167.
Ainsworth et al.
[AT] reported that fractionation of a gamma dose of
838 rad into 2h doses of 35 rad administered over 23 weeks produced a sparing
effect by approximately three-fold.
On the contrary, a similar regime of frac-
tionation with fission spectrum neutrons produced an increased life-shortening,
which is a rather unusual observation in fractionation experiments.
Histo-
pathological observations on pulmonary tumours could not explain all of the
increased mortality resulting from neutron dose fractionation.
Tentative ex-
planations were offered, based on the differential acceleration of the lung
tumour appearance or on the differential killing of potentially transformed
cells in the lung.
168.
Storer et al. [S44] exposed RFM and Balb/c mice to 7 weekly doses of
fission neutrons of 6.7 rad for 7 weeks (total of 47 rad) or to 23.5 rad once
every 4 weeks over 28 weeks (total 188 rad).
Animals exposed to 47 rad frac-
tionated had a life-span not different from those given single high dose-rate
exposures or exposures at 1 rad/day.
Balb/c animals receiving 188 rad had a
survival time significantly shorter than that following a single exposure. RIM
animals given fractionated treatment up to 188 rad experienced the same survival as after single exposure and a significantly longer survival than following exposure at 1 rad/day.
It was concluded that the cause of death may cri-
tically determine the effectiveness of the fractionated exposure, although the
authors were unable to provide a basis for their observations.
169.
Data on the effect of dose fractionation on life-~shortening were de-
scribed in the Wistar male rat by Hursh et al.
[H5| irradiated with x rays.
While the life-shortening caused by an acute exposure of 600 R was marked (19
per cent of the control) animals receiving 20 R x 30 days had a life-span similar to that of the controls.
Sixty R x 10 days produced an intermediate ef-
fect between the previous two conditions.
A pneumonia infection occurring
about one year after the start of the experiment complicated the survival picture of the animals.
Also, since the range of control values in these experi-
ments was extremely variable and different from previous data, all this series
seems rather inconsistent.
170.
Data on x-ray dose fractionation were reported in the same animal spe-
cies by Lamson, Billings and Gambino [L4, L11].
Increasing the number of
fractions from 1 to 3 to 6 for the same total exposures of 120, 240, 480 R
caused an increase of life-span, compared to the same exposure in one frac-