ESTRO 2021 Abstract Book

S229

ESTRO 2021

PH-0319 Alanine dosimeters for determining linear energy transfer in proton beams R. Nattudurai 1 , D. Arous 1,2 , N. F.J. Edin 1 , E. Malinen 1,2 1 University of Oslo, Department of Physics, Oslo, Norway; 2 Oslo University Hospital, Department of Medical Physics, Oslo, Norway Purpose or Objective To investigate microwave power characteristics (MPC) of alanine dosimeters irradiated with low-energy protons and identify associations between MPC and the proton linear energy transfer (LET). Materials and Methods Thin-film alanine dosimeters were irradiated with 16 MeV protons at a research cyclotron and with Co-60 g rays. Five different water-equivalent depths, with 3 dosimeters at each depth, were used in the proton experiments. A dose of 45 Gy was delivered at all depths. Monte Carlo simulations using the FLUKA code gave corresponding dose-averaged LET values at each depth of 4.7, 25.1, 33.0, 37.0, and 40.4 keV/mm. Using electron paramagnetic resonance spectroscopy, the alanine signal was recorded for six different microwave powers (range: 1.6-50.6 mW). The MPC was defined as the change in the alanine dosimeter signal per increase in applied microwave power. Regression analysis was used to evaluate the association between MPC and LET. Results Minimum and maximum MPC of 4.3±0.1 and 7.0±0.1 (mean±SD), respectively, were found for LETs of 4.7 and 37.0 keV/mm. For comparison, the MPC for Co-60 g-irradiated dosimeters was 4.7±0.3. The mean sample-to- sample relative standard deviation in MPC was 2.2 %, indicating rather high precision. Plotting the MPC against the LET show a highly significant linear association (Figure 1; r2=0.986, P= 0.007). Data for LET of 40.7 keV/mm was omitted in the linear regression due to a suspected saturation in MPC at this LET level.

Conclusion We found a significant positive association between alanine MPC and proton LET, which to our knowledge is the first demonstration of the potential use of alanine dosimeters as an ‘LET detector’. The findings may open for novel applications using alanine in proton beam dosimetry. The increase in MPC with LET is most likely due to a decrease in T2 relaxation time with increasing local radical spin density in the alanine crystal lattice. The saturation in MPC at the very extreme proton LET of 40.7 keV/mm indicate limited applicability for carbon ion beams, but further studies are needed for elucidation. PH-0320 Influence of variable beam quality on dosimetry for light ion beams relevant for ocular treatments A. Resch 1 , D. Georg 1 , H. Palmans 2,3 1 Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria; 2 MedAustron Ion Therapy Centre , Medical Physics, Wiener Neustadt, Austria; 3 National Physical Laboratory, Medical Physics, Teddington, United Kingdom Purpose or Objective Light ion beams used for ocular treatments exhibit two main characteristics: small fields with high dose gradients at the edges and low energies with ranges in water of less than or equal to about 3 cm. Due to the low penetration depth and small modulation widths, the energy distribution within the SOBP is less wide as in conventional ion therapy leading to increased beam quality effects. In this study, measurements were combined with Monte Carlo (MC) simulations to characterize the response of several detectors in proton and carbon ion beams. Materials and Methods