The Role of Dose Rate, Irradiation Technique, and LET in Determining Radiosensitivities at Low Oxygen Concentrations

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Abstract
AS 02 concentration increases from O, the rate of increase in radiosensitivity may be measured by K in the expression r = (m[O] + K)/([O] + K), r being the sensitivity relative to that in complete anoxia, [O] the O2 concentration, and m and K constants. The value of K depends on the dose rate and the irradiation technique. For organisms in suspension, K was greater at high than at low dose rates and, at high dose rates, it was greater for organisms in aqueous suspension than for those exposed directly to the ambient gas. Changes in the value of K are much more dependent on those variables than on the LET [linear energy transfer] of the radiation. Values of K were compared for 250-kVp X-rays and fast neutrons from the Medical Research Council Cyclotron. The value of K was higher with the radiation of higher LET, supporting the hypothesis that reduction in O2 enhancement ratio with increasing LET may be ascribed to the formation of O2 in the irradiated material in the tracks of heavily ionizing particles.

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