RECOVERY FROM LATENT RADIATION INJURY
THE SHORTER-TERM BIOLOGICAL HAZARDS OF A FALLOUT FIELD
cells. In any case, at this time, it is safe to
assume only that recovery as determined
experimentally by paired doses may be faster
RADIATION DOSE
114
than that of the slowest recovering tissue and
that the use of this recovery to predict. the levels
of injuryfor prolonged or intermittent exposures
muy underestimate them eonsiderably. For
this reason in clioosing a tentative value for
man probably it is advisable, in the absence
of other information, to select a recovery half-
TEST OQOSE
time somewhat in excess of the longest known
~— ln.
TIME
a
AFTER INITIAL DOSE
Frover I—Following an initial whole body dose, about
one-hatf LDDs in this case, recovery of the fastest
segment is represented by curve A and of the slorest
by curve BL Test doses adjusted to produce LDtill
define a recavery curve somewhere between A and B
strongly influenced hy A initially and later tending to
run parallel to B. There are implications in these
Statements with respect lo the summation of radiation
effects produced in different segments of the body.
problem haa been discussed (if).
will later run parallel to, but below, B.
This
The
effect will be to make recovery appear too
fast in the carly stages and too complete in
later stages. The first will indicate too short
a half-time and the second will tend to obscure
the residual injury.
If curves A and B are
simply exponential the experimental curve will
not be.
There is suggested, perhaps, somepeculiarity
of abdominal radiation which alters its relative
importance in some strains or species or under
certain conditions since some observers find a
fast early, presumably abdominal, component
of recovery from whole bodyirradiation while
others do not. Possibly recovery of segments
is less independent in some specics or under
in mammals, which, at present, is that of the
guinea pig---some 20 days.
There is another problem raised by Mole [6],
who asserta that recovery rate per unit of
injury is not a constant, as required by an
exponential recovery hypothesis, but is a function of the level of injury and is slower with
highinitial dores, a result perhaps contrary to
that of Storer discussed above. Mole’s analysis
of his data is not definitive butif bis conclusion
should be correct much more investigation of
recovery from different dose levels would be
required before the results could be applied to
unknown situations. The bulk of the present
evidence indicates that recovery is not a function of initial dose for doses less than about
one-half LD.
Since this paper was presented verbally
Davidson [7] has issued a report in which he
shows a linear relation between whole body
recovery rates in various mammals and the
time course, following irradiation, of changes
in white cells of the blood. Because blood data
are available in man this relation permits a
prediction of recovery half-time in man to be
aboul.one month.
Although there is no known
biological basis for Davidson's correlation it
may be @ sound one and also it may give a
certain conditions, one of which may be dose
size. In the gastro-intestional tract, for example, latent injury, as defined here, may have
lead for search of similar empirical relation-
normally undergoing rapid replacement.
The objective for which the recovery halftime is used for human exposure calculations
is that of avoiding a level of acute injury which
will be dangerous or lethal. If recovery went
no meaning with respect to those doses which
are sufficiently great to kill cells whieh are
Res-
toration of normal eell division and proliferation is presumably a process quite different
from those involved in recovery of persisting
ships. In any case the half-recovery time of
28 days chosen by Davidson appears to be a
fairly conservative choice for man, even in the
lightof the problems raised above.
to completion this factor alone would be wholly
determining and it would be reasonable, when
necessary, to permit exposures to levels as high
as possible without incapacitation.
Thepertial irreversibility of radiation injury
precludes adoption of this simple point of view
and. also raises the question whether it is more
practical to adopt a total dose as a permissible
level independently of the time, for at least a
month or two, over whichit is austained.
It is reasonable in comparison to other species
and is indicated by the Rongelap incident (8)
that 200 roentgens of whole body gamma radiation is sublethal for young adult man and
probably for most. of the very young and for
the moderately old. In the young adult man
this dose is not seriously incapacitating even
when received promptly.
It appears worth-
while then to consider the probable effects of
200 roentgens as a permissible dose in single
episodes lasting for durations of minutes up to
a month or more.
Observations on rodents [1] indicate that life
is shortened about 7 percent per LD, or about
1 percent per 100 roentgens for accumulated
doses whose daily components do not exceed
120 roentgens. The effect with doses administered in less than a few hours is about 3 per~
cent per 100 roentgensin the 200 to 500 roentgen
Tange and is greater with larger doses.
This difference is not attributable to dose
rate per se but to total dose within a given
time.
For example, using the same dose rate,
Hurshet al (9] showed that 600 r shortened the
life of the rat some 20 percent when administered in one day but gave a much smaller
effect when administered in 10 daily doses of
60 r. These relationships require much additional study but in the rodents, at least, it
appears safe to assume that doses less than
100 r per day give the smaller effect on life
apan and that doses of 200 r per day or more
give the larger effect.
The only evidence that man may suffer
fractional shortoning of life spen similar to
115
comparison to unexposed physicians. The av- .
erage ages of death are 60.5 and 65.7 years,
respectively. If these radiologists dying in the
period 1930 to 1954 sustained on the average
the equivalent of about 800 roentgens of whole
body radiation in divided doses their loss of
life span wouldbe similar to that in the rodent.
Because this dose is in the range to be expected
it is unlikely that man and rodent can differ
by more than a small factor such as 2 or 3.
Theeffect on life-span of large prompt doses
in man is not known but presumably it will be
greater than that of distributed doses as in the
rodents.
Assuming man and rodent to be alike 200
roentgens will shorten life about 2 percent when
delivered at rates not exceeding about 100 r
per day and shorten it as much as 6 or 7 percent
when delivered promptly.
Existing data indicate that the after effects
of successive exposures are additive. There-
fore, two exposures of 200 r widely separated
would shortenlife twice as much as one. However, 400 r in a single prompt dose, if this is
very near LD,» for man, would be expected to
shorten life as much as 30 or 40 percent because
life-shortening in rodents increases rapidly with
the magnitude of the single prompt dose as the
dose approaches the lethal range.
CONCLUSIONS
It appears that a limit of 200 roentgens for
emergency exposures for any period up to 30
days will not entail acute lethality or significant
incapacity. Congequent life-shortening would
be as much as 6 percent, about 4 years, if man
is like the rodent andif the doseis received over
a short period. If the dose is less than a given
amount, possibly about 100 r on any one day,
life shortening will be about 2 percent.
How-
ever, there are no definitive data for any species
on how small the daily level must he to cause
the lesser effect.
The suggested use of recovery with half-time
that of rodents is that presented by Warren
of 28 days by Davidson to determine “effective
years in longevity of American radiologists in
It is not clear at present, however, what effec-
[10] whose data show an average loss of 5.2
dose” appears to be a conservative practice.