ESTRO 36 Abstract Book

S397 ESTRO 36 2017 _______________________________________________________________________________________________

1 German Cancer Research Center, Medical Physics in Radiation Oncology, Heidelberg, Germany 2 National Center for Radiation Research in Oncology, Heidelberg Institute for Radiation Oncology, Heidelberg, Germany 3 ViewRay, Inc, Oakwood Village, USA Purpose or Objective Ionization chambers exhibit an altered dose response in a magnetic field of an MR-linac due to the deflection of secondary electrons by the Lorentz force. The actual dose response depends on the magnetic field strength as well as on the orientation between chamber axis, beam and magnetic field [Meijsing PMB 54 2009, Reynolds Med Phys 40 2013, Spindeldreier DGMP 47 2016]. The purpose of this study is to investigate the influence of dead volumes, known to exist at the chamber base, on the response of a thimble ionization chamber in the presence of a magnetic field. Material and Methods The response of a Farmer chamber (PTW 30013) subject to a 6 MV beam was measured in a small water tank [Bakenecker Uni Heidelberg 2015] embedded in an experimental magnet for magnetic field strengths between 0.0 and ±1.1 T in the two magnetic field orientations perpendicular to the beam and to the chamber axis. The experimental setup was simulated using the EGSnrc [Kawrakow Med Phys 27 2000, NRC PIRS 898 2009] user code egs_chamber [Wulff Med Phys 35 2008]. In addition to computing the total dose deposited in the chamber cavity for different sensitive volumes, a high resolution dose map inside the cavity was obtained. Results A maximum of 8.1% and 7.0% increase in chamber response was measured for the two orientations at a field strength of ±0.9 T. In contrast, the calculated response was only marginally different, when the entire air volume was considered as a sensitive volume in the simulations. It was possible to reproduce the experimentally observed differences in dose response using a small dead volume close to the chamber stem. The simulated dose distribution within the chamber cavity was found to be highly non-uniform with hot and cold spots at the chamber stem and chamber tip, depending on the field orientation (see Fig. 1).

concentration (4 wt%) and RI concentrations ranging from 6-16%. These formulations were cast in spectrophotometer cuvettes and stored at <3°C prior to irradiation. A passively scattered 225 MeV proton beam with a 10 cm SOBP was selected and each formulation of dosimeters was irradiated in a solid water phantom at four depths along the beam profile: one in the dose plateau and three along the SOBP. The photo-absorption spectra were measured for each formulation. The optical attenuation coefficients of the PRESAGE® samples were compared to ion chamber measurements to determine the quenching magnitudes. Results The photo-absorption spectra demonstrated consistent absorption peaks, and all formulations responded linearly with dose. The dose sensitivity of the dosimeters changed by as much as 42% across all formulations. All formulations with RI concentrations between 10-21% showed quenching less than 3% at the proximal SOBP dose point but increased quenching at other measurement points along the SOBP. Formulations outside this RI concentration range had greater quenching across all measurements. The distal-most points of all formulations showed the greatest quenching. When comparing these points, high LMG formulations had lower quenching than those with low LMG while RI concentrations were 12% or lower, but quenching was greater when RI concentration was above this range. The least quenching in the low LMG formulations was 14.6% which occurred at 12% RI, while in the high LMG formulations this occurred at 10% RI with a maximum under-response of 8.4%. The highest quenching observed was 73.8% in the low LMG, 30% RI formulation.

Conclusion Previous studies have the only investigated the effects of changing RI concentrations on the quenching magnitude of PRESAGE® in a proton beam, but this study has demonstrated that the quenching process is additionally limited by LMG concentration. While a quenching reduction limit for low LMG formulations was before it could be fully eliminated, further reduction of quenching by increasing LMG demonstrates that additional study into PRESAGE® optimization of both of these components may continue to improve accuracy in proton dosimetry. PO-0761 Dosimetry with Farmer ionization chambers in magnetic fields: Influence of the sensitive volume C.K. Spindeldreier 1,2 , I. Kawrakow 3 , O. Schrenk 1,2 , S. Greilich 1,2 , C.P. Karger 1,2 , A. Pfaffenberger 1,2

Conclusion In the presence of a magnetic field perpendicular to the axis of thimble ionization chambers, the amount of electrons entering the cavity from the tip and stem is increased or decreased, depending on the field orientation. The chamber response is therefore influenced in a significant way by the presence of a dead region known to exist at the chamber base near the stem. Measurements with the chamber axis parallel to the magnetic field are thus advantageous, as in this case the dead volume has less impact due to the Lorentz force acting radially. An optimized chamber design that