Biological effectiveness of isolated short electron tracks: V79-4 cell inactivation following low dose-rate irradiation with Al(K) ultrasoft X-rays

PURPOSE: To investigate the biological effect of single, isolated, short electron tracks (<70 nm) relevant to practical human exposures to low-linear energy transfer radiation. MATERIALS AND METHODS: An irradiation rig was constructed that allowed environmentally controlled, protracted irradiations with an individually prescribed dose to up to 20 samples over a period of days. Inactivation of V79-4 mammalian cells by Al(K) ultrasoft X-rays was studied at high and low dose-rates with a maximum exposure time of 42 h. RESULTS: A significant increase in clonogenic survival was observed at the higher doses when the exposure time was increased from <6 min to 21 h, with no further increase observed for 42-h exposures. Despite the short range of the low-energy electrons produced (<70 nm), significant cell inactivation was observed for these low dose-rate exposures. CONCLUSIONS: The results are consistent with the hypothesis that even individual tracks can be biologically effective.