The dependence in bystander cellular effects between carbon-ion and X-ray microbeams

Radiation effects are yielded in bystander cells not directly irradiated as well as in those undergone with direct irradiation. The bystander effect has been investigated mostly using the low-energy alpha particles from 238Pu and micro-beam of helium ion. This paper describes results of studies on the effect induced by heavier ions of the broad beam of low-energy ion (C or Fe) accelerated by Heavy Ion Medical Accelerator in Chiba (HIMAC) (non-targeted irradiation), and of micro-beam of C ion by Takasaki Ion Accelerators for Advanced Radiation Application (TIARA) (targeted irradiation), for the purpose to know the quality dependency of the effect. The HIMAC experiment by 6 MeV ion is done with cultured human fibroblasts using such experimental conditions as irradiation to all or half cell-culture surface, use of an inhibitor of inter-cellular signal exchange, and mixing of irradiated and non-irradiated culture, to see cell lethality and mutagenesis. The TIARA experiment by micro-beam of 220 MeV C particle (linear energy transfer (LET)=103 keV/micro-m) or monochromatic 5.35 keV X-ray (orbital radiation yielded in High Energy Accelerator Research Organization (KEK) is performed with the same cell culture and end point as above. The bystander effect is found to occur by irradiations of these heavy ions, and to differ dependently on the nuclide accelerated and energy: the positive quality dependency. The mechanism of the effect can be elucidated more exactly with the micro-beam radiation, which is expected to contribute for making the bases of future studies on radiation effects by low dose radiation, and of medical application like cancer diagnosis and treatment. (R.T.)