Dose‐mean lineal energy values for electrons by different Monte Carlo codes: Consequences for estimates of radiation quality in photon beams

Abstract

Background:The microdosimetric quantity lineal energy and its mean values have proven useful for quantifying radiation quality in many situations. The ratio of dose-mean lineal energies is perhaps the simplest quantity for quantifying differences between two radiation qualities. However, published dose-mean lineal energy values from different codes may differ significantly with potential influence on radiation quality estimates. Purpose: The purpose was to compare dose-mean lineal energy values from different track-structure data sets for condensed water vapor and liquid water, and to evaluate the influence on radiation quality estimations for some photon sources. Methods: Published dose-mean lineal energy values for 0.1 keV to 1 MeV electrons in spheres with diameters 2 nm to 1 μm, calculated with water vapor and liquid water track structure codes and proximity functions, were collected, analyzed, and compared. Data for cylinders were converted to spheres using a theoretical transformation published by Kellerer. A new set of dose-mean lineal energy values was calculated to cover the whole range of volumes of interest here using the GEANT4-DNA code. The influence from the differences between codes on radiation quality calculations was estimated using dose-mean lineal energy ratios for the photon sources 125 I, 169 Yb, and 192 Ir relative to 60 Co. Results: The theoretical relation for converting the dose-mean lineal energy between different geometrical volumes, results in differences up to 10% between cylinders and spheres depending on electron energy and target size, in agreement with published simulated results. For spheres with diameter above 100 nm, dose-mean lineal energy values for condensed water vapor and liquid water are with few exceptions within ±10%. Below 100 nm, the difference increases with decreasing diameter reaching a factor of two at 2 nm. The values from water vapor codes are in general larger than from liquid water codes. If the dose-mean lineal energy ratio is based on condensed water vapor instead of liquid water, the ratio differs less than 9% for the nuclides 125 I, 169 Yb, and 192 Ir relative to 60 Co independent of the volume simulated. However, a specific value of the dose-mean lineal energy ratio, is found at a larger target diameter in liquid water than in condensed water vapor. Conclusions: When ratios of the dose-mean lineal energy are used as a measure of the radiation quality it is important to compare values for geometrically equal target shapes. A practical method of converting values for cylinders of equal diameter and height to spheres was demonstrated. Although dose-mean lineal energy values calculated with water vapor and liquid water codes may differ significantly, the radiation quality, in terms of ratios of dose-mean lineal 1286

Technical note: Evaluation of a mean value of the number of ionizations in a single event distribution by the variance method in microdosimetry

“…Values of $${\bar{\varepsilon }}_{D,s}$$ down to a few tens of nanometers are possible to measure with the variance‐ or the variance‐covariance method 5 and computations may be used at even smaller volumes. The dimension of the volume to study was recently discussed 15 . Although the results relate to ratios of $$\alpha \ $$‐values, it is the dominating component in the L‐Q relation and both techniques may become useful in investigations of numerical values of relative biological effectiveness in clinical therapy beams.…”

ICRU Report 98, Stochastic Nature of Radiation Interactions: Microdosimetry

“…Good correlation between true y ¯ D ratios for particle therapy beams compared with 60 Co γ-rays and clinical RBEs (Lindborg et al , 2013) emphasizes the potential of experimental nanodosimetry in particle radiotherapy treatment planning as has also been discussed in Lindborg and Waker (2017) and Lindborg et al (2022). Under the condition that the W values in two beams are close, ratios of true n ¯ i , D , s will be close to ratios of true ε ¯ D , s and would also be expected to correlate with changes in clinical RBE.…”

“…For spherical volumes of diameters 2 to 3.4 nm, comparable to DNA, frequency- and absorbed-dose mean values of lineal energy have been evaluated by Margis et al (2020) for tracks of electrons of energies 100 eV to 100 keV simulated by the Geant4-DNA code. Famulari et al (2017) present dose mean values in spheres of diameter 2 to 1000 nm for electrons from 100 eV to 1 MeV, as do Lindborg et al (2022), who also make comparisons between Monte Carlo codes. For protons of energies 0.5 to 100 MeV, frequency and dose mean lineal energies in nanometric cylinders representing DNA-related target structures were evaluated by Mokari et al (2021).…”

“…They are particularly well suited to microdosimetry as they can describe the stochastic nature of particle interactions and energy depositions. They have been used to characterize detectors for microdosimetry, including tissue equivalent proportional counters (TEPCs), silicon and diamond microdosimeters, and thermoluminescence detectors (TLDs), and to calculate microdosimetric quantities in radiation fields of interest (e.g., Bachiller-Perea et al , 2022; Baratto-Roldán et al , 2021; Bianchi et al , 2023; Böhlen et al , 2011; Bolst et al , 2017; 2018; 2020b; Bortot et al , 2018; Burigo et al , 2013; 2014; Conte et al , 2020; Dos Santos et al , 2020; Fattori et al , 2022; Hugtenburg et al , 2007; Kyriakou et al , 2017; Lindborg et al , 2022; Markovic et al , 2020; Matsuya et al , 2022; Mazzucconi et al , 2018; Missiaggia et al , 2023; Nikjoo et al , 1989; 1991; 1994a; 2007; Northum et al , 2012; Oliver and Thomson, 2018; Parisi et al , 2019; 2022; Petringa et al , 2020; Poignant et al , 2022a; Rollet et al , 2004; 2010; Sato et al , 2021b; Selva et al , 2022; Stewart et al , 2002; Vohradsky et al , 2021; among others).…”

Accessing radiation damage to biomolecules on the nanoscale by particle-scattering simulations

“…Lineal energy is defined as the energy deposited in matter in a given volume by a single energy deposition event, and should be applied for dosimetric analysis in subcellular volumes with characteristic length below 100 nm [21]. Extensive collections of microdosimetric parameter for particles such as x-rays, electrons [89] and ions can be found in the monographs of published by the Karolinska institute as listed by Nikjoo et al [90]. Examples and extensions to calculate microdosimetric quantities with Geant4 and TOPAS are described by Ivanchenko et al [91] and Zhu et al [92], respectively.…”