FIGURE 1.

Comparison of the inhibitory effects on colony formation by the three types of radiation under normoxia and hypoxia. Tumor cells were irradiated with 4Gy (physical dose) photon, proton, or carbon-ion radiation under normoxia or hypoxia conditions. Survival fraction (SF) was calculated as: colony formation rate in the irradiated group/colony formation rate in the control group. The representative images of colony formation for each tumor cell group are shown in the upper panels. The histograms indicate the SF of tumor cells exposed to different types of radiation under normoxic and hypoxic conditions. Results are presented as mean ± s.d. Statistical significance of each irradiated group relative to the corresponding control group (0Gy) was indicated by asterisks. *p < 0.05, **p < 0.01, and ***p < 0.001.