Low-dose ionizing radiation exposure: Understanding the risk for cellular transformation L.DE SAINT-GEORGES SCK•CEN, Department of Radiobiology, Mol, Belgium

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by L DE SAINT-GEORGES – 2004

ABSTRACT:Radiation is energy transfer.When radiation has sufficient energy to remove an orbital electron from its
atom, an ionized atom is formed, and radiation with the capacity to do this is called ionizing radiation.The primary
effect of radiation is the induction of free radicals and Reactive Oxygen Species (ROS). All the molecules in every cell
of the body are potential targets,but the final effect of radiation will be mainly of concern if the molecule impaired is a
molecule critical for life. ROS are also generated as a result of the aerobic respiration (metabolic ROS) in much larger
quantity than from the natural radiation background. During evolution, life has developed powerful control and repair
mechanisms that greatly contribute to minimize the risks associated with the generation of free radicals and ROS. At
low irradiation doses the probability of the risk is therefore proportional to the dose, and the ALARA (As Low As
Reasonable Achievable) principle seems to be a valuable goal in radioprotection policies. (JBiol Regul Homeost
Agents 2004;18:96-100)

….Radiation dose: Threshold or no threshold?
An important question is about a hypothetical dose
threshold. Does a threshold dose, below which the
risk is nil, exist? According to what has been
previously said on, the essentially random interaction
of radiation with various biological molecules, it
appears to make sense to consider a decrease of the
risk with a decrease of the dose. However, no data
exist which allows us to define a threshold value.
However, at low dose not only harmful effects but
also possibly beneficial effects of radiation could
occur. Here it is essential but not always evident, to
clearly differentiate possibly beneficial effects from the
lack of noxious effects.

Adaptive response and hormesis are often
mentioned to minimize the risk of radiation or
sometimes to deny any adverse outcome below a
dose threshold, as detailed below.
Hormesis is a hypothesis that emphasises the
possible beneficial effect of low doses of radiation and
claims the necessity of a low-dose exposition to get
some benefits while excluding any risk. However, this
concept is controversial. According to the hormesis
model, people should be exposed to low radiation
dose unless it is demonstrated with certitude that
there is no benefit from such exposure. The possibility
of adverse effects is not even considered.
We may wonder why the proponents of the
hormesis model acknowledge a radiation threshold
value for harmful effects, but reject it for beneficial
effects. Considering the essentially random interaction
between radiation and target molecules leading to
unpredictable molecular damage, it appears
surprising that at low doses only beneficial effects
would occur while noxious effect would require a dose
above a certain threshold. To consider hormesis as an
argument against actual dose limits would only be
valid if the efficacy of hormesis could be demonstrated
for the effects against which one wants to
protect at low radiation doses, i.e. cancer and genetic
damages. Unfortunately this is not yet demonstrated
in an unequivocal way. Therefore, the hormesis model
is currently not considered in radioprotection.
The theory of “adaptive response”, (not to be
confused with hormesis) shows that a low dose can
reduce the effect of a higher dose when administered
after a short time delay. This theory is based on
substantial evidence. To reduce a risk appears
beneficial, but it does not mean that the risk is
eliminated. According to the “adaptive response”
model, a first low dose (conditioning dose) is
considered to stimulate the DNA repair mechanisms
that contribute to reduce the effect of a subsequent
higher dose. But the initial low dose can only stimulate
the limited number of cells actually hit, the total of
which in function with the dose. This situation never
excludes the possibility of a transformation of one of
the cells. The next higher dose concerns all cells.
Some of them having the repair mechanisms
stimulated by the first conditioning dose, and may
repair the damage more easily. The other cells, that
were not previously hit, are not protected. The total
damage can be reduced by a factor depending on the
number of the cells conditioned but will always be
dependent on the total number of the cells exposed to
both doses.
Would the conditioning of all cells solve the
question? No, because to reach such a goal we have
to increase the conditioning dose and the risk remains
proportional to the dose and to the number of cells
irradiated.
Therefore the adaptive response does not appear
to be a relevant mechanism for radiation protection
because the (low) conditioning dose that defines it,
also generates a risk of transformation. On the other
hand the challenging dose is not a low dose. We
suggest that natural background irradiation and
metabolic ROS are already stimulating toward some
adaptive response by a constant stimulation of the
repair mechanisms. Then it would appear that there is
no need to add to this radiation burden.
Evolution, in our natural radioactive environment, is
often used as an argument to support such beneficial
effects of low-dose radiation. We should remember
that if Evolution has led to the current scala of
successfully living species, the eliminated species are
unavailable to analyse the non-beneficial aspect of
evolution.

Conclusion: Is a low dose radiation safe or not?
The possible different interactions between rays
and target atoms, the different types of ROS and free
radicals produced, the different molecules as end
points of ROS and the heterogeneity of damage in the
target molecules makes the effect of ionization
essentially unpredictable. Therefore, it does not seem
to be appropriate to predict either deleterious or
beneficial effects. Any issue from primary radiation
effects remains theoretically possible. The probability
of having an effect, whatever it is depends on the
dose and the number of cells being hit. However, the
biological consequences can vary greatly and a
distinction must be made between the organism as a
whole, and the fate of individual cells. Indeed the
worst issue for the cell, the cell death, is probably the
best issue for the organism which in this way remains
protected from transformed cells. Cellular transformation
does not threaten the life of the as it results
in unlimited proliferation, but it may lead to the death
of the entire organism.
Even if the probability of a transformation is no
more important than the probability of any other
effect, we must consider that the final outcome for the
organism will be more determined by cellular
transformation than by any other effect on cells. For
radiation protection the fate of the organism should
prevail above the fate of individual cells.
The biological effects of ROS and free radicals (and
hence of radiation) are ultimately determined by the
effect on the biosystem that controls the enormous
burden of oxidative damage that is essentially
resulting from metabolic ROS. Checking the
radiosensitivity is checking the integrity and efficiency
of the control biosystems, e.g. the DNA repair system,
the immune system, cell cycle control, and apoptosis.
Due to space limitations, we have not considered
all elements dealing with low-dose exposition. For
example, Linear Energy Transfer, dose rate,
bystander effect, genomic instability and questions
related to internal radiation emitters, were not
considered. Nevertheless, this review should
contribute to the understanding of the relative risk
linked to low-dose and low-dose rate radiation
exposure.

The Linear No Threshold hypothesis should remain
so far the basic guide line for the radioprotection
authorities. It appears clearly that the ALARA (As
Low As Reasonably Achievable) principle, which is
currently the basis of radiation protection policies,
should be followed as long as no relevant scientific
facts provide other insights. The very weak probability
of oncogenic events at low dose should reassure
everybody. If any beneficial effects from low-dose
radiation should exist, we can not exclude them,
there is no reason to expect a higher occurrence
probability for them than for cell transformation and
the one would never exclude the other possibility in
other cells. Therefore, such concepts aimed to
attenuate the risk perception, will only lead toward
more confusion, which in turn will generate more
unwarranted anxiety and will finally be totally
counterproductive.
end partial quote.

5 Responses to “Low-dose ionizing radiation exposure: Understanding the risk for cellular transformation L.DE SAINT-GEORGES SCK•CEN, Department of Radiobiology, Mol, Belgium”

  1. Whoopie Says:

    I will post this at HP.

    A new one just popped up THERE

    Japan’s Parliamentary Panel On The Nuclear Crisis Should Draw Methods From Other National Investigations
    http://www.huffingtonpost.com/sunil-chacko/japan-nuclear-panel_b_1155105.html?show_comment_id=124470766

  2. captdd Says:

    Canada Medical Journal: Experts calling for wider Japan evacuation­s — Officials would have to evacuate 1,800 km² using Chernobyl standard
    http://ene­news.com/?­p=21907
    snip
    CMAJ: Public health fallout from Japanese quake, Canadian Medical Associatio­n Journal, DOI:10.150­3/cmaj.109­-4083, Dec. 21, 2011 :

    [… S]ome experts are calling for evacuation of people from a wider area, which they say is contaminat­ed with radioactiv­e fallout. […]

    Following the 1986 nuclear disaster at the Chernobyl nuclear power plant […] clear targets were set so that anybody anticipate­d to receive more than five millisieve­rts in a year were evacuated, no question […]

    Were Japan to impose similar strictures­, officials would have to evacuate some 1800 square kilometres and impose restrictio­ns on food produced in another 11 100 square kilometres­, according to estimates of the contaminat­ion presented by Dr. Kozo Tatara for the Japan Public Health Associatio­n at the American Public Health Associatio­n’s 139th annual meeting and exposition in November in Washington­, District of Columbia.

    “It’s very difficult to persuade people that the level [of exposure set by the government­] is okay,” Tatara told delegates to the meeting. He declined requests for an interview. […]

    • nuclearhistory Says:

      That’s really important stuff Capt D. RPH and Joe’s latest paper will get attacked – as usual – however taking on Canada is going to be more difficult.

  3. Reference lists for exposing radiation hormesis and quack radiation “science” « nuclear-news Says:

    […] https://nuclearhistory.wordpress.com/2011/12/21/low-dose-ionizing-radiation-exposure-understanding-th… […]

  4. Reference list on ‘radiation hormesis’ and ‘adaptive radiation’ « nuclear-news.info Says:

    […] https://nuclearhistory.wordpress.com/2011/12/21/low-dose-ionizing-radiation-exposure-understanding-th… […]

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