Determination of the Mean Energy Required to form an Ion-Pair (w-value) in Gases of relevance in Reference Dosimetry for Protontherapy.

TESSARO, VERÓNICA BELÉN; MICAEL BEUVE; POIGNANT FLORIANE; GALASSI, MARIEL ELISA; BENOIT GERVAIS

Santiago de Chile

Conferencia; 24th International Conference on Medical Physics 8th Latin American Congress of Medical Physics 2nd Chilean Congress of Medical Physics; 2019

International protocols for reference dosimetry in radiotherapy,as TRS-398 [1],provide a methodology to determine the absorbed dose inwater using ionization chambers filled with air. The percentage of gas ionization can be determinedusing an electrometer that collects the electrons generated. The dosimeter lecture isproportional to the absorbed dose in liquidwater, butconversion factors likeW-values(mean energy required to form an ion-electron pair after the complete dissipation of the projectile initial energy) are required.For swift protons used in protontherapy, W-values are not accessible precisely by experiments; therefore,they must be obtainedusing theoretical models able to simulate all the physical processes involved in the ion-matter interaction.As experimental data are scarce and theoretical calculations are very complex,international dosimetry protocols for protons and heavy ionstake constant values corresponding to the trend observed by electron impact.Uncertainties in W-values for hadrontherapydominates theglobal uncertainties of the absorbeddose.In a recent work [2], we calculated W-values by electron and proton impact on vapour and liquid water. We used two different methodsconsideringallthe processes involved in the energy depositionby the primary and secondary particles. These are:the Monte Carlo code MDM [3], which allous torepresent the stochastic nature of the ion-matterinteractions, and the Fowler Equation [4], basedin the Continuous Slowing Down Approximation. The results obtained for vapour water are in very good agreement with experimental data and with simulations results reported in the literature from other authors. Here, we present an extension of thesemodels in air gasesby proton impact. We present resultsin the middle and high-energy range, reaching energies higher than 100 MeV where no experimental values exit.Results are in good agreement withexperimental data and with recommended values at intermediate impact energies.[1] IAEA-TRS 398 (2005).[2] Tessaro, Poignant, Gervais, Beuve, Galassi. Nuclear Inst. and Methods in Physics Research B,https://doi.org/10.1016/j.nimb.2018.11.03(2018)[3] Gervais et al, Chem. Phys. Lett. 410, 330-334 (2005) and Gervais, et al. Rad. Phys. And Chem.75, 493 (2006).[4] M.Inokuti, Radiat. Res. Vol.64 6-22 (1975).