Purpose: Today we have two carbon-ion beam cancer treatment facilities in Japan. One is at the National Institute of Radiological Sciences (Chiba), the other is at Hyogo Ion Beam Medical Center (Hyogo). Carbon-ion beam therapy is a promising technology because of its superb biologic effects, even for radioresistant tumors, and excellent dose distribution. However, serious adverse effects have also been reported due to the difficulty of avoiding irradiating normal tissues. Moreover, it is an expensive technology, and requires a lot of human resources and time. Mixed beam irradiation with X-ray may provide an answer to these problems. Therefore we researched the biologic effects of mixed beam irradiation of carbon-ion and X-ray at Hyogo Ion Beam Medical Center to assess its possible clinical applications.Materials and Methods: Cultured cells from human salivary gland cancer (HSG cells) were used for all experiments. The following conditioned cells were prepared: cultured under standard condition (Normal), cultured under hypoxic condition for 24 hours before irradiation (Hypoxia), and synchronized in late S-phase of cell cycle by serum starvation technique (Synchronized); Hypoxia and Synchronized are both radioresistant conditions. Cells were irradiated with 320 MeV carbon-ion only (CC) at the center of the 6 cm spread-out Bragg peak, 4 MV X-ray only (XX), or mixed beam of both (carbon-ion followed by X-ray; CX or X-ray followed by carbon-ion; XC). When utilizing mixed beam, two irradiations were done within 15 minutes. Irradiated doses were determined according to our previous assessment of biologic equivalent doses. Next, 72-hour-interval fractionated irradiation was performed with cells cultured under standard condition (Interval) to observe the difference of sublethal damage repair. Cell survival was assessed with the usual colony formation assay and survival curves were fitted by linear-quadratic model. Results: In all experiments, the survival curves for cells irradiated with carbon-ion showed the steepest curves with the smallest shoulders, X-ray-irradiated cells showed the gentlest curves with the largest shoulders, and mixed beam irradiation showed intermediate curves. The difference of cell survival in the irradiation sequence of carbon-ion and X-ray (CX or XC) was not significant. In Hypoxia, Synchronized, and Interval conditions, surviving fractions were generally higher than in Normal condition, but not statistically significant. In mathematical analyses, mixed beam irradiation of carbon-ion and X-ray had no synergistic effect, and its cell-killing effect could theoretically be estimated from survival curves of carbon-ion and X-ray by using geometric internal dividing point method. These findings were observed in Hypoxia, Synchronized, and Interval conditions as well as in Normal condition. Conclusions: The therapeutic effect of mixed beam irradiation of carbon-ion and X-ray is intermediate between carbon-ion only and X-ray only, and can be estimated without any complicated calculations. This provides very important information for the clinical use of mixed beam irradiation.45th ASTRO Annual Meetin
