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Thirteen percent of the total radiation dose received by
the UK population is from manmade radiation. Ninety two percent of this
artificial radiation is due to medical exposure of patients. The other
sources are nuclear discharges, fallout, miscellaneous and occupational (NRPB,
1989). The dose from medical exposure of patients is set to increase as new
technology in radiography is introduced and existing technology is made more
readily available (e.g. CT scanners) (NRPB, 1991).
Radiation
used for medical purposes is mainly gamma (g)
and X radiation. Both
g-rays and X-rays are electromagnetic radiation
just like radio waves and light waves but with a much shorter wavelength.
This type of radiation consists of quanta or packets of energy being
transmitted in waves. The energy of the radiation is inversely proportional
to the wavelength. The wavelength of X-rays is 2.5*10-11 and that
of g-rays
is 1.2*10-12. Although
g
and X-radiation are both of the electromagnetic type, they differ in their
origin. While g-radiation
originates from changes in the nucleus, X-radiation results when atomic
electrons change their orbit (Martin and Harbison, 1993).
Ionisation
happens when an atom loses an electron and consequently becomes positively
charged. The atom now becomes a positive ion (an atom with a missing
electron) and a negative ion (the electron). The resulting ions are known as
the ion pair.
Effects of
radiation on cells
When ionisation
occurs, one or more electrons are removed from atoms, bonds between atoms
can be broken, and the chemical constitution of the system is changed.
Radiation can therefore damage cells through the process of ionisation.
Deterministic
effects (formerly non-stochastic)
High doses of
radiation that damage many cells produce effects that can be related
specifically to the radiation exposure. Theses effects occur in the period
from a few hours up to a few weeks after being subjected to a high dose of
radiation. Cell populations are killed and cell division is slowed down due
to the radiation. Doses above 100 rads absorbed will give rise to nausea and
vomiting. This radiation sickness occurs a few hours after exposure and is
due to the damage of the mucosa of the intestines.
Skin will receive
the largest dose of radiation as it is on the outside of the body. Erythema
is a reddening of the skin that happens after an acute exposure to radiation
of about 300 rads of X-rays. Larger exposures may lead to blistering and
ulceration.
Medical
applications of radiation use much lower doses of radiation than those
required to produce such early effects of radiation (Martin and Harbison,
1993).
Stochastic
effects
Low doses of
radiation may affect only a few cells or possibly a single cell. Such damage
may not cause any symptoms in an organism and may subsequently be repaired.
Any radiation damage may not become apparent until years or even decades
later and therefore stochastic effects of radiation are also known as late
effects of radiation. Since all effects of low dose radiation can occur
spontaneously (with no exposure to radiation) or be caused by other agents,
it may be then difficult or impossible to link the observed abnormality with
the exposure to radiation.
Somatic and
genetic effects
Stochastic effects
of radiation can be classified as somatic or genetic. If radiation damage
results in a disease in the organism itself, the effect is said to be
somatic. Carcinogenesis is the somatic effect of radiation most likely to be
associated with diagnostic levels of radiation (Dowd, 1994). Leukaemia,
thyroid and breast cancer being the most common forms. Another somatic
effect of radiation is cataract formation in the lens of the eye (Martin and
Harbison, 1993).
If the pathology
appears in the descendants of the organism, the effect is said to be
genetic. Mutations may occur in genetic cells of exposed parents. These
effects may then be expressed in the offspring of the parents (Wooton,
1993).
Foetal effects are
related to the developing embryo, when it receives a dose during
organogenesis with the possible effects of intrauterine growth retardation,
malformation and cancer induction (Moore and Persaud, 1993).
Human responses to ionising radiation
A.
Early effects of radiation on humans
1.
Acute radiation syndrome
a.
Hematologic syndrome
b.
Gastrointestinal syndrome
c.
Central nervous system syndrome
2.
Local tissue damage
a.
Skin
b.
Gonads
c.
Extremities
3.
Hematologic depression
4.
Cytogenetic damage
B.
Late effects of radiation on humans
1.
Leukaemia
2.
Other malignant diseases
a.
Bone cancer
b.
Lung cancer
c.
Thyroid cancer
d.
Breast cancer
3.
Local tissue damage
a.
Skin
b.
Gonads Eyes
4.
Life span shortening
5.
Genetic damage
a.
Cytogenetic damage
b.
Doubling dose
c.
Genetically significant dose
C.
Effects of foetal irradiation
1.
Prenatal death
2.
Neonatal death
3.
Congenital malformation
4.
Childhood malignancy
5.
Diminished growth and development
(Bushong,
1993 p. 523) |
