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

V. TESSARO; M. BEUVE; B. GERVAIS; F. POIGNANT; M. E. GALASSI

Santiago de Chile

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

International Organization on Medical Physics, Asociación Latinoamericana de Física Médica y Sociedad Chilena de Física Médica

International protocols for reference dosimetry in radiotherapy, as TRS-398, provide a methodology to determine the absorbed dose in water using ionization chambers filled with air. The percentage of gas ionization can be determined using an electrometer that collects the electrons generated. The dosimeter lecture is proportional to the absorbed dose in liquid water, but conversion factors like W-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 obtained using 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 ions take constant values corresponding to the trend observed by electron impact. Uncertainties in W-values for hadrontherapy dominates the global uncertainties of the absorbed dose. In a recent work (Tessaro et.al, NIMB 2018), we calculated W-values by electron and proton impact on vapour and liquid water. We used two different methods considering all the processes involved in the energy deposition by the primary and secondary particles. These are: the Monte Carlo code MDM, which allow us to represent the stochastic nature of the ion-matter interactions, and the Fowler Equation, based in 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 these models in air gases by proton impact. We present results in the middle and high-energy range, reaching energies higher than 100 MeV where no experimental values exit. Results are in good agreement with experimental data and with recommended values at intermediate impact energies.