2.
e0T.
Conclusions
Exeept for a study of thermal column radiation
MeV protons
[S17]
and another on 60
[C12] - for which, as expected, the RBE was found to be near unity -
experience on life-shortening refers only to neutron irradiations.
Fission and
degraded-fission spectrum neutrons, weapon and monoenergetic neutrons were used
in the various cases.
The only species on which data are available is the mouse,
where inferences may be drawn about the RBE changes as a function of dose fractionation and protraction and, to some extent, as a function of the dose level.
208.
For acute doses in the region of a few tens to a few hundred rad of x and
gamma rays (and equivalent doses of neutrons) RBE figures of 2 to 3 have generally been found [U5, S18, S44].
Direct comparisons of single and brief daily
courses of irradiation were reported by Vogel, Frigerio and Jordan [V1] and by
Vogel and Jordan [V3, V4, V5, V6] at neutron dose rates of between 1 and 35 rad/
min: under these conditions the RBE is very similar to that applying to acute
single doses (2.8) with maximum oscillations between 2 and 5.
These figures
are also similar to those of Upton, Randolph and Darden [U10! (2.7 to 5.4) obtained by changing the instantaneous dose-rate form 7 rad/min to 5 rad/day, for
total doses of up to 600 rad.
The data of Curtis and Gebhard [C8, C17] are in
this respect an exception in that the RBE is similar for single and for frac-
tionated doses and, on the whole, very low (1.7).
The evidence reviewed shows
some increase of the RBE upon short fractionation courses and with decreasing
dose-rate down to a few rad of neutrons per day, from figures of 2 - 3 applying
to acute treatments to figures of about
5 - 6.
More consistent changes are
found for extremely long courses of irradiation.
RBE values of between 9 and
15 were quoted under similar conditions by Neary, Munson and Mole [N3!/ and
Neary, Hulse and Mole [N8].
These latter are in fair agreement with the figure
of 13 that may be obtained from the 1958 report of the Committee [U1].
209.
For more precise evaluations the data of Upton, Randolph and Conklin [U7]
are of particular interest because they examine the RBE changes as a function
of the dose-rate, with a spectrum of dosages between the acute and the long
chronic exposures.
The RBE changes in this series from figures of about 3 at
high dose rates to figures of about 8 for protracted exposures or even up to
about 14 for very prolonged treatments.
Ainsworth et al.
[A7! also reported
differences in the RBE of fission neutrons between 2.0 - 2.4 up to 10 - 15 for
weekly dose fractionation over 23 weeks and total doses a few hundred rad of
gamma rays and a few tens of rad of fast neutrons.