ESTRO 2022 - Abstract Book

S26

Abstract book

ESTRO 2022

Based on the blueprints the Farmer-type chamber PTW 30013 geometry was modelled in PENELOPE (2014). Two DVs were modelled; one based on published data (Pojtinger et al, 2019 ) (DV 1 ) and one determined using FEM simulations (DV 2 ). Radiation fields (10x10 cm 2 ) were based on phase space files of a 60 Co source and a 7MV MRI-linac. The model was validated against measurements with a setup using an electromagnet and a 60 Co source (0 – 1.5 T) and an MRI-linac (0 and 1.5 T) (de Prez et al , 2019). Simulations were performed with the PENMAIN routine with magnetic field routines implemented for both parallel and perpendicular orientations of the chamber and the magnetic (B) field, and for several B-field strengths between 0 T and ±1.5 T. To study the dose contribution to the sensitive volume (SV = cavity – DV) from the electrons produced in certain components of the chamber (wall, central electrode (CE), guard, stem, phantom), the labelling of the particles was implemented in PENMAIN. A separate model with each solid component of the chamber modelled as liquid water (indicated by PTW 30013 (water)) was used to investigate the impact of material choice on k B,Q . Results The simulated k B,Q results agree better with the measured k B,Q when the DV is considered. The difference between measured k B,Q and simulated k B,Q reduces from at maximum 0.6% for DV 1 to 0.4% for DV 2 at B = 0.75 T in 60 Co. This demonstrates the accuracy of the model, and it shows that small changes in the DV may have a small impact in the k B,Q . Figures 1 and 2 show the relative contribution of each component to the effect of k B,Q (i.e., k B,Q -1). The negative value in the figure means an increase in the contribution from that component when the B is present.

Figure 1 demonstrates that the contribution to the SV dose from the bodies closer to the SV is higher than without B. For parallel orientation, the guard and the stem have higher impact on the dose to the SV. For the perpendicular orientation the wall contribution is more important. For all orientations the CE has a similar increased contribution