity of clinical linear accelerators requires re-calibration of the exposed systems. After re-calibration the performance is similar as before installation of a 1.5 T MRI system. For future site planners, the use of non-magnetic rebar material is suggested when installation of a MRI in the direct vicinity is considered. Award lecture