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1.3.3 Radiological signature of cancers
It is currently impossible to distinguish a radiation-induced
cancer from a cancer that is not radiation induced. The
reason for this is that the molecular lesions caused by
ionising radiation seemno different to those resulting from
the normal cellular metabolism, with the involvement of
free radicals – oxygenated in particular – in both cases.
Furthermore, to date, neither anatomopathological
examinations nor research for specific mutations have
been able to distinguish a radiation-induced tumour
from a sporadic tumour.
It is known that in the first stages of carcinogenesis a cell
develops with a particular combination of DNA lesions
that enables it to escape from the usual verification of
cellular division, and that it takes about ten to one hundred
DNA lesions (mutations, breaks, etc.) at critical points
to pass through these stages. All the agents capable of
damaging cellularDNA (tobacco, alcohol, various chemical
substances, ionising radiation, high temperature, other
environmental factors, notably nutritional and free radicals
of normal cellular metabolism, etc.) contribute to cellular
aging, and ultimately to carcinogenesis.
Consequently, in amulti-risk approach to carcinogenesis,
can we still talk about radiation-induced cancers? Yes
we can, given the large volumes of epidemiological data
which indicate that the frequency of cancers increases
as the dose increases, but the approach is undoubtedly
more complex, since in certain cases cancer results from
an accumulation of lesions originating fromdifferent risk
factors. However, the radiation-induced event can also
in certain cases be the only event responsible (radiation-
induced cancers in children).
Highlighting a radiation signature of cancers, that is to
say the discovery of markers that could indicate whether
a tumour has a radiation-induced component or not,
would be of considerable benefit in the evaluation of
the risks associatedwith exposure to ionising radiation.
The multifactorial nature of carcinogenesis pleads in
favour of a precautionary approach with regard to all
the risk factors, since each one of them can contribute
to DNA impairment. This is particularly important in
persons displaying high individual radiosensitivity and
for the most sensitive organs such as the breast and the
bonemarrow, and all themore so if the persons are young.
Here, the principles of justification and optimisation are
more than ever applicable (see chapter 2).
2. THE DIFFERENT SOURCES
OF IONISING RADIATION
2.1 Natural radiation
In France, exposure to the different types of natural
radioactivity (cosmic or terrestrial) represents on average
about 65% of the total annual exposure.
2.1.1 Natural terrestrial radiation (excluding radon)
Natural radionuclides of terrestrial origin are present
at various levels in all the compartments of our
environment, including inside the human body.
They lead to external exposure of the population
owing to gamma rays emitted by the uranium-238
and thorium-232 daughter products and by the
potassium-40 present in the soil, but also to internal
exposure by inhalation of particles in suspension and
by ingestion of foodstuffs or drinking water.
The levels of natural radionuclides in the ground are
extremely variable. The highest external exposure
dose rates in the open air in France, depending on the
region, range from a few nanosieverts per hour (nSv/h)
to 100 nSv/h.
The dose rate values inside residential premises are
generally higher owing to the contribution of construction
materials (about 20% higher on average).
Based on assumptions covering the time individuals
spend inside and outside residential premises (90%
and 10% respectively), the average effective dose due
to external exposure to gamma radiation of terrestrial
origin in France is estimated at about 0.5 mSv per
person per year.
The doses due to internal exposure of natural origin
vary according to the quantities of radionuclides of the
uranium and thorium families incorporated through the
food chain, which depend on each individual’s eating
habits. According to UNSCEAR (2000), the average dose
per individual is about 0.23 mSv per year. The average
concentration of potassium-40 in the organism is about
55 Becquerels per kilogram, resulting in an average
effective dose of about 0.18 mSv per year.
Waters intended for human consumption, in particular
groundwater and mineral waters, become charged in
natural radionuclides owing to the nature of the geological
strata in which they spend time. The concentration of
uranium and thorium daughters, and of potassium-40,
varies according to the resource exploited, given the
geological nature of the ground. For waters displaying
high radioactivity, the annual effective dose resulting
from daily consumption (2 litres/inhabitant/day) may
reach several tens or hundreds of microsieverts (µSv).
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CHAPTER 01:
NUCLEAR ACTIVITIES: IONISING RADIATION AND HEALTH AND ENVIRONMENTAL RISKS
ASN report on the state of nuclear safety and radiation protection in France in 2015