Microdosimetry calculations in situ for clinically relevant photon sources and their correlation with the early DNA damage response
Abstract
Background: Radiobiological data suggests variations in relative biological effectiveness (RBE) between clinically used photon-based sources. A microdosimetric formalism using Monte Carlo (MC) methods can mechanistically describe the photon RBE. Experimentally derived RBE based on DNA double-strand breaks ( RBEDSB$\mathrm{RBE_{DSB}}$ ) has been shown to scale with the microdosimetry quantity dose-mean lineal energy ( yD$\mathrm{y_{D}}$ ).
Purpose: To calculate microdosimetric spectra for clinically relevant photon sources, spanning from soft x-rays produced by a 50 kVp x-ray source through various brachytherapy sources up to a 6 MV medical linac. Furthermore, we investigated the correlation between RBEDSB$\mathrm{RBE_{DSB}}$ and yD$\mathrm{y_{D}}$ of different photon sources.
Methods: Photon sources simulated include low-energy x-rays (50 kVp), orthovoltage x-rays (225 kVp), high-dose-rate brachytherapy sources (75Se, 192Ir and 60Co), and a 6 MV medical linac. Secondary electron spectra at the cellular level were calculated for in vitro cell irradiation setups using Geant4 MC-based packages, RapidBrachyMCTPS and RapidExternalBeam. The obtained spectra were used in MicroDose, a microdosimetry simulation software, to obtain microdosimetric quantities, including single-event lineal energy ( y$y$ ) and specific energy ( z$z$ ) spectra, and dose-mean and frequency-mean quantities ( yF$\mathrm{y_{F}}$ , yD$\mathrm{y_{D}}$ , zFs$\mathrm{z_{F}^{s}}$ , zDs$\mathrm{z_{D}^{s}}$ ).…
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