Presentation Modeling of recombination characteristics in heavy-ion dosimetry with a track structure model (2)

松藤, 成弘  ,  佐藤, 眞二  ,  松山, 哲大

(Aim) Ionization chambers (IC) have been used as a standard dosimetry device in routine radiotherapy. For heavy-ion dosimetry, due to denser energy deposition, initial recombination of produced charge in the IC has been regarded noteworthy but still not yet formulated. The aim of this study is at first to estimate the extent of the initial recombination for ion beams of various radiation quality, then to model the characteristics from mechanistic viewpoint.(Methods) We have reported our experimental result on the initial recombination measured with parallel-plate ICs for the beams of proton 40 to 160 MeV, carbon 21 to 400 MeV/n and iron 23.5 to 500 MeV/n at the Heavy Ion Medical Accelerator in Chiba (HIMAC) and the Cyclotron Facility of NIRS. The result showed the initial recombination factor reached up to 10 % depending on the incident ion energy and species, and varied to be about 1 % even in the 60 mm SOBP region of therapeutic carbon-ion beam. In order to model this recombination characteristics, at first the spatial energy deposition (track structure) formed by the incident ions was estimated. Commonly-used Kiefer-Chatterjee’s amorphous track model was used there, and the initial production of electron and ion pairs, and consecutive recombination, was assumed to be dependent on the energy density. The diffusion of electrons was additionally taken into consideration there.(Result) With the track structure approach, the observed general trend of the initial recombination, i.e., ions of less kinetic energy or higher atomic number lead more significant recombination, was well reproduced while quantitatively overestimated for less energetic ions. This systematic discrepancy was improved by the electron-energy dependent diffusion process.(Conclusion)The initial recombination was measured for various ions in therapeutic energy range. The extent of the recombination was well reproduced by the track-structure model coupled with electron diffusion.

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