Dosimetric effects of inserted non-radioactive elements in a tumor area in proton therapy

Abstract

Introduction: Various prompt gamma (PG) ray-based techniques have been proposed to monitor the proton range during treatment, but the poor PG statistics produced entangle their clinical application. Recently, we developed a Monte Carlo (MC) simulation model for the enhancement of PG production based on the nuclear cross section of protons with the material transversed, via inserting the non-radioactive elements 19F, 17O, and 127I in a hypothetical tumor area. This work aimed to study the dose distribution changes caused by the addition of specific % weight fractions of PG enhancers in a water medium using MC simulations.Methods: Our MC model was created using the TOPAS MC package and retaining identical geometries, mixture compositions, and incident proton energies (75, 100, and 200 MeV, respectively) previously tested for the PG statistics enhancement. The total dose deposition in water and in mixture compositions that have been found to maximally increase the PG production was scored and evaluated based on the important dosimetric metrics R90, Bragg peak (BP) width, and full width at half maximum (FWHM). Furthermore, the spatial correlation of PG emissions relative to BPs was also studied and compared on the basis of the BP decrease at R90.Results and Discussion: There is no significant change in total dose deposition except for 127I. However, dose curve shifts in R90 toward shallower depth, followed by steeper BP and reduced FWHM, were observed in all cases. The percentage changes vary with incident proton energies and mixture compositions. The addition of the stable elements had no effect on the PG spatial emission. The dosimetric study reveals that the addition of the tested stable elements did not change the dose distribution and did not alter the dose deposited by secondary particles. Since their addition increases the electron density of the medium, the shift of BP to shallower depths is linked with the mixture composition changes. Furthermore, a steeper BP value is observed that could be beneficial for OAR avoidance. Since this theoretical study of using 19F, 17O, and 127I as PG enhancers is promising from a dosimetric point of view, experimental studies are necessary to determine their clinical application feasibility.