Monte Carlo modelling of a prototype small-body portable graphite calorimeter for ultra-high dose rate proton beams

John Cotterill; Sam Flynn; Russell Thomas; Anna Subiel; Nigel Lee; David Shipley; Hugo Palmans; Ana Lourenço

doi:10.1016/j.phro.2023.100506

Monte Carlo modelling of a prototype small-body portable graphite calorimeter for ultra-high dose rate proton beams

Phys Imaging Radiat Oncol. 2023 Nov 8:28:100506. doi: 10.1016/j.phro.2023.100506. eCollection 2023 Oct.

Authors

John Cotterill¹, Sam Flynn^{1

2}, Russell Thomas^{1

3}, Anna Subiel^{1

4}, Nigel Lee¹, David Shipley¹, Hugo Palmans^{1

5}, Ana Lourenço^{1

4}

Affiliations

¹ Medical Radiation Science Group, National Physical Laboratory, Teddington TW11 0LW, UK.
² Particle Physics Group, University of Birmingham, Edgbaston B15 2TT, UK.
³ University of Surrey, Faculty of Engineering and Physical Science, Guildford GU2 7XH, UK.
⁴ Department of Medical Physics and Biomedical Engineering, University College London, London WC1E 6BT, UK.
⁵ Medical Physics Group, MedAustron Ion Therapy Center, A-2700 Wiener Neustadt, Austria.

Abstract

Background and purpose: Accurate dosimetry in Ultra-High Dose Rate (UHDR) beams is challenging because high levels of ion recombination occur within ionisation chambers used as reference dosimeters. A Small-body Portable Graphite Calorimeter (SPGC) exhibiting a dose-rate independent response was built to offer reduced uncertainty on secondary standard dosimetry in UHDR regimes. The aim of this study was to quantify the effect of the geometry and material properties of the device on the dose measurement.

Materials and methods: A detailed model of the SPGC was built in the Monte Carlo code TOPAS (v3.6.1) to derive the impurity and gap correction factors, $k_{i m p}$ and $k_{g a p}$ . A dose conversion factor, $D_{w}^{MC} / D_{g}^{MC}$ , was also calculated using FLUKA (v2021.2.0). These factors convert the average dose to its graphite core to the dose-to-water for a 249.7 MeV mono-energetic spot-scanned clinical proton beam. The effect of the surrounding Styrofoam on the dose measurement was examined in the simulations by substituting it for graphite.

Results: The $k_{i m p}$ and $k_{g a p}$ correction factors were 0.9993 ± 0.0002 and 1.0000 ± 0.0001, respectively when the Styrofoam was not substituted, and 1.0037 ± 0.0002 and 0.9999 ± 0.0001, respectively when substituted for graphite. The dose conversion factor was calculated to be 1.0806 ± 0.0001. All uncertainties are Type A.

Conclusions: Impurity and gap correction factors, and the dose conversion factor were calculated for the SPGC in a FLASH proton beam. Separating out the effect of scatter from Styrofoam insulation showed this as the dominating correction factor, amounting to 1.0043 ± 0.0002.

Keywords: Calorimetry; Dosimetry; FLASH; Monte Carlo; Proton; UHDR.