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  Ionizing
radiations are electromagnetic waves or subatomic particles capable of
ionizing matter. The most common non corpuscolar ionizing radiations are
represented by X-rays that have long been employed in radiodiagnostics
and that, nowadays, are especially used in Computer Tomography (CT).
In the health sector, corpuscolar radiations are essentially represented
by beta and gamma radiations that are generated by employing
radionuclides such as Technetium and Iodine 131 for diagnosis and
treatment. They entail both an external exposure risk and an internal
contamination risk linked to the absorption and incorportation of
radionuclides.
Among the fundamental units of measure in radiobiological dosimetry, it
is very important to acknowledge the unit of exposure. This measures the
quantity of ionization produced in a unitary mass of air. The previous
unit of measure for exposure was the Rontgen (R) which has been gradually
replaced by the Coulomb/Kg of air (C/Kg): 1 C/Kg=3876 R.
The absorbed dose (D) measures the quantity of energy given up in a
unitary mass of tissue: the current unit is the Gray (Gy) that has
replaced the RAD (Radiation Absorbed Dose) corresponding to 1/100 Gy (1
Gy=100 RAD).
The equivalent dose (H) is a conventional quantity obtained by multiplying
the absorbed dose D by a weighting factor for the radiation WR (H = D x
WR).
H is the ability of the radiation to produce biological effects in the
tissues with respect both to the energy given up and to the radiation
type. The unit of measure is the Sievert (Sv) that has replaced the REM
(Radiation Equivalent Man) corresponding to 1/100 Sv (1Sv=100REM). The
weighting factor for gamma, beta and X-rays equals the unity for which
the equivalent dose of such radiations corresponds to their absorbed dose
(H=D).
The introduction of a further unit of measure, the Effective Dose (ED),
was necessary because the human body's irradiation is not always
homogeneous as different tissues have different susceptibilities to
radiobiological damage. The effective dose is the probability of an
irradiated organ undergoing a stochastic effect with respect to the
entire body. It is measured in Sv and it takes into account other
weighting factors and WT specific for the different organs (ED = H x WT).
The health personnel exposed to ionizing radiations works in the
following wards and departments:
- radiology and radiotherapy
- nuclear medicine
- cardiovascular haemodynamics
- orthopaedics (plaster room and operating theatre)
- digestive endoscopy
- urologic endoscopy
- anaesthesia
Health personnel assisting patients who undergo diagnostic and
therapeutic procedures involving ionizing radiations may also be
occasionally exposed.
The ionizing process may cause DNA damage. If not adequately repaired
this damage may lead to cell death or alteration. In the first instance,
if the number of dead cells is high enough a serious, clinically evident,
functional damage may occur in an organ or tissue. In the second case,
the modified cell is still able to reproduce itself and this can lead,
after a variable latency period, to a neoplastic condition (in the case
of a somatic cell) or to a damage to the offspring (in the case of a
germinal cell). Effects of the first kind are defined non-stochastics,
whereas those of the second kind stochastics.
Non-stochastic effects
Such effects can follow exposure of the whole body or partial
irradiations.
Whole body exposure (external panirradiation or internal contamination)
gives rise to a very serious syndrome linked to irreversible damage
especially of highly proliferating tissues. The bone marrow and the
digestive system mucosae are worst damaged. The bone marrow shows a
reduced haemopoiesis which is followed by peripheral pancytopenia. Damage
to the digestive system mucosae is followed by diarrhoea, intestinal
bleeding, septicaemia and shock. The worst cases also involve cerebral
tissue damage followed by coma and death. Prognosis is related to the
absorbed dose:
- absorbed dose over 5-6 Gy
survival impossible
- absorbed dose between 2 and 4,5 Gy
survival possible
- absorbed dose between 1 and 2 Gy
survival likely
- absorbed dose below 1 Gy
survival virtually certain
Whole body irradiation with lower doses split in time causes early aging,
reduction of average life in the exposed subjects, alterations of skin,
reproductive organs, bone marrow and crystalline lens. The skin shows
dystrophic alterations especially to the hands where it is possible to
observe flat fingerprints, hair loss, telangiectasia, wart formation. The
blood is affected with chronic anaemia, leucopenia and thrombocytopenia. The
crystalline may be affected by cataract formation.
Partial irradiation can cause an acute effect (that arises immediately
after the irradiation) or a more delayed effect (in the case of prolonged
exposure to small doses). The latter is usually the case for occupational
exposure. High dose partial irradiation mainly affects the skin and the
reproductive organs. Skin alterations are represented by erythema,
blisters and desquamation with ulcer formation. Exposure of the
reproductive organs to doses of 0,1-1 Gy is responsible for temporary
sterility, whereas exposure to doses over 5 Gy causes definitive
sterility.
Stochastic effects
Stochastic, i.e. statistical and casual in nature, effects occur when a
cell that has been modified by the ionization maintains the capacity to
divide and is therefore able to give rise to a malignant neoplasy. No
threshold dose exists for such effects.
The most likely neoplasies linked to chronic exposure to ionizing
radiations are leukaemias and skin cancer. Furthermore, epidemiological studies have shown an increase in
thyroid carcinoma following external irradiation and/or contamination
with I131. Similarly, an excess of bone and breast tumours has been found
in the exposed subjects.
Hereditary effects
Ionizing radiations' effects can affect both the exposed subject and
his/her offspring. Such effects follow the DNA damage induced by the
radiations in the germinal cells or the irradiation of the embryo or
fetus during intrauterine life.
Genetic effects include:
- genetic mutations
- chromosomic aberrations
Genetic mutations can be dominant or recessive; in the first case the
effect will show itself in all the offspring, whereas in the second
instance it will appear only in part of them. Chromosomic aberrations can
concern structure (translocations, deletions) or number.
Should the embryo or fetus be exposed during intrauterine life, the
following outcomes can result:
- death of the embryo or fetus
- malformations and growth alterations
- mental retardation
- malignant tumours induction
- hereditary effects.
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