1.
103.
Mouse
Henshaw [H1]| carried out two small experiments with C3H mice with 200 kV
x rays (0, 5, 10, 20 and 25 R/daY, 5 times per week).
He showed a decrease of
the average survival time down to 58 per cent of control at the highest exposure rate and an increase of the death rate.
In a later re-examination of
these data Boche [B11] showed that the excess death rate over the control death
rate divided by the exposure rate was a constant within the range of exposures
considered and could be taken as a characteristic of the mouse sensitivity to
chronic x-radiation.
104.
More complete experiments by Henshaw [H6] and Henshaw, Riley and Staple-
ton [H7] were reported on four strains of mice (CF1, ABC, C58 and A), selected
for special characteristics, like, for example, the high spontaneous incidence
of leukaemia and lung tumours.
Daily whole-body
exposures to fast neutrons
(0.115 to 13.0 n) and of gamma rays (1 to 17.2 R) were administered 6 times
weekly at high instantaneous dose rates.
Most of the animals died either in a
state of atrophy or with mediastinal lymphomatosis.
Doses and LET dependencies
were noted, as well as changes in sensitivity between strains and special pat-—
terns of response of the various diseases.
In those days the main interest
was in establishing the existence of a threshold dose: in this respect that of
neutrons was set below 0.115 n/day and below about
1 R/day for the gamma rays.
It was also noted that life-span was a sensitive indicator of damage since
about ten times more radiation was required to produce a threshold pathological change than to shorten appreciably the life-span.
105.
Daily doses of 0.014 n of fast neutrons (equivalent to 0.1 R/day of
x rays on the basis of the relative effect of single exposure) were shown to
have no definite effects on mice treated for 87 weeks.
Survival time was de-
finitely reduced at exposures of 0.14 n/day for a total of 60 n, while doses
of neutrons ten times higher produced over 50 per cent death of the animals
by about the 29th week at a total dose of approximately 200 n [E2}.
106.
Boche [B11] studied various species of animals.
Wistar rats were
treated with 250 or 1000 kVp x rays at exposure levels of 0.1 — 10.0 R/day.
Rabbits received 1 year of irradiation and beagle dogs or monkeys were
treated for 2 years at similar exposure levels.
The life-span~shortening
effect was shown in various ways but the different energy of radiation or