(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: Microdosimetry theory provides tools to calculate the dose-average LET corresponding to a charged particle beam, L_D, from the distributions of energy imparted to a given structure (site), chord length and energy imparted per collision. However, this approach is limited to the case in which LET can be considered constant within the site. We present an extension of this theory by means of the concept of segment-averaged LET.
Methods: In a particle beam, the frequency-distribution of LET represents the relative presence of particles of a given value of LET in the beam. This requires a well-defined LET for each particle across the site. However, if a particle’s LET changes in the site, its path can be divided into smaller portions, in a way that LET can be considered constant at each portion. These LET values at each segment within the site generates a “segment-distribution� of LET. The dose-weighted average of the “segment-distribution�, L_(D,s), can be obtained from the distributions of energy imparted in the site, energy imparted per collision and actual length of the particle track within the site instead of chord length. We have performed Monte Carlo simulations with Geant4-DNA in order to assess the values of L_(D,s) in water for monoenergetic proton beams in spherical sites of diameters from 1 μm to 10 μm.
Results: Figure 1 shows the stopping power for protons in water and the L_(D,s) values for our simulations. As the site becomes larger, the segment-average value tends to decrease since a longer range of energies are considered into the average of the stopping power curve.
Conclusion: The concept of segment-averaged LET extends the current validity of microdosimetry theory and provides a way to obtain a characteristic value of LET for a particle beam even for those cases in which macroscopic LET is not well-defined.
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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