Validating a Monte Carlo approach to absolute dose quality assurance for proton pencil beam scanning

Abstract

For radiotherapy, it is crucial to guarantee that the delivered dose matches the planned dose. Therefore, patient specific quality assurance (QA) of absolute dose distributions is necessary. Here, we investigate the potential of replacing patient specific QA for pencil beam scanned proton therapy with Monte Carlo simulations. First, the set-up of the automated Monte Carlo model is presented with an emphasis on the absolute dose validation. Second, the absolute dose results obtained from the Monte Carlo simulation for a comprehensive set of patient fields are compared to patient specific QA measurements. Absolute doses measured with the Farmer chamber are shown to be 1.4% higher than the doses measured with the Semiflex chamber. For single energy layers, Monte Carlo simulated doses are 2.1% ± 0.4% lower than the ones measured with the ionization chamber and 1.1% ± 1.0% lower than measurements compared to patient field verification measurements. After rescaling to account for this 1.1% discrepancy, 98 fields (94.2%) agree within 2% to measurements, the maximum difference being 2.3%. In conclusion, an automated, easy-to-use Monte Carlo calculation system has been set up. This system reproduced patient specific QA results over a wide range of cases, showing that the time consuming measurements could be reduced or even replaced using Monte Carlo simulations without jeopardizing treatment quality.

Daily adaptive proton therapy – the key to innovative planning approaches for paranasal cancer treatments

“…As for fast planning and plan QA, we have previously demonstrated that the complete optimization of plans can be performed in just a few seconds [13], whereas efficient independent dose calculations, based on machine control files, provided a fast and sensitive alternative to plan specific QA measurements [14,33]. Finally, although accurate contour propagation and dose accumulation for deformable geometries are still challenging [34], for the paranasal sinuses spatial transformations between daily datasets are predominantly rigid, simplifying both processes considerably.…”

Pitfalls in the beam modelling process of Monte Carlo calculations for proton pencil beam scanning

“…In contrast however, for PBS proton therapy, even though there is a low-dose spray due to interactions within the beam line and Gantry, 19,22 MC simulations can be started at a defined point before the patient, and not all hardware needs to be explicitly modelled. [17][18][19][20][21] At this starting point, each pencil beam is described by its number of protons, energy, energy spread, and initial optical phase space (beam size, angular spread and correlation). In this study, proton numbers per monitor units have been defined based on Faraday cup measurements (see Winterhalter et al 17 for more details on the absolute dose validation of this approach), nominal energy and energy spreads have been iteratively adjusted to reproduce depth-dose curves in water and the initial optical phase space characterised by subtracting the air scattering contribution from the measured beam data (see Grevillot et al 20 for more detail).…”

“…[17][18][19][20][21] At this starting point, each pencil beam is described by its number of protons, energy, energy spread, and initial optical phase space (beam size, angular spread and correlation). In this study, proton numbers per monitor units have been defined based on Faraday cup measurements (see Winterhalter et al 17 for more details on the absolute dose validation of this approach), nominal energy and energy spreads have been iteratively adjusted to reproduce depth-dose curves in water and the initial optical phase space characterised by subtracting the air scattering contribution from the measured beam data (see Grevillot et al 20 for more detail). Additionally, to deliver pencil beams with energies below the lowest range that can be transported through the beam line (70 MeV for the PSI Gantry 2), a pre-absorber (range shifter) is typically required.…”

“…Additionally, to deliver pencil beams with energies below the lowest range that can be transported through the beam line (70 MeV for the PSI Gantry 2), a pre-absorber (range shifter) is typically required. This pre-absorber can either be modelled as a physical component, whereby all protons are tracked through the pre-absorber as part of the MC simulations, 17,19,23 or as a descriptive component by adjusting the beam energy, energy spread and phase space after the pre-absorber accordingly. 18 These data are then used to perform a pre-absorber specific beam modelling of the MC.…”

“…Both allow the user to define complex simulation setups in text based parameter files, and MC calculations for proton PBS using these toolkits have been set up at multiple institutes. [17][18][19][20] Indeed, with steadily increasing numbers of proton treatment centres, more and more will set up institute specific MC calculations. As such, GATE and TOPAS facilitate the use of Geant4 by users (such as Medical Physicists) without requiring them to have developer level experience of Geant4.…”