. Cellular basis and aetiology of late somatic effects of ionization radiation. Radiation -- Physiological effect. 108 R. H. MOLE ages at whicli they were irradiated. It had already been shown that the leukaemic response of mice of the strain involved varied fairly sharply with age (Kaplan, 1948). Therefore fractionation experiments have been repeated covering a much wider range of fractionation of dose and keeping constant the over-all exposure-tune and therefore the age of the mice duruig irradiation. lOOr o a E o. ⢠SOr daily X 25r daily X .jO O ' .. 50r daily y """â X 1000
. Cellular basis and aetiology of late somatic effects of ionization radiation. Radiation -- Physiological effect. 108 R. H. MOLE ages at whicli they were irradiated. It had already been shown that the leukaemic response of mice of the strain involved varied fairly sharply with age (Kaplan, 1948). Therefore fractionation experiments have been repeated covering a much wider range of fractionation of dose and keeping constant the over-all exposure-tune and therefore the age of the mice duruig irradiation. lOOr o a E o. ⢠SOr daily X 25r daily X .jO O ' .. 50r daily y """â X 1000 2000 Total dose (r) 3000 Fig. 1. Leukaemia incidence in female CBA mice given daily irradiation by X- or y-rays for limited periods. X-Rays 5 days a week at about 1 r/sec for 1-24 weeks (Mole, 1959a). "^"Co y-rays 5 nights a week at about 3 r/hr for 4-12 weeks. (Unpublished data.) With a total dose of 1,000 r the optimum fractionation was found to be very similar to that reported earher by Kaplan and Brown, the effect of dose- rate being evident as well (Fig. 2). However with a total dose of 2,000 r given in just the same over-all exposure-time and in just the same way by the same radiation source, the optimum fractionation was clearly quite different (Fig. 3). It is worth emphasizing that when a dose was given every hour for 672 hours (28 days) 1,000 r produced very little leukaemia. However when each individual fraction was doubled in length from 5 minutes to 10 minutes, there was roughly a 50% incidence of leukaemia. Thus the first half of each fraction had little effect, whereas the second half of each fraction did a great deal. Conversely when the dose was given once a day, 5 days a week, for 4 weeks, the incidence with 2,000 r was about the same as with 1,000 r, so that with this system of fractionation the second half of each fraction had no additional effect at all. Clearly the leukaemogenic effect of the y-rays depended far more on the circumstances of the
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