(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: A new method to calculate dose and LET distributions in voxelized geometries in proton therapy is presented. These calculations employ analytical models of the distributions of energy deposition in a microdosimetric structure (site) for monoenergetic beams.
Methods: Monte Carlo (MC) simulations of monoenergetic protons crossing spherical sites in liquid water with diameters from 1 μm to 10 μm have been performed with the Geant4-DNA code. The average and the variance of the distributions of single-event energy imparted, chord length and energy imparted per proton electronic collision have been modeled with analytical functions. Further, the spectral fluence of a clinical proton beam has also been modeled analytically using Monte Carlo calculations carried out with the Geant4 toolkit in liquid water as reference. The integration of these spectra over the curve of average energy imparted provides the dose imparted to the site by the clinical beam. According to the Kellerer’s work1, the dose-average restricted LET distribution, L_D, can be also obtained from the integration of the spectral fluence over the modeled functions. The microdosimetric model is validated against the proton convolution-superposition (PCS) algorithm presently used in our TPS.
Results: Figure 1 shows the dose and restricted L_D distributions, derived from microdosimetry calculations carried out with sites of 1 μm diameter, for a clinical beam of 115.4 MeV in a water box. Lateral and longitudinal characteristics of the dose distributions calculated by PCS are reproduced. LET increases, as expected, laterally and longitudinally as the dose decreases.
Conclusion: A new analytical approach based on the microdosimetric physics of protons has been developed. This provides simultaneous calculations of 3D distributions of both dose and LET. Energy depositions due to other particles can be included in this method by performing the equivalent MC simulations and generating the corresponding models for the necessary quantities.
Funding Support, Disclosures, and Conflict of Interest: This project is supported by Varian Medical Systems, Palo Alto, California; M. A. Cortes-Giraldo has been funded by the Spanish Ministry of Economy and Competitiveness under Grant No. FPA2016-77689-C2-1-R. The Monte Carlo simulations were carried out at the FIS-ATOM computing cluster hosted at CICA (Seville, Spain).
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