ESTRO 36 Abstract Book

S401 ESTRO 36 _______________________________________________________________________________________________

For depth dose measurement, both substrates show a great agreement within ±2% when compared to the IC response. Conclusion The difference detector Si substrates show the difference in degradation of the detector sensitivity. The M512-Epi demonstrate 3.5 times better radiation hardness in comparison with M512-Bulk while show more the dose per pulse dependence. However, for typical treatment when SSD <150 cm for all beam angles the sensitivity of the detector decreases within 2% for both substrates making M512 -Epi more preferable choice as QA detector for dosimetry in SRS and SBRT. PO-0760 Investigation of PRESAGE formulation on signal quenching in a proton beam M. Carroll 1,2 , M. Alqathami 2 , G. Ibbott 2 1 University of Texas at Houston, Graduate School of Biomedical Sciences, Houston, USA 2 The University of Texas MD Anderson Cancer Center, Radiation Physics, Houston, USA Purpose or Objective PRESAGE®, a radiochromic polyurethane dosimeter, has shown potential as a 3D dosimetry system for conventional radiotherapy systems. When irradiated by protons, however, signal quenching is observed in high-LET regions. This quenching may result from either (or both) the local saturation of the Leucomalachite green (LMG) or recombination of the radical initiator (RI) along proton tracks. This work studied the magnitude of these quenching mechanisms and the effects of changes to formulaic concentrations of these components to further minimize or eliminate the quenching effect. Material and Methods Ten formulations of PRESAGE® were manufactured under standardized conditions but with RI concentrations ranging from 3-30 (wt%) and low LMG concentration (2 wt%). Six more formulations were then manufactured with high LMG 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 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.