ESTRO 36 Abstract Book

S416 ESTRO 36 _______________________________________________________________________________________________

detectors. A rotational symmetric Gaussian horizontal beam profile and exponential decaying depth dose profile in the vicinity of the pellet was fitted to the measured profiles. Both 6 MV and 6 MV FFF beams were considered. Results The fit of the beam profile in three dimensions was based on two parameters: the variance for the Gaussian profile and the gradient of the depth profile. The parameters in turn were both changing as function of FS. Using the fitted beam profiles, an analytical model was developed for the calculation of volume correction factors k V for given FS (see Table 1). Table 1: Calculated volume correction factors k V , temperature and volume corrected output factors (OF) with SD being one standard deviation (SD) are displayed for the 6 MV and 6 MV FFF beams as function of the field size FS.

charged particle drifting were accounted by means of a look-up table (LUT) approach. Longitudinal dose depositions were estimated from the LUT and corrected using a water-equivalent depth scaling. In a first step proton beams in the clinical required energy range 60 – 250 MeV with transverse external magnetic fields ranging from 0 – 3T were analyzed in a 40x40x40 cm 3 water phantom. Next validation simulations were performed for different phantom configurations, e.g. using a simple water box or slab-like geometries with inhomogeneities of different materials and volumes. Percentage depth dose curves (PDD) and two-dimensional dose distributions were calculated to assess the performance of the PBA. Results For PDD in water discrepancies between the PBA and MC of less than 1.5% were observed for all the depth values before the Bragg-Peak (see Figure 1). An increasing value of up to 6% was found in the distal energy falloff region, where dose values represents around 1% of the maximum dose deposition. In all cases, maximum range deviations of the results were less than 0.2 mm. Deviations between two dimensional dose maps obtained with PBA and GATE remained below 1% for almost all the proton beam trajectory, reaching a maximum value up to 4% in the Bragg-Peak region, see Fig. 2. As expected, agreement became worse for high energy protons and high intensity magnetic fields.

Conclusion Volume averaging was found to influence the alanine measurements by up to 6 % for the smallest field size. For a cylindrical detector irradiated along the symmetry axis of the detector, simple analytical expressions of the volume correction factors were obtained. The analytical expression gives valuable insight in the volume correction factor k V as function of field size and the radius of the sensitive volume of the detector. The method presented here would be applicable for other detectors. With a defined geometry of the sensitive volume of the detector relative to the central axis of the beam the volume correction factor can either be calculated analytically or numerically as function of FS. Poster: Physics track: Dose measurement and dose calculation PO-0785 A pencil beam algorithm for protons including magnetic fields effects F. Padilla 1 , H. Fuchs 1 , D. Georg 1 1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria Purpose or Objective Magnetic Resonance Image (MRI) has the potential to increase the accuracy and effectiveness of proton therapy. Previous studies on that topic demonstrated that corrections in dose calculation algorithms are strictly required to account for the dosimetric effects induced by external magnetic fields. So far, a real dose calculation possibility including a trajectory corrected approach was missing. In this study, we developed a pencil beam algorithm (PBA) for dose calculation of a proton beam in magnetic fields. Material and Methods MC simulations using the GATE 7.1 toolkit were performed to generate first benchmarking data and subsequent validation data for the PBA. The PBA was based on the theory of fluence weighted elemental kernels. A novel and non-symmetric exponential tailed Gauss fitting function was used to describe the lateral energy deposition profiles in water. Nuclear corrections, multiple scattering and

Fig. 1. PDD curves comparing the PB algorithm with MC simulations for proton beams in water. Relative discrepancies are shown in the top region of the graph.

Fig. 2 Relative dose difference map for a 240 MeV proton beam in water exposed to a 3T transverse field. Conclusion The proposed pencil beam algorithm for protons can accurately account for dose distortion effects induced by external magnetic fields. Corrections of dose distributions using an analytical model allows to reduce dose