Track-average LET of secondary electrons generated in LiF:Mg,Ti and liquid water by 20–300 kV x-ray,¹³⁷Cs and⁶⁰Co beams

Abstract

Electrons generated in matter by photons could be a fundamental basis for an adequate analysis of radiation effects and damage. We have studied separately the 'primary electrons' generated directly by photons from the 'secondary electrons' (SE) produced by electron-electron interactions. In this work, track-average linear energy transfer, [Formula: see text], of SE in LiF:Mg,Ti and liquid water produced by twelve photon energy beams from 20 kV x-ray to Co gamma rays have been investigated using the EGSnrc Monte Carlo Code. The exposure of LiF:Mg,Ti in different phantom materials has been considered. Depending on the photon energy, SE represent 40%-90% of the total electron fluence (TEF) between 1 keV and 10 keV, being higher when the photon energy increases. Independent of the medium, [Formula: see text] versus mean photon energy displays a local minimum at around 40 keV, followed by a local maximum at ~80 keV-100 keV. The [Formula: see text] of SE generated by the x-ray beams are of order of 11 keV µm to 19 keV µm in LiF:Mg,Ti and 5 keV µm to 9 keV µm in liquid water which represent 3-5 times those produced by Co gamma rays in both media. These values were considerably greater than those of TEF, by factors of 3-8. Furthermore, [Formula: see text] of SE generated in liquid water by 20 kV-200 kV x-rays are similar to those of 76 MeV-120 MeVHe ions. Contrary to the TEF, where [Formula: see text] were independent of the phantom material, at low photon energies [Formula: see text] of SE was found to be sensitive to the surrounding medium showing higher values within the phantom than in air. This result, which agrees with published experimental results, implies the importance of the SE ionization density for an understanding of dosimeter response induced by photon beams.

Track and dose-average LET dependence of Gafchromic EBT3 and MD-V3 films exposed to low-energy photons

“…In the EGSnrc code, it is possible to score the "total electron fluence" (TEF) where all electrons generated by photons and by electron-electron interactions are counted; the "primary electrons" (PE) that compute all electrons generated by photons during the interaction and "secondary electrons" (SE) which include all electrons produced due to electron-electron interactions. We are particularly interested on the SEs since our previous work for LiF suggested that the LET of SEs better describes the experimental observation of the dosimeter response than the LET of TEF 8 . This is explained by the fact that SEs have very short range (order of nanometers) and cannot move far away from their origin, so consequently deposit their energy locally and activate more colour centers within the film sensitive volume.…”

“…This is explained by the fact that SEs have very short range (order of nanometers) and cannot move far away from their origin, so consequently deposit their energy locally and activate more colour centers within the film sensitive volume. The simulation was performed considering the same parameters selected in our previous work 8 , i.e. 512 keV and 1 keV electron transport cut-off (ECUT) and photon transport cut-off (PCUT), respectively with Bound Compton, spin effects and Rayleigh scattering turned on.…”

“…To support that statement, the relationship between the film response and LET should be established. Besides, in a recent study about the secondary electron (SE: electrons generated by electron-electron interactions) spectra generated in LiF by low photon energies and their corresponding track-average LET, we found that SE spectra could contribute to a better understanding of the dosimeter response induced by photon beams in terms of ionization-density 8 . Thus, to determine the role of the ionisation density in the polymerization process, investigation on the average-LET dependence of radiochromic film's response after exposure to photons is needed.…”

“…Besides, increase up to 12% on the response of TLD-100 situated outside of the field was observed comparing to 1% variation within the treatment field of a 6 MV x-ray linear accelerator 30 . Furthermore, secondary electrons generated by low photon energies have been found to be mostly of energies below 10 keV and have track-average LET as high as of 19 keVμm −1 and 9 keVμm −1 in LiF:Mg,Ti and liquid water, respectively similar to those of 76 MeV-120 MeV 3 He ions 8 .…”

Absorbed Dose to Water Rate in a Cyberknife VSI System Reference Field Using Ionization Chambers and Gafchromic Films

“…In particular, the use of small radiation fields (60 mm down to 5 or 4 mm diameter) should impart dose to the target volume in a sure manner. The physical concept of small radiation fields in radiotherapy is strongly correlated to the high-ionization density problem caused by the variation of the electron fluence spectra in the lateral direction and the small range of the electrons generated in the field [1]. Consequently, the determination of the absorbed dose in these fields remains a challenging task compared to the fields used in conventional radiotherapy (square fields from 4 cm to 40 cm).…”