Monte Carlo calculated stopping-power ratios, water/air, for clinical proton dosimetry (50 - 250 MeV)

Abstract

Calculations of stopping power ratios, water to air, for the determination of absorbed dose to water in clinical proton beams using ionization chamber measurements have been undertaken using the Monte Carlo method. A computer code to simulate the transport of protons in water (PETRA) has been used to calculate sw.air-data under different degrees of complexity, ranging from values based on primary protons only to data including secondary electrons and high-energy secondary protons produced in nonelastic nuclear collisions. All numerical data are based on ICRU 49 proton stopping powers. Calculations using primary protons have been compared to the simple continuous slowing-down approximation (c.s.d.a.) analytical technique used in proton dosimetry protocols, not finding significant differences that justify elaborate Monte Carlo simulations except beyond the mean range of the protons (the far side of the Bragg peak). The influence of nuclear nonelastic processes, through the detailed generation and transport of secondary protons, on the calculated stopping-power ratios has been found to be negligible. The effect of alpha particles has also been analysed, finding differences smaller than 0.1% from the results excluding them. Discrepancies of up to 0.6% in the plateau region have been found, however, when the production and transport of secondary electrons are taken into account. The large influence of nonelastic nuclear interactions on proton depth-dose distributions shows that the removal of primary protons from the incident beam decreases the peak-to-plateau ratio by a large factor, up to 40% at 250 MeV. It is therefore emphasized that nonelastic nuclear reactions should be included in Monte Carlo simulations of proton beam depth-dose distributions.

Water equivalent properties of materials commonly used in proton dosimetry

“…The contribution to the depth-dose 25 distribution due to the secondary protons agrees rather well with the simulation 26 presented by Medin and Andreo [Medin and Andreo, 1997] where the transport of these 27 secondary protons are included, except near the target surface . In 28 our approach, we assume that the energy transferred to neutrons or photons leaves the 29 target without contributing to the dose distribution [Medin and Andreo, 1997]. However, 30 a Monte Carlo study of secondary neutrons generated in proton therapy in several 31 phantom materials has been reported in Ref.…”

“…Only a fraction of 23 the energy of the secondary particles is deposited locally according to the ICRU tables 24 for protons or for heavier particles [ICRU, 2000]. The contribution to the depth-dose 25 distribution due to the secondary protons agrees rather well with the simulation 26 presented by Medin and Andreo [Medin and Andreo, 1997] where the transport of these 27 secondary protons are included, except near the target surface . In 28 our approach, we assume that the energy transferred to neutrons or photons leaves the 29 target without contributing to the dose distribution [Medin and Andreo, 1997].…”

Lateral spread of dose distribution by therapeutic proton beams in liquid water

“…65 In order to achieve high precision in hadron therapy treatment [9], PHITS [10], GEANT4 [11], FLUKA [12], KURBUC [13] or MCHIT 96 [14]. Monte Carlo calculations prove that proton dose distribution 97 not only depends on electromagnetic interactions with target elec-98 trons, but also on nuclear interactions, such as fragmentation reac-99 tions and multiple scattering interactions [14][15][16]. 100 The aim of this work is to obtain accurate three-dimensional is made [19,20].…”

Condensed-history Monte-Carlo simulation for charged particles: what can it do for us?

“…3, taken from a very recent IAEA code of practice on electron dosimetry [40]. MC simulation has also been employed to compute stopping-power ratios for proton beams [41].…”