Abstract
Nanoparticles (NPs) containing high atomic number (high-Z) materials have been shown to enhance the radiobiological effectiveness of ionizing radiation. This effect is often attributed to an enhancement of the absorbed dose in the vicinity of the NPs, based on Monte Carlo simulations that show a significant local enhancement of the energy deposition on the microscopic scale. The results of such simulations may be significantly biased and lead to a severe overestimation of the dose enhancement if the condition of secondary particle equilibrium is not met in the simulation setup. This current work shows an approach to estimate a ‘realistic’ dose enhancement from the results of such biased simulations which is based on published photon interaction data and provides a way for correcting biased results.
Highlights
Since the pioneering work of Hainfeld et al (2004), who demonstrated that better tumour control could be achieved by injecting mice with gold nanoparticles (NPs) prior to ionizing radiation exposure, so-called radioenhancing effects have been observed in vitro and in vivo for a variety of NPs containing high atomic number elements, as recently reviewed in the literature
For the low-energy x-ray spectra considered in the EURADOS exercise, the results shown in Figure 5, Figure 6, Table 1 and Table 2 clearly indicate that the dose enhancement factor (DEF) derived by taking the ratio of results from simulations in a narrow-beam geometry significantly overestimate the real dose enhancement by orders of magnitude, at distances from the NP exceeding 100 nm
In this paper we have demonstrated that radiation transport simulations performed with a narrow-beam geometry that does not assure secondary electron equilibrium (SEE) can result in a substantial overestimation of the spatial range around high-Z NPs where absorbed dose is significantly enhanced, and the magnitude of the DEF
Summary
Since the pioneering work of Hainfeld et al (2004), who demonstrated that better tumour control could be achieved by injecting mice with gold nanoparticles (NPs) prior to ionizing radiation exposure, so-called radioenhancing effects have been observed in vitro and in vivo for a variety of NPs containing high atomic number (high-Z) elements, as recently reviewed in the literature (Cui et al 2017, Her et al 2017, Kuncic and Lacombe2018). A widely accepted assumption in studying photon irradiation of high-Z NP, is that the number of secondary electrons emitted by NPs is higher than in soft tissue and that a higher energy deposition within and around NPs is expected, due to the enhanced photo-absorption. These effects are usually quantified by the so-called dose enhancement factor, which is essentially the ratio of absorbed dose to water with and without the presence of the NP. In order to better understand the observed dose enhancement and radiobiological efficiency at the gold/tissue interface, the authors performed Monte Carlo simulations with the PARTRAC code to compare with their experimental results
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