damaging than g-emitting radionuclides. Radioiodine administered orally
to pregnant women during the gestation period of 15 to 22 weeks can cross
the placenta and localize in the fetal thyroid to the extent of 50% to 75%.
The fetal thyroid dose at 6 weeks of gestation is of the order of 2.1 Gy/MBq
(7.8 rad/mCi) (Watson, 1991).
In most cases, radiologic procedures are avoided in pregnant women by
proper screening such as asking them prior to the procedure if they are
pregnant or when they had their last menstrual period. However, at times,
it is discovered after the procedure that the women is pregnant. In such sit-
uations, steps should be taken to estimate the dose received by the embryo
or fetus based on the dosimetry parameters of the radiopharmaceutical.
Depending on the period of pregnancy, the question of therapeutic abor-
tion may be considered if the dose is excessive. Some experts believe that
a dose of 10 cGy (10 rad) is a reasonable value above which therapeutic
abortion should be considered. However, the decision to abort depends on
a number of socio-personal factors.
In radionuclide therapy, pregnant women are absolutely excluded
because of the anticipated excessive fetal dose.^131 I treatment of pregnant
women is almost prohibited unless benefit outweighs the risk of the fetus
from therapy. Besides the in utero effects, there is a small probability of
thyroid cancer induced by the^131 I therapy of hyperthyroidism.
Dirty Bombs
A dirty bomb, also called a Radiological Dispersal Device (RDD), is a mix
of explosive, such as dynamite, with radioactive materials. After the explo-
sion, in addition to the immediate devastating effects of the explosive mate-
rial causing injury and property damage, radioactive dust and smoke spread
the radioactive contamination into the surrounding areas. Radioactive dust
and smoke, if inhaled, can cause ill health effects. The use of dirty bombs
by perpetrators is to spread radioactive contamination and create fear and
panic, more than anything else. Subsequent decontamination could involve
considerable time and cost.
A dirty bomb is not an atomic bomb and is primarily used to disrupt and
not destroy the human life. Another type of RDD might involve a very high
level of radioactive source hidden in a bus, train, or subway station, where
people passing close to the source might get a significant dose of radiation.
Prompt detection of these devices (bomb or radioactive source) is essen-
tial in order to take protective measures.
The sources of radioactive materials are the hospitals, research facilities,
and industrial and construction sites where radioactivity is used for various
purposes (diagnosis and treatment at hospitals, research work, sterilizing
equipment, and check of welding). Some of the highly radioactive sources
are cobolt-60, strontium-90, cesium-137, and iridium-192 used in industrial
260 15. Radiation Biology