Experiments on the Mechanism of the Biological Action of Fast Neutrons

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Abstract
As a preliminary, the now well recognized different dependence upon dosage of the frequency of production of chromosome interchanges by X- and [gamma]-radiation and by fast neutrons is reviewed. The objects of the expts. were (1) to determine the identity or difference in the relevant mechanism of the biological action of fast neutrons and [gamma]-radiation and in particular whether the 2 radiations are completely additive, (2) to measure the relative biologic efficiency of the 2 radiations for unit energy absorption and to determine whether this value increases with overall time of exposure. The fast neutrons were from a polonium-beryllium source (described by Graham and Spinks[long dash]1945), the [gamma]-rays from a standardized Ra source. There was total body exposure of [male] mice. The summation method, well recognized in pharmacology and used in radiobiology by Liechti and Muller (1936) and more recently by Gray and Read (1944), was employed. For dosimetry small ionization chambers were used. The mean energy of the fast neutrons used lies between 4 and 4.5 Mev. In 2 series of biologic summation expts. significant departures from additivity are found for deaths in 0 to 20 days after exposure. The mixed radiations consecutively applied are less effectively lethal than comparable doses of either fast neutrons or [gamma]-radiation alone, indicating a difference in the mechanism of biologic action of the two. The relative biologic efficiency of the fast neutrons in terms of the efficiency of the [gamma]-radiation as unity was found, in the expt. with mean exposure time 48 hrs., to have the value 32.0 [plus or minus] 8.6. This is significantly higher than for short exposure times. A plausible explanation of the difference in mechanism of the biologic action of fast neutrons and [gamma]-radiation appears that the 2 radiations act at different times. Proliferating cells are more sensitive to [gamma]-radiation during a short interval in the prophase stage of mitosis. It is likely that the proliferating cells exhibit a much more uniform sensitivity to neutrons throughout both resting stage and mitosis and because of the length of the former the resting cells show the main effects. This supports the hypothesis that the lethal effects of [gamma]-radiation on normal mammalian cells are produced at least in part by chromosome exchanges.
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