tion.
The effect was dose-dependent in that it could be seen at the two
highest but not at the low exposure.
Spacing of the fractions at intervals of
3, 5, 7 or 14 days did not influence longevity.
It was concluded that frac-
tionation after exposure in the 1 to 6 fraction range led to 30-46 per cent of
the sparing effect obtained by halving the total exposure in the 0 - 446 R exposure range.
171.
In the Wistar rat Reincke et al.
[R3]
examined the effects on tumour
frequency and life-span-shortening of two different fractionation regimes of
whole-body x-irradiation (300 R in 3 exposures over 2 months or 10 R in 90
exposures at daily intervals)
and compared these regimes between themselves
and with control non-irradiated rats.
No significant difference in the over-
all incidence of tumours or on tumour types were observed.
vival times were 20 and 25 months, respectively.
The average sur-
Tumour induction rates and
death rates were not different between the two groups, but some differences
became apparent when age-specific rates due to neoplastic and non-neoplastic
causes were compared.
In general, all radiation effects were more pronounced
in the group receiving 300 R x 3.
There was no complete summation of the
effects of the single doses and recovery processes were more effective after
small daily exposures than after greater radiation exposures given at longer
intervals.
172.
Various schedules of fractionation as well as single exposures to 250
kV xX rays were given to female beagle dogs by Andersen and Rosenblatt
[A2}.
For total exposures of 100 or 300 R, 4 equal fractions or 2 equal fractions
given at 7, 14 and 28 days interval were administered.
In general, differen-
ces between subgroups receiving fractionated exposures were apparent only in
groups totalling 300 R.
In these animals life-shortening was increased when
the total treatment time increased from 7 to 84 days.
It was estimated that
the decrease in life-shortening produced by 300 R would be reduced from 23 to
10 per cent as treatment time increased from 7 to 84 days.
3.
173.
Chronic terminated exposures
The papers discussed in the next few paragraphs (173 - 180) pertain to
chronic exposures carried out with different radiations on various animals.
The
heterogeneity of this work does not detract from the quality of some of the con-
tributions in which many important factors such as the protraction of dose ad-