Biological dose distribution analysis with microdosimetry; experiment and monte carlo simulation

We investigated biological dose distribution of heavy ion beam with microdosimetry technique in experiment and simulation. In experiment, which was performed at heavy ion medical accelerator in Chiba: HIMAC, we evaluated physical dose and lineal energy or radiation quality distribution with tissue equivalent proportional counter. The counter includes tissue equivalent gas as a detector and has tissue equivalent size of 1 mum, which makes it possible to imitate dosimetry in a region as small as a cell. Also we developed a technique to evaluate lineal energy and biological dose with Monte Carlo simulation. We installed a new feature to model microdosimetry into general purpose transport code named PHITS. We made comparison between experimental and calculation results, and found that PHITS code overestimated biological effect by 20% because of misestimating of energy dispersion in counter, although it estimates physical dose distribution in good agreement.