Abstract

Ravikumar Nattudurai¹, Delmon Arous^1,2, Nina F.J. Edin¹, Eirik Malinen^1,2

¹University of Oslo, Department of Physics, Oslo, Norway; ²Oslo University Hospital, Department of Medical Physics, Oslo, Norway

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).

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.

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.

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.