The response of alanine dosimeters in small fields and low kV X-rays

Abstract

Alanine/EPR dosimetry has been used for many decades for dosimetry QA in radiotherapy. The Nuclear Technology Centre (NuTeC) from the University of Hasselt is using alanine/EPR dosimetry for providing audit services for MV photon beams and MeV electron beams. Lots of improvements are happening recently in the preclinical research for radiotherapy where they are working with small animal irradiators. These machines use medium and lower kV X-rays and small fields. Besides, electronic brachytherapy sources using miniature X-ray sources are also gaining interest. To use alanine/EPR dosimetry in these conditions, its response must be characterised. The goal of this thesis is to investigate the relative response of alanine/EPR dosimetry at 50 – 225 kV X-rays and small fields. This way, NuTeC could extend its audit services to small animal irradiators.

Alanine and film dosimeters were irradiated on two different small animal irradiators: the SARRP and the X-RAD 225CX. The detectors were irradiated with beams of 50 – 225 kV X-rays and with field sizes of 13.6 x 13.6 cm² to 1 x 1 cm². Both detectors were placed at the same location and depth as close as possible. The dose measured by the alanine pellet was compared to the dose measured by the film. This was defined as the relative response of the alanine detector. The film detectors were investigated for energy dependence by measuring PDD’s in water and RW3. MC simulations with the DOSRZnrc user code from the EGSnrc code system were performed to compare with the experimental results.

The relative response of the alanine dosimeters drops to 0.795 at photon beams with a mean energy of 78 kV and it drops further to 0.665 at photon beams with a mean energy of 35 kV. The relative response increases with 1.5% when the field size decreases to 1 x 1 cm². The simulated results are approximately 4.5 % higher than the experimental results. The intrinsic efficiency of the alanine detectors should be known to reproduce the experimental results correctly.

The relative response of the alanine detectors drops significantly with decreasing photon energy in the kV range while the relative response increases slightly with decreasing field sizes. It is possible to characterise this behaviour in function of parameters such as the mean energy or the HVL of the photon spectra. It is feasible to use alanine detectors for dosimetry QA of small animal irradiators and therefore it is possible to use it for auditing purposes of these machines.