Abstract ID: 196 Relation between dose average linear energy transfer and dose mean lineal energy calculated for proton therapy beams off axis: A study with the Geant4 toolkit.

Abstract

In a previous work we compared various methods to score, in a voxelized geometry, dose average linear energy transfer (LETd) distributions produced by proton therapy beams in water [1] . In that work, in order to determine which method provided more reliable results, we resorted to numerical stability against changes of geometry and production cuts, and to comparison against LETd values obtained from microdosimetry calculations according to a formula proposed in [2] . Our simulations, with which we could come up with a robust LETd computation method in good agreement with microdosimetry calculations, were done with the Geant4 toolkit [3] , [4] , [5] at central axis and for primary protons only. The aim of this work is to further prove the validity of the method, extending the calculation to off-axis voxels and including the contribution of secondary protons. With this purpose, we compared LETd distributions (2D) calculated for clinical proton beams with those computed from microdosimetry spectra obtained for sites of various sizes, whose typical dimension ranged from 0.25 to 10 microns. The microdosimetry calculations were done with the Geant4-DNA physics package, that simulates step by step interactions of particles in liquid water down to the eV scale [6] , [7] . Our preliminary results confirmed the overall validity of the method at increasing off-axis distances. However, we could also find a significant dependence on the site size of the deviations reported between our “macroscopic” LETd values and those obtained from microdosimetry calculations.