A new approach to modeling the microdosimetry of proton therapy beams

Abstract

Introduction: To revisit the formulation of the mean chord length in microdosimetry and replace it by the particle mean free path appropriate for modelings in radio-biology.Methods: We perform a collision-by-collision following by event-by-event Geant4 Monte Carlo simulation and calculate double-averaged stepping-length, l , for a range of target sizes from mm down to µm and depth in water. We consider l to represent the particle mean free path.Results: We show that l continuously drops as a function of depth and asymptotically saturates to a minimum value in low energies, where it exhibits a universal scaling behavior, independent of particle nominal beam energy. We correlate l to linear density of DNA damage, complexities of initial lethal lesions and illustrate a relative difference between predictive RBEs in model calculations using mean chord length vs. the proposed mean free path. We demonstrate consistency between rapid increase in RBE within and beyond the Bragg peak and l , a decreasing function of depth.Discussion and conclusion: An interplay between localities in imparted energy at nano-meter scale and subsequent physio-chemical processes, causalities and pathways in DNA damage requires substitution of geometrical chord length of cell nuclei by mean-free path of proton and charged particles to account for a mean distance among sequential collisions in DNA materials. To this ends, the event averaging over cell volume in the current microdosimetry formalism must be superseded by the collision averaging scored within the volume. The former, is fundamentally global attribute of the cell nuclei surfaces and boundaries and is characterized by their membrane diameters, hence such global indices are not appropriate to quantitatively represent the radio-biological strength of the particles and their RBE variabilities that is associated with the sensitivities to local structure of the collisions and their spatio-temporal collective patterns in DNA materials.

A Novel Hybrid Microdosimeter for Radiation Field Characterization Based on the Tissue Equivalent Proportional Counter Detector and Low Gain Avalanche Detectors Tracker: A Feasibility Study

“…So far, only the calculation for a uniform isotropic randomness could be successfully applied to experimental methodologies. Estimating the path length l is a critical parameter in microdosimetry that will influence the accuracy of the radiation field quality characterization [17]. In fact, for a given energy ϵ deposited in the detector, the resulting y value can assume a wide range of values depending on the l. For example, if ϵ 10 keV in a 2 μm diameter sphere made of tissue, y can varies from 5 keV/μm to 1,000 keV/μm just considering l values ranging from the sphere diameter to 0.01 μm.…”

A novel hybrid microdosimeter for radiation field characterization based on TEPC detector and LGADs tracker: a feasibility study

“…So far, only the calculation for a uniform isotropic randomness could be successfully applied to experimental methodologies. Estimating the path length l is a critical parameter in microdosimetry that will influence the accuracy of the radiation field quality characterization [Abolfath et al, 2020]. In fact, for a given energy deposited in the detector, the resulting y value can assume a wide range of values depending on the l. For example, if =10 keV in a 2 µm diameter sphere made of tissue, y can varies from 5 keV/µm to 1000 keV/µm just considering l values ranging from the sphere diameter to 0.01 µm.…”