Impact of DNA Geometry and Scoring on Monte Carlo Track-Structure Simulations of Initial Radiation-Induced Damage.

Track structure Monte Carlo simulations are a useful tool to investigate the damage induced to DNA by ionizing radiation. These simulations usually rely on simplified geometrical representations of the DNA subcomponents. DNA damage is determined by the physical and physicochemical processes occurring within these volumes. In particular, damage to the DNA backbone is generally assumed to result in strand breaks. DNA damage can be categorized as direct (ionization of an atom part of the DNA molecule) or indirect (damage from reactive chemical species following water radiolysis). We also consider quasi-direct effects, i.e., damage originated by charge transfers after ionization of the hydration shell surrounding the DNA. DNA geometries are needed to account for the damage induced by ionizing radiation, and different geometry models can be used for speed or accuracy reasons. In this work, we use the Monte Carlo track structure tool TOPAS-nBio, built on top of Geant4-DNA, for simulation at the nanometer scale to evaluate differences among three DNA geometrical models in an entire cell nucleus, including a sphere/spheroid model specifically designed for this work. In addition to strand breaks, we explicitly consider the direct, quasi-direct, and indirect damage induced to DNA base moieties. We use results from the literature to determine the best values for the relevant parameters. For example, the proportion of hydroxyl radical reactions between base moieties was 80%, and between backbone, moieties was 20%, the proportion of radical attacks leading to a strand break was 11%, and the expected ratio of base damages and strand breaks was 2.5-3. Our results show that failure to update parameters for new geometric models can lead to significant differences in predicted damage yields.

Development of advanced skin dose evaluation technique using a tetrahedral-mesh phantom in external beam radiotherapy: a Monte Carlo simulation study.

Incorrect prediction of skin dose in external beam radiotherapy (EBR) can have normal tissue complication such as acute skin desquamation and skin necrosis. The absorbed dose of skin should be evaluated within basal layer, placed between the epidermis and dermis layers. However, current treatment planning systems (TPS) cannot correctly define the skin layer because of the limitation of voxel resolution in computed tomography (CT). Recently, a new tetrahedral-mesh (TM) phantom was developed to evaluate radiation dose realistically. This study aims to develop a technique to evaluate realistic skin dose using the TM phantom in EBR. The TM phantom was modeled with thin skin layers, including the epidermis, basal layer, and dermis from CT images. Using the Geant4 toolkit, the simulation was performed to evaluate the skin dose according to the radiation treatment conditions. The skin dose was evaluated at a surface depth of 50 µm and 2000 µm. The difference in average skin dose between depths was up to 37%, depending on the thickness and region of the skin to be measured. The results indicate that the skin dose has been overestimated when the skin is evaluated using commercial TPS. Although it is not possible with traditional TPS, our skin dose evaluation technique can realistically express the absorbed dose at thin skin layers from a patient-specific phantom.