940 
GENETIC AND BIOLOGICAL EFFECTS OF RADIATION 
els have not been documented, there are some 
qualitative observations which appear to be in- 
dependent of species and genetic or environ- 
mental influences. These are outlined below. An 
example of a useful quantitative generalization 
would be the relation of cell replication rate to 
the probability of tumor induction following 
irradiation of a given tissue. If this relationship 
were independent of strain and species, then 
determination of cell replication rates in human 
tissue could be used to predict tumor incidence 
in man for that tissue. While no such quantita- 
tive generalizations are obvious at present, their 
potential importance is sufficient to justify a 
continuing search, for as the mechanisms of 
radiation carcinogenesis are clarified, quantita- 
tive generalizations may also be reached. 
The useful qualitative generalizations drawn 
from experimental radiation data can be sum- 
marized as follows.^ 
1. Radiation shortens the mean life-span. 
2. Such life-shortening can be explained 
as principally due to induction and ac- 
celeration of neoplastic diseases, espe- 
cially for low doses and low dose rates 
of radiation. 
3. Dose-response curves for the induction 
of neoplasms are characterized by an 
increase in incidence with increasing 
dose up to a maximum, followed by a 
decline as the dose continues to increase. 
Where a tumor system is easily induced 
by small amounts of radiation (e.g., 
thymic lymphoma in mice, breast tu- 
mors in rats), an absolute threshold 
(i.e., a dose below which there is no 
effect) has not been demonstrated due 
to the large numbers of animals re- 
quired for statistical proof; but for 
more resistant tumor systems (e.g., 
kidney tumors induced by local organ ir- 
radiation in rats, skin tumors by local 
irradiation in rats) an absolute thres- 
hold seems to be established. 
4. High-LET radiation (e.g., fission neu- 
trons) is more effective for a given dose 
than low-LET radiation (e.g., gamma 
rays). 
5. For the same total accumulated dose. 
high dose rates are more effective than 
low dose rates. 
6. Low-LET radiation shows a more pro- 
nounced dose-rate effect than high-LET 
radiation. 
7. Juvenile animals are generally more 
susceptible than adults to the induction 
of neoplasms. 
If these generalizations are indeed independ- 
ent of species, then they should be applicable 
to irradiated human populations. The irradiated 
human population most similar to the experi- 
mental populations discussed above is the Japa- 
nese atom-bomb survivors. Contrary to most 
other human populations studied, these individ- 
uals were not selected on the basis of illness or 
a single parameter such as occupation, and they 
received whole-body rather than localized ir- 
radiation. The qualitative generalizations 
drawn from animal data apply to this popula- 
tion as follows.® 
1. Radiation does shorten the life-span. 
2. This life-shortening can be explained 
on the basis of induced or accelerated 
neoplasia. 
3. The dose-response curve for leukemias 
is similar to the curves seen with the 
experimental animal data. 
4. Differences in the induction of leukemia 
between the irradiated populations of 
Hiroshima and Nagasaki can be ex- 
plained on the basis of increased effi- 
ciency of the radiation from the bomb 
exploded at Hiroshima, radiation which 
contained a larger component of neu- 
trons than was received by the Naga- 
saki population. 
5,6. There are no dose-rate data available 
for this population. 
7. Young members of the irradiated pop- 
ulation appear to be more sensitive to 
the induction of thyroid carcinoma 
than their adult counterparts. 
Thus, in human populations exposed to whole- 
body radiation there do not appear to be any 
peculiar responses that would invalidate the 
qualitative generalizations developed from ex- 
perimental animal data. 
In general, evaluation of the scanty data from 
irradiated human populations in the light of the 
