The influence of nuclear interactions on ionization chamber perturbation factors in proton beams: FLUKA simulations supported by a Fano test

Abstract

Purpose In all recent protocols for the reference dosimetry of clinical proton beams, ionization chamber perturbation factors are assumed to be unity. In this work, such factors were computed using the FLUKA Monte Carlo code for three ionization chamber types, with particular attention to the influence of nuclear interactions. Methods The accuracy of the transport algorithms implemented in FLUKA was first evaluated by performing a Fano cavity test. Ionization chamber perturbation factors were computed for the PTW‐34001 Roos® and the PTW‐34070 and PTW‐34073 Bragg peak® chambers for proton beams of 60–250 MeV using the same transport parameters that were needed to pass the Fano test. Results FLUKA was found to pass the Fano test within 0.15%. Ionization chamber simulation results show that the presence of the air cavity and the wall results in dose perturbations of the order of 0.6% and 0.8%, respectively. The perturbation factors are shown to be energy dependent and nuclear interactions must be taken into account for accurate calculation of the ionization chamber's response. Conclusion Ionization chamber perturbations can amount to 1% in high‐energy proton beams and therefore need to be considered in dosimetry procedures.

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

“…This increase is most pronounced for large cylindrical chambers like the two Farmer chambers NE 2571 and PTW 30013, where the f Ref Figures 6 and 7 show the simulated perturbation correction factors for the investigated cylindrical and plane-parallel ionization chambers, respectively. The perturbation correction factors for the PTW Roos chamber are compared to a study by Lourenço et al 11 Of all the perturbation factors p dis shows the greatest deviation from unity. Figure 6 shows an increase in p dis towards low proton energies for cylindrical ionization chambers.…”

“…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][9] All The approaches to determine p Q factors have been presented in various studies. 8,11,21,22 The simulated absorbed dose values that were determined to calculate the various p Q for cylindrical chambers are illustrated in Fig. 1.…”

“…12 In these studies, all factors were determined for various incident proton energies. Furthermore, the perturbation correction factors p Q were individually investigated by Lourenço et al 11 and Baumann et al 8 Gomà et al 6 determined k Q for a set of three cylindrical and nine plane-parallel ionization chambers with the Monte Carlo Code PENH 13 in combination with GAMOS, 14 which is based on Geant4. 15 Wulff et al 9 investigated ionization chamber calculations in proton beams with the Monte Carlo Code TOPASv3.1.p1 16 based on GEANT4.10.3.p1 15 and determined f Q factors for a Farmer type and a plane-parallel ionization chamber.…”

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

“…All simulations were performed with full electromagnetic transport (photon and electron transport down to 1 keV) and with the physics models set to the highest precision level (e.g., full Rayleigh and Coulomb scatter corrections, heavy fragment evaporation and coalescence). Recently, a Fano cavity test performed by Lourenco et al [35] showed that the FLUKA code passes the test within 0.15% if the step size in the multiple Coulomb scattering algorithm is set small enough compared to the dimensions of the cavity of interest. Therefore, in order to maximize the transport precision for the simulations FIGURE 2 | FLUKA simulation of the irradiation of a PTW 30013 Farmer ionization chamber in a water phantom with 1 GeV/u 56 Fe ions.…”