Chromosome aberration yields induced in human lymphocytes by 15 MeV electrons given at a conventional dose-rate and in microsecond pulses.

Yields of unstable chromosome aberration were analysed in human lymphocytes after in vitro exposure to 15 MeV electrons. Two dose-effect curves were prepared. In one, doses of 44 to 742 rad were given at 100 rad/min, and in the other doses of 53 to 764 rad were each delivered in single microsecond pulses. No significant difference could be found between the two sets of data when analysed in terms of the quadratic model of aberration production. Good agreement was observed with other dose-response studies in this laboratory, in which human lymphocytes were exposed to 250 kVp X-rays and 60CO gamma-rays at conventional rates of 100 and 50 rad/min, respectively. Comparison with the results of a low-LET dose-rate experiment shows that the yield of dicentric aberrations remains constant overa wide range, i.e. 25 to 6X 10(9), 100 to 1-5 X 10(10), and 150 to 3 X 10(10) rad/min, respectively, for doses of 100, 250 and 500 rad. Radiochemical consumption of oxygen occuring in the lymphocytes during the single microsecond exposures may amount to less than 5 per cent of the total oxygen present in the blood samples, immediately before irradiation. The data also indicate that the ultra-high dose-rates currently available are insufficient to overcome the therapeutic problem of hypoxic radioresistant tumour cells.