Monte Carlo calculation of beam quality correction factors in proton beams using PENH

Abstract

This work calculates beam quality correction factors ( ) in both modulated and unmodulated proton beams using the Monte Carlo (MC) code . The latest ICRU 90 recommendations on key data for ionizing-radiation dosimetry were adopted to calculate the electronic stopping powers and to select the mean energy to create an ion pair in dry air ( ). For modulated proton beams, factors were calculated in the middle of a spread-out Bragg peak, while for monoenergetic proton beams they were calculated at the entrance region. Fifteen ionization chambers were simulated. The factors calculated in this work were found to agree within 0.8% or better with the experimental data reported in the literature. For some ionization chambers, the simulation of proton nuclear interactions were found to have an effect on the factors of up to 1%; while for some others, perturbation factors were found to differ from unity by more than 1%. In addition, the combined standard uncertainty in the MC calculated factors in proton beams was estimated to be of the order of 1%. Thus, the results of this work seem to indicate that: (i) the simulation of proton nuclear interactions should be included in the MC calculation of factors in proton beams, (ii) perturbation factors in proton beams should not be neglected, and (iii) the detailed MC simulation of ionization chambers allows for an accurate and precise calculation of factors in clinical proton beams.

Monte Carlo simulated beam quality and perturbation correction factors for ionization chambers in monoenergetic proton beams

“…are also referred to as the so-called f Q and f Q 0 factors, respectively. [6][7][8][9] In this case the proton beam specific factor f Q is defined by:…”

“…Fano cavity test passed with 0.1% accuracy Kawrakow 24 Source description 10 × 10 cm 2 parallel beam with 60 Co spectrum Mora et al 25 Cross sections and transport parameters Brems cross sections BH Medical Physics, 0 (0), xxxx D w,Q 0 , a cylinder with a radius of 1 cm and a thickness of 250 μm was used as scoring volume and positioned with its center at the measurement depth z Q 0 . [6][7][8] The source was defined as parallel beam using the 60 Co spectrum from Mora et al 25 An overview on the simulation settings used in EGSnrc is provided in Table I according to the recommendations of Sechopoulos et al 26…”

“…The depth was chosen to slightly differ from the recommendations in the two mentioned codes of practice because this allows a comparison to f Q and k Q determined in the literature. [6][7][8][9]11 Simulations were performed for incident proton energies between 70 MeV (R res = 2.18 cm) and 250 MeV (R res = 36.69 cm). The influence of the positioning method of cylindrical ionization chambers was investigated by simulating the f Q factors under consideration of the EPOM (reference point at depth z Q þ Δz Q ) as well as by placing the chambers with their reference points at the measurement depth z Q .…”

“…[6][7][8] Depending on the respective beam quality, this ratio is referred to as f Q or f Q 0 and can also be described by the product of ðs w,a Þ Q=Q 0 and p Q=Q 0 . [6][7][8][9] Current dosimetry protocols, like the IAEA TRS-398 1 and the German DIN6801-1 10 assume that the perturbation correction factor p Q can be approximated as unity for proton beams such that f Q is described solely by ðs w,a Þ Q and the ionization chamber specific perturbations are only considered by a relatively large uncertainty for f Q of up to 1.3%. 1 The German code of practice DIN6801-1 10 states that the uncertainty in assuming p Q being equal to unity is 0.1%.…”

Beam Monitor Calibration for Radiobiological Experiments With Scanned High Energy Heavy Ion Beams at FAIR

“…For protons and heavy ions, TRS-398 assumes the detector perturbation to be negligible (p Q = 1). Today advanced computational methods, in particular powerful Monte Carlo codes, are available to study the response of ionization chambers in different radiation fields [29][30][31][32]. The calculation of k Q factors by means of Monte Carlo simulation can be also described by Equation (7) [33,34]:…”