Monte Carlo calculated microdosimetric spread for cell nucleus-sized targets exposed to brachytherapy¹²⁵I and¹⁹²Ir sources and⁶⁰Co cell irradiation

Abstract

The stochastic nature of ionizing radiation interactions causes a microdosimetric spread in energy depositions for cell or cell nucleus-sized volumes. The magnitude of the spread may be a confounding factor in dose response analysis. The aim of this work is to give values for the microdosimetric spread for a range of doses imparted by (125)I and (192)Ir brachytherapy radionuclides, and for a (60)Co source. An upgraded version of the Monte Carlo code PENELOPE was used to obtain frequency distributions of specific energy for each of these radiation qualities and for four different cell nucleus-sized volumes. The results demonstrate that the magnitude of the microdosimetric spread increases when the target size decreases or when the energy of the radiation quality is reduced. Frequency distributions calculated according to the formalism of Kellerer and Chmelevsky using full convolution of the Monte Carlo calculated single track frequency distributions confirm that at doses exceeding 0.08 Gy for (125)I, 0.1 Gy for (192)Ir, and 0.2 Gy for (60)Co, the resulting distribution can be accurately approximated with a normal distribution. A parameterization of the width of the distribution as a function of dose and target volume of interest is presented as a convenient form for the use in response modelling or similar contexts.

Microdosimetric considerations for radiation response studies using Raman spectroscopy

“…which is based on equations (2.19) and (A.12) from ICRU Report 36, 35 and is in agreement with Villegas et al 36 . The curve_fit function from the optimize package of the SciPy library of the Python programming language (http:// www.python.org) is used to determine different C 1 and C 2 fit parameters for each incident photon spectrum.…”

“…(2) for 125 I, 192 Ir and 1.25 MeV photons (with the latter approximating a 60 Co spectrum). 44 Comparison of r z =z calculated with our fit parameters (Table I, for cubic voxels) and those of Villegas et al 44 (determined for spheres) generally differ by about $ 10% for 60 Co, likely due to differences in calculation approach, target geometries, scatter conditions, and source spectrum, as well as uncertainties in fit parameters. Recent work observed discrepancies between track structure and condensed history physics models as target diameter decreased below 1 lm (the smallest dimension considered herein) into the nanometer range.…”

“…Our simulation approach involved using EGSnrc, a class II condensed history MC code, to score specific energy within populations of lm-scale targets. Villegas et al 36 used a different approach to estimate the microdosimetric spread. They used a modified version of PENELOPE to carry out track structure (TS) MC simulations to calculate the single track frequency distribution, f 1 (z), which was repeatedly convolved with itself to obtain the (multi-event) specific energy distribution at the desired dose level.…”

“…Their initial work considered cubic voxel targets, but these results must be disregarded due to a critical bug in their MC transport code. 44 They later considered spherical targets, providing fit parameters to Eq. (2) for 125 I, 192 Ir and 1.25 MeV photons (with the latter approximating a 60 Co spectrum).…”

Proton and light ion RBE for the induction of direct DNA double strand breaks

“…This variation is due to the stochastic probability of tracks interacting with DNA targets and should be distinguished from the spread in microdosimetric energy deposition for populations of cells irradiated to the same macroscopic dose. 48 In fact, to obtain a full distribution of the average number of DSBs per cell in a macroscopic cell population, the yields shown here have to be convolved with the microdosimetric spread in the dose to cells.…”