Lethal effect and potentially lethal damage recovery in cultured mammalian cells irradiated by neutron-capture beams

Cell killing and potentially lethal damage (PLD) recovery in Hela cells were examined after irradiation in suspension using a 10B neutron-capture beam. Thermal-neutron irradiation in the medium containing from 0.09 to 9 mM boron-10 (0.9-90 micrograms/g 10B, 90 per cent enriched boric acid) resulted in steeper survival curves than in the boron-free medium. The relative biological effectiveness (r.b.e.), expressed as the Do ratio, increased from 2.2 +/- 0.1 for the control without boron-10 to 5.4 +/- 0.3 in the medium containing 0.9 mM boron-10. The r.b.e. value was 5.2 +/- 0.5 at a boron-10 concentration of 9 mM, and did not increase when the boron-10 concentration was increased from 0.9 to 9 mM. It was suggested that the observed plateau of r.b.e. at boron-10 concentrations higher than 0.9 mM was due to the dominancy of the 10B(n, alpha)7Li reaction for dose accumulation. PLD recovery after the neutron irradiation containing 0.9 mM boron-10 was not observed, while a small effect was observed after irradiation in the thermal neutron beam, although much less than after gamma rays. The results show that the neutron-capture beam is an excellent beam for radiotherapy if the boron-10 concentration in tumour cells could be elevated to about 1 mM.