childhood leukemia in the first 10 to 15 years by a factor of 1.5 to 2. Mental
retardation in this period has been reported with doses as low as 10 to
20 rad (10 to 20 cGy), if given in the 8 to 15 weeks of gestation.
Because of these radiation effects, radiological procedures are con-
traindicated in pregnant women, and practitioners must exercise caution in
determining the woman’s status of pregnancy before these procedures.
Before a procedure, it is a common practice to inquire of the patient if she
is pregnant or when she had her last menstrual period, and thus unneces-
sary fetal exposure can be avoided. If the patient is pregnant and the pro-
cedure is essential, then the risk versus benefit to the patient from the
procedure should be weighed by the practitioner with due consideration to
the stage of pregnancy.
Genetic Effects
As mentioned above, ionizing radiations can cause changes in the DNA
structure, which ultimately are expressed in gene mutations. Through
the affected germ cells, these mutations propagate to future generations.
Genetic effects are not expressed in the individual whose germ cells have
been affected by radiation, but are expressed in future generations. Genetic
effects appear as Down syndromes, achondroplasia, retinoblastoma, cystic
fibrosis, sickle cell anemia, Tay-Sachs disease, and other chromosome
disorders.
Spontaneous Mutations
In normal cells, genes occasionally undergo natural mutations even without
radiation exposure. Such mutations are called spontaneous mutations, and
their frequency is about 10−^5 per gene per generation. This means that the
chance of spontaneous mutation is 1 in 100,000. This frequency is increased
by various mutagens such as chemicals and radiation.
In a given generation, radiation does not produce any new mutations and
simply increases the frequency of spontaneous mutations. BEIR VII (2005)
estimated the total risk for all classes of genetic diseases to be 3.0 × 10 −^5 to
4.7 × 10 −^5 per rem (3.0 × 10 −^3 to 4.7 × 10 −^3 per Sv) per generation. It indi-
cates that the genetic risks are relatively small. The dose-response rela-
tionship is linear without threshold, indicating that no dose is safe and any
dose, however relatively small. Furthermore, genetic damage is a function
of the dose rate and the LET of ionizing radiations. High-LET radiations
and high-dose rates cause more mutations. Genetic mutations may appear
in future generations long after exposure has occurred.
Doubling Dose
The doubling dose is a measure of the increase in genetic mutations by radi-
ation. It is the amount of radiation dose that doubles the spontaneous muta-
256 15. Radiation Biology