Presentation Proton-nucleus interactions in non-water materials for proton radiotherapy treatment planning

Inaniwa, Taku

To exploit the advantages of proton beam therapy fully, it is necessary to perform three-dimensional treatment planning and optimization. A prerequisite for this is an algorithm for accurate dose calculation. Recently, Monte Carlo (MC)-based dose-calculation systems have been developed for proton radiotherapy treatment planning. The accuracy of the calculation have been confirmed by comparing the MC-simulated and the measured dose distributions in water. This may satisfy the need for proton radiotherapy treatment planning, since tissue heterogeneity is conventionally modeled as water with various densities to reproduce its stopping power, i.e., the stopping effective density. Dose calculation based on this approximation, however, can be erroneous due to the water nonequivalence of body tissues, mainly in nuclear interactions. In absolute dosimetry, similar errors have been reported to occur when, for convenience, solid phantom materials are used instead of water. These errors can be corrected by introducing a fluence correction factor, which is the predetermined ratio of dose-in-water to dose-to-water in the material. The fluence correction factor varies among beams, materials, and depths and can be specifically determined by MC simulation. The accuracy of the MC-based patient dose calculation as well as that of the fluence correction factor depends heavily on the validity of the nuclear model employed by the MC codes. The purpose of this study is to validate the nuclear model employed by Geant4 MC code for proton radiotherapy treatment planning. We performed the irradiation experiments with a 235-MeV proton beam using five non-water materials. These materials were placed in front of a water phantom to measure the planner integrated dose distribution (PIDD) behind the materials. An in-house parallel-plate ionization chamber was used for the measurements. The measured PIDDs were compared with the MC-simulated PIDDs for each non-water materials. We will report the results of the comparisons.
International conference on Monte Carlo techniques for medical applications (MCMA2017)

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