Consistency checks of results from a Monte Carlo code intercomparison for emitted electron spectra and energy deposition around a single gold nanoparticle irradiated by X-rays

H Rabus,W.B Li,H Nettelbeck,J Schuemann,C Villagrasa,M Beuve,S Di Maria,B Heide,A.P Klapproth,F Poignant,R Qiu,B Rudek

doi:10.1016/j.radmeas.2021.106637

Abstract

Organized by the European Radiation Dosimetry Group (EURADOS), a Monte Carlo code intercomparison exercise was conducted where participants simulated the emitted electron spectra and energy deposition around a single gold nanoparticle (GNP) irradiated by X-rays. In the exercise, the participants scored energy imparted in concentric spherical shells around a spherical volume filled with gold or water as well as the spectral distribution of electrons leaving the GNP. Initially, only the ratio of energy deposition with and without GNP was to be reported. During the evaluation of the exercise, however, the data for energy deposition in the presence and absence of the GNP were also requested. A GNP size of 50 nm and 100 nm diameter was considered as well as two different X-ray spectra (50 kVp and 100 kVp). This introduced a redundancy that can be used to cross-validate the internal consistency of the simulation results. In this work, evaluation of the reported results is presented in terms of integral quantities that can be benchmarked against values obtained from physical properties of the radiation spectra and materials involved. The impact of different interaction cross-section datasets and their implementation in the different Monte Carlo codes is also discussed.

Highlights

Gold nanoparticles (GNPs) have been shown to enhance the biolog ical effectiveness of ionizing radiation in-vitro and in-vivo (Hainfeld et al, 2004; Her et al, 2017; Cui et al, 2017; Kuncic and Lacombe, 2018; Bromma et al, 2020)
Due to Auger cascades following the creation of inner shell holes, a larger number of low-energy secondary electrons may lead to additional energy deposi tion in the vicinity of a GNP (McMahon et al, 2011)
Participants in the exercise were to implement this geometry and the given photon energy spectra into their simulation and report the following results for each combination of GNP size and X-ray spectrum: (a) the spectral distribution of electrons emitted from the GNP per pri mary photon emitted from the source, (b) the dose enhancement ratio (DER) in spherical shells around the GNP, i.e. the ratio of the energy deposited per primary photon in the presence and absence of the GNP

Summary

Introduction

Gold nanoparticles (GNPs) have been shown to enhance the biolog ical effectiveness of ionizing radiation in-vitro and in-vivo (Hainfeld et al, 2004; Her et al, 2017; Cui et al, 2017; Kuncic and Lacombe, 2018; Bromma et al, 2020) This effect is often attributed to a dose enhancement due to the higher absorption of radiation by the high-Z material gold as compared to other elemental components of tissue. Due to Auger cascades following the creation of inner shell holes, a larger number of low-energy secondary electrons may lead to additional energy deposi tion in the vicinity of a GNP (McMahon et al, 2011) This results in an additional local enhancement of absorbed dose around a GNP, compared to the case when the GNP volume is filled with water. Since this local dose enhancement is limited to microscopic dimensions, Monte Carlo (MC) simulations are needed to determine its value

Methods

Results

Conclusion