163.
According to Vogel, Frigerio and Jordan [V11 fractionating 275 rad of
neutrons into 3, 4 or 10 separate daily exposures did not result in any significant difference of the mean survival time of CF1 female mice.
Kohn and
Guttman [K6] studied the effect of 520 R of 250 kVp rays given as a single exposure or in two fractions administered 8 days apart on male and female CAF1
mice.
Regarding specifically the effect of fractionation, here again the re-
sults were unclear in showing any significant improvement in survival.
64.
Vogel and Jordan [V5] examined on CF1 female mice the effect of frac-
tionating a weekly dose (300 rad of 600, gamma rays or of 60 rad of fission
neutrons) into 1, 3 or 6 equal dose fractions per week.
Both radiations were
delivered at aprroximately 1 rad/min and the treatments were continued for a
total of 13 consecutive weeks, so that the mice were exposed to almost 4000
rad of gamma rays or 780 rad of neutrons.
The mean survival times of the
gamma-irradiated mice were not significantly different whether they were
exposed 1 or 3 times per week.
There was some indication that a further
dilution of the dose to 6 fractions per week might increase survival but the
significance of the data could be questioned.
No indication of a sparing ef-
fect of fractionation was, however, found in the neutron-irradiated mice.
165.
Silini and Metalli [S27] showed in a small experimental series that sur-
vival time could be improved by a schedule of fractionation where a condition-
ing dose of 150 was followed by 350 R given at seven time intervals between
zero and 48 hours.
A positive regression of the data amounting to an 8 per
cent improvement in survival time was detected.
The kinetics of the phenomen
followed a pattern reminiscent of the short~term intracellular type of re-
covery described in cultured cells by Elkind and collaborators [E44].
166.
Grahn and Sacher [G1] tested the effects of 450 and 750 R of 604, gamma-—
radiation delivered as single exposures or in two equal fractions separated by
increasing time intervals from 3 hours to 28 days.
The regression of survival
time versus fractionation interval was negative in 3 our of 4 cases (2 doses
x 2 sexes) but none of the regressions were significantly different from zero.
The incidence of leukaemia was not consistently modified by fractionation and
this disease was not specifically associated with life-shortening, in contrast
to what seen in other studies [G4, U11].