(from a Po-Be source) were essentially as effective as the 1-MeV eyclotrongenerated neutrons in spite of the much lower dose-rate (0.000003 - 0.01 rad/
min as opposed to 85 rad/min) it was felt that neutrons of still higher energy
might prove less effective.
20k.
Clapp et al.
[C12] published a large experiment in which the effects of
acute doses of 300 kVp x rays (50 to 400 rad) were compared with those of 60
MeV protons (47 to 372 rad).
LET values were obtained.
Irradiation conditions were such that comparable
The proton mean LET was in fact estimated to be ap-
proximately 1.5 keV/yum within the animal's body.
As expected on the basis of
LET considerations, protons were found to be 0.36 times as effective as x rays
for life-shortening and were in general slightly less effective on the basis
of all parameters, excluding the induction of ovarian tumours.
Thymic lymphoma,
myeloid leukaemia and ovarian tumours were increased significantly by radiation,
while other alterations in the incidence and severity of diseases were minimal
by comparison with non-irradiated mice.
An RBE of 1.0 or slightly less would
thus be applicable for most pathological parameters examined.
205.
The experiments of Ainsworth et al.
[A7] showed differences in the shape
of the dose-response relationships which were linear with gamma rays and convex
upwards with fission neutrons.
The RBE of this latter radiation was therefore
dose-dependent and changed between 2.0 and 2.4 for doses causing between 25 per
cent and 45 per cent of life-shortening.
For doses causing 4 per cent life re-
duction the RBE was between 6.8 and 7.6.
Fractionation of the treatment into
24 doses given over 23 weeks, up to a total dose of 80 rad of neutrons or 838
rad of gamma rays, caused an increase of the RBE between 13.5 and 10.5 in male
and female animals, respectively.
206.
The RBE of fission neutrons for life-shortening, relative to '3T og gamma
rays varied in the experiments of Ullrich and Storer [U8, Sk4] according to dose
level.
In the RFM female mice treated at high dose rates the two-component na-
ture of the gamma-ray relationship produced RBE values increasing for progresSively lower doses as the inverse of the square root of the dose in the dose
range below 50 rad.
Such a trend would be predicted by the dual radiation ac-
tion theory of Kellerer and Rossi
[K10].
At doses above 50 rad the RBE estimate
(based on the ratio of the slopes of the linear regressions fitted to the gammaray and neutron experimental points) was 2.9.
This value was similar to that
observed within the same range of doses on the Balb/c mice (3.0).