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Systematic Microdosimetric Data for Protons Calculated with Geant4-DNA

O Vassiliev*, C Peterson , W Cao , D Grosshans , R Mohan , UT MD Anderson Cancer Center, Houston, TX


(Sunday, 7/14/2019) 2:00 PM - 3:00 PM

Room: Stars at Night Ballroom 1

Purpose: To generate a data set that contains all quantities needed for fast interpolation-based calculation of frequency and dose mean lineal energies, yF and yD, for spherical volumes of diameters from 2 to 1000 nm, irradiated with 0.1 to 100 MeV proton beams. The maximum energy was limited by the physics models of Geant4-DNA. These data will simplify implementation of microdosimetry-based radiobiological models for proton therapy.

Methods: We used Geant4-DNA, release 10.5. The tracking cut off energy was set to 11 eV. A spherical volume was irradiated by a parallel spatially uniform beam of monoenergetic protons. The beam originated a short distance from the volume surface chosen to mitigate nanoscale dose buildup effects, while limiting the energy loss over this distance to 0.1%. The beam was circular with a radius exceeding by 10 nm the volume radius, to account for indirect effects, where protons miss the volume but electrons reach it. This margin was sufficient to capture > 99% of those electrons.

Results: We present tables of the first two moments of the frequency distribution of lineal energy, yF and (y²)F, vs incident proton energy and volume diameter. The tables also include the average kinetic energy of protons when they reach the depth of the volume center. For monoenergetic beams yD is the ratio of tabulated values, (y²)F/yF. For polyenergetic beams, yD=<(y²)F>/, where <...> denotes the fluence-spectrum-weighted average. The latter equation is why, in contrast to previous studies, we tabulated (y²)F , not yD.

Conclusion: We present systematic data that permits fast calculation of frequency and dose mean lineal energies for 0.1-100 MeV protons and volumes of 2-1000 nm in diameter. Statistical and interpolation uncertainties of our data add only a fraction of a percent to uncertainties of physical models of Geant4-DNA.


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