210.
Only the most recent reports allow an analysis of the RBE changes as a
function of dose.
Ainsworth et al.
[AT] pointed out that the effectiveness of
neutrons increased from values of 2.0 - 2.4 at doses causing 25 - 45 per cent
of life reduction to values of 6.8 - 7.6
tion.
at doses producing 4 per cent reduc-
Ullrich and Storer [U8, S44] reported that the RBE figures of 2.9 - 3.0
observed for doses about 50 rad would increase for progressively lower doses as
the inverse of the square root of the dose, in accordance with theoretical pre-
dictions [K10].
There is therefore a tendency of the RBE to increase as the
dose decreases and in both the cases cited this effect is brought about by the
upper convexity of the neutron curve at low doses, rather than by the upper
concavity of the dose relationship applying to the low-LET radiation.
211.
From the analysis of the mouse data in Figures II and JII the RBE values
that would be obtained for acute single doses of neutrons by pooling all available experience would differ at different doses.
The effectiveness of the neu-
tron treatment obtained by dividing the doses of the two radiations producing
50 per cent life-shortening would be 1.2.
At 20 per cent of the effect the
corresponding value of the RBE would be 3.2 and it would further increase to
6.6 at 10 per cent of the effect.
For doses in the region of 1 rad of neutrons
or less the efficiency would be about 50.
These are of course average figures
obtained from the best fit of the data, as specified wnder paragraphs 90-95.
eile,
The average RBE values applicable to exposures for the duration-of-life
may be obtained from a comparison of the curves as a function of the dose-rate
shown in Figure IX.
At 50 per cent life-shortening the neutron dose-rate that
would give the same effect as the x and gamma rays would be about 10 times
smaller than that of the gamma rays; this same value would go to about 17 at
20 per cent of effect and to about 25 at 10 per cent of effect.
At dose rates
corresponding to 1 rad/day of low-LET radiation (and proportionately lower neutron intensities) the effectiveness of the neutron treatment would increase to
about 40.
Alternatively, the RBE as a function of total accumulated dose might
be calculated from the curves in Figure X.
If the assumptions made in para-—
graph 148 are utilized to this effect, the relative neutron effectiveness values applying to 50, 20 and 10 per cent of effect would be, respectively, about
11, about 14 and about 16.
213.
It may be concluded that the RBE values derived in the mouse by the in-
dependent and comprehensive review of the Committee are in fair agreement with
"