I Jornadas RSEF / IFIMED de Física Médica

On the Monte Carlo calculation of unrestricted dose average linear-energy-transfer (LETd) distributions in proton therapy

No programado

Parque Científico. Salon de Actos Edificio de Cabecera (IFIC-Valencia)

Descripción

We compare three Monte Carlo scoring strategies to calculate unrestricted dose average linear energy transfer (LETd) maps obtained in voxelized geometries irradiated with proton therapy beams. Simulations were done with the Geant4 (Geometry ANd Tracking) toolkit, version 9.6.2 [1]. The first method consists in the step-by-step computation of LET, reported in the literature [2, 3], using the proton energy loss and step length. We found that this strategy is influenced by spurious high LET values, which significance increases as the voxel size becomes smaller. Unrestricted LETd values calculated for primary protons in water with a voxel size of 0.2 mm were a factor ~1.8 higher than those obtained with a size of 2.0 mm at the plateau region for a 160 MeV beam [4]. The aforementioned spurious high LET values are produced by proton steps defined by a “hard” collision with an electron, so that the condensed-history algorithm determines an energy transfer near the maximum value, while the step length remains limited due to voxel boundary crossing. To overcome this problem we derived two alternative methods. The first alternative method scores the LET along the entire path described by each proton within the voxel. The other one followed the same approach of the first method, but using directly the LET computed from stopping power tables, according to the proton kinetic energy at the step, instead of the LET calculated from the actual energy loss and step length. We also carried out microdosimetry calculations with the aim of deriving reference LETd values from microdosimetric quantities. Significant differences between the methods were reported either with pristine or spread-out Bragg Peaks (SOBP). We found that the second alternative method proposed gave the most consistent performance, since it returned stable LETd values against simulation parameter changes and also gave the best agreement with LETd values estimated from microdosimetry calculations.