ESTRO 36 Abstract Book

S408 ESTRO 36 _______________________________________________________________________________________________

Material and Methods In the convolution model, the 1D lateral dose response function K ( x-ξ ) is defined as the convolution kernel transforming the true dose profile D ( ξ ) into the disturbed signal profile M ( x ) measured with a detector. For an air- filled ionization chamber, type T31021 (PTW Freiburg, Germany), the lateral dose response functions were determined by Monte-Carlo simulation using 0.25 mm wide 60 Co and 6 MV slit beams. The chamber was modelled according to manufacturer’s detailed specification and placed at 5 cm water depth in three different orientations, i.e. axial, lateral and longitudinal. For each chamber orientation, a magnetic field oriented perpendicular to the beam axis was applied. Simulations were performed for magnetic fields of 0, 0.5, 1 and 1.5 T using the EGSnrc package and the egs_chamber code. To verify the simulation results, the lateral dose response functions without magnetic field were compared against measurements with a 5 mm wide collimated 6 MV photon slit beam using tertiary lead blocks following the approach of Poppinga et al 2015. Results Fig. 1 shows good agreement between the simulated and measured dose response functions K ( x-ξ ) of the investigated ionization chamber in the three investigated orientations. The structures of the measured functions are not as evident as those of the simulated functions possibly due different scanning step widths used in the experiment and the calculation. Fig. 2 shows the lateral dose response function K ( x-ξ ) with and without magnetic field obtained exemplary for the detector in lateral orientation. The distortion of the dose response function K ( x-ξ ) corresponds to the change in the trajectory lengths of the secondary electrons in the air of the ionization chamber due to the Lorentz force, as compared to the trajectories in a small sample of water.

the EGSnrc package, and 0.5, 1.0 and 1.5 T magnetic fields were applied. Results Fig. 1 shows the derived kernels K(x-ξ) without and with magnetic field for the three detector densities and two beam qualities. The shape of K(x-ξ) without magnetic field has been discussed in Looe et al 2015 in terms of the electron density of the detector material. The effect of the magnetic field on the secondary electrons’ trajectories in a non-water equivalent medium is manifested as a distortion of K(x-ξ). It is worth mentioning that function K(x-ξ) for water with normal density (middle panels) does not vary in the presence of a magnetic field, and the shape of this function merely represents the geometrical volume-averaging effect.

Fig. 1. Area normalized K(x-ξ) for the cylindrical detector voxels of 'low”, 'normal”, and 'enhanced” density without and with, 0.5, 1.0 and 1.5 T magnetic field. Conclusion It has been shown for the first time that the lateral dose response functions K(x-ξ) of non-water equivalent detectors will be distorted by a magnetic field, showing asymmetrical detector response, even if the detector’s construction is symmetrical. The distortions are attributed to the differences in charged particle trajectories within the detectors having electron density other than of normal water. The effect of a magnetic field on a detector’s response can be characterized by the area-normalized convolution kernel K(x-ξ, y-η). As previously proposed (Looe et al 2015), corrections based on the convolution model can be applied to account for the detector’s volume effect in the presence of magnetic field: PO-0771 The dose response functions of an air-filled ionization chamber in the presence of a magnetic field B. Delfs 1 , D. Harder 2 , B. Poppe 1 , H.K. Looe 1 1 University Clinic for Medical Radiation Physics, Medical Campus Pius Hospital Carl von Ossietzky University, Oldenburg, Germany 2 Prof em. Medical Physics and Biophysics, Georg August University, Göttingen, Germany Purpose or Objective The development of therapeutic devices combining clinical linear accelerators and MRI scanners for MR guided radiotherapy leads to new challenges in the clinical dosimetry since the trajectories of the secondary electrons are influenced by the Lorentz force. In this study, the lateral dose response functions of a clinical air- filled ionization chamber in the presence of a magnetic field were examined depending on beam quality and magnetic field following the approach of a convolution model (Looe et al 2015, Harder et al 2014).

Fig. 1. Area-normalized simulated and measured dose response functions K ( x - ξ )

Fig. 2. Area-normalized dose response functions K ( x - ξ ) for the T31021 in lateral orientation for magnetic fields of 0, 0.5,1 and 1.5 T Conclusion The distortions of the lateral dose response function K ( x- ξ ) will alter the measured signal profile M ( x ) of a detector in magnetic field, as demonstrated in this study. The variety of the possible combinations of detector orientation and magnetic field direction and the strong dependence of the distortion on the magnetic field strength require careful consideration whenever a non- water equivalent detector is used in magnetic field.