Reoxygenation and recovery phenomena after potentially lethal damage of tumor cells EMT6 following thermal and epithermal neutron irradiation. Comparison with those after gamma-ray irradiation

A lot of hypoxic cells exist usually in solid tumor. The humor cells, which need commonly oxygen for breeding, are killed selectively by irradiation, but almost all of the hypoxic cells survive. Fraction of hypoxic cell in the solid tumor after the irradiation increases relatively. The fraction of the survived hypoxic cell, however, decreases with time. Radiation sensitivity of the solid tumor increases again as a whole, Recovery phenomena after the potentially lethal damage (PLD) are recognized in normal culture cells, also. The reoxygenation in the tumor cells is considered as a relatively large recovery phenomena after the PLD. Tumor cells EMT6, which have chronically a large hypoxic fraction, are irradiated for investigating of the reoxygenation and the recovery phenomena by thermal, epithermal neutron and Co-60 gamma-ray in the heavy water neutron irradiation facility of Kyoto University Research Reactor (KUR). Absorbed doses of Balb/c female mice injected the EMT6 tumor cells are 15 Gy for thermal neutron, 27 Gy for epithermal neutron, and 15 Gy for gamma-ray, respectively. Surviving fraction and hypoxic fraction of the EMT6 tumor cells after the irradiation are determined by colony formation ratio. LET dependency of the recovery phenomena, however, is not recognized in the experiments. Reoxygenation phenomena caused by thermal and epithermal neutron irradiation have a tendency of more rapid than that by gamma-irradiation. (M. Suetake)