|
FYI Jerry Cuttler --------------------------------------------
November 27, 2001 Medical
irradiation, radioactive waste, and disinformation: The
position of the French Academy of Medicine The
French Academy of Medicine, preoccupied by the concerns that arose in the public
regarding medical exposure to X rays and radioactive waste in the environment,
and by erroneous information that these topics give rise to, wishes to express
its position. Humanity
is exposed to ionizing radiation From
the beginning, life developed in a bath of ionizing radiation to which it has
adapted. This radiation has a
cosmic origin or originates in the earths crust where, since the creation of
the earth, the unstable isotopes of the elements of very long physical
half-lives remain: thorium, uranium, potassium, and rubidium. Natural exposure results therefore from
internal and external sources, both characterized by various physical properties
and different effects on the human body. The
presence of radionuclides in the environment results in an average radioactivity
of 10.000 Bq in the human body, essentially from carbon-14, and potassium-40
whose concentration is regulated by homeostatic control of intracellular
potassium content. Humans are
exposed to natural sources of ionizing radiation and the effective dose they
receive is on the average equal to 2,4 mSv per year. This dose varies according to the
altitude and nature of the soil, from 1 to 10 mSv, and attains 100 mSv in large
regions such as the Kerala in India, or the city of Ramsar in Iran (1). Various tissues are specifically
irradiated, such as the lung by radon, the kidney by uranium, bones by radium,
and bones, hepatic and reticulo-endothelial system by thorium whose metabolic
and radiological characteristics are similar to those of plutonium. The dose the tissues receive varies
widely throughout the world. Since
the end of the 19th century, to natural irradiation has been added diagnostic
medical irradiation delivering an average of 1 mSv per year, but with variations
going from less than 1 mSv to more than 20 mSv per year. Since
1950, it is necessary to add irradiation of industrial origin - notably that of
nuclear energy for the production of electricity (extraction and treatment of
uranium, functioning of reactors) corresponding to an exposure of about 0.01 to
0.02 mSv per year. Coal, a natural
source, represents 0.01 mSv per year.
Nuclear testing in the atmosphere contributes to an average exposure of
0.005 mSv/yr and the Chernobyl accident to 0.002 mSv/yr (1).
For
equal doses, the biological effects of the different types of ionizing radiation
are identical whether their origin is natural or artificial.
Exposure
of workers to ionizing radiation (200,000 in France of which more than half is
in the medical sector) results, in France, in an average exposure of 2 mSv per
year (OPRI annual 2report) with less than 1% surpassing the average statutory
limit of 20 mSv per year. With the
exception of diagnostic irradiation, these exposures are characterized by a low
dose rate, close to types of chronic irradiation. The dose rate clearly distinguishes them
from accidental and therapeutic irradiations which have a high dose-rate,
leading to an instantaneous accumulation of damaged molecules that perturb the
cell repair mechanism components, from a few mGy absorbed in a few minutes (2).
The
dismantling of nuclear power plants and nuclear waste storage facilities are
activities that contribute to small increases of delivered doses to the
population at very low dose rates (1) (about 0.005 ΅Sv (1) per year for
iodine-129 for example). This is
due essentially to the transfer to the food chain of various man-made
radionuclides of very long half lives leading to either homogenous exposure of
the entire body, (as in the case of the potassium natural 40), or to selective
organ exposure, especially to the digestive tract, bone, liver and kidney, as in
the case of the natural isotopes of uranium and thorium. It is therefore legitimate to infer
their possible effects on human health from those known to result from natural
sources, which expose populations of several millions. The
health consequences of the exposure of humans to some mSv. There
exists data (3) establishing that high natural exposure is associated in adults
to an increased rate of chromosome aberrations of the circulating lymphocytes, a
biological indicator of exposure.
It cannot be concluded, however, that it is an index of harm since no
global increase of cancer risk (4), or increase of congenital malformations (5),
or abnormalities in newborns induced by cytogenetic effects (6) were detected in
the well studied population of Kerala region, which is highly exposed to
external irradiation and to contamination.
Identical conclusions are obtained in the exposed Chinese populations
(7-8). As the NCRP reported in the
United States (9): «It is important to
note that the rates of cancer in most of the exposed populations to low level
radiation have not been found detectably increased and that, in most cases, the
rates have appeared to be decreased. » The
hypothesis of the risks of cancer induced by low doses and dose rates is founded
on the extrapolation from data of highly exposed human groups, postulating that
the global risk is constantly proportional to the received dose without being
limited by a threshold (LNT). This
hypothesis raises many scientific objections (10) and is contradicted by the
experimental data (11) and epidemiology. In
the groups having received more than 200 mSv in adults and 100 mSv in children,
an increase in cancer incidence has been observed: the survivors of Hiroshima
and Nagasaki, irradiated patients, nuclear workers, and residents of the former
USSR contaminated by nuclear waste.
No cancer excess was observed for doses lower than 100 mSv; a doubt
remains nevertheless in the case of irradiation by X-ray in-utero from 10 mSv,
as the epidemiological data are contradictory (12). Even
though no excess of cancer has been observed, effects from low doses cannot be
excluded because of statistical limitations. Nevertheless, it is necessary to recall
that the linear theory with no threshold is contradicted by the observation of
thresholds for bone cancers induced by radium-226 and cancers of the liver
induced by Thorotrast. It is also
not compatible with induced leukemias in A-bomb survivors and with patients
treated by radioactive iodine (1,10,13). Furthermore, the epidemiological study
of British radiologists for the period 1897-1997 (14) showed that after 1954
there is no excess of cancers in these practitioners in comparison with their
non-radiologist colleagues. On the
contrary, the incidence tended to be lower, as in the case of populations
described by the NCRP (9). Similar
findings were observed for many groups of exposed professional workers to
ionizing radiation, notably the radiological technicians (12). While the frequency of cancers was
increased during the period when no radioactive protection measures were taken,
the excess of cancers disappeared when regulatory limits were reduced to 50
mSv/year, as enforced up to 1990 (12). These
observations, as well as recent biological data, show the complexity and the
variety of molecular and cellular mechanisms governing cell survival and
mutagenesis according to the dose and dose rate (1,2,11,13), remove all
scientific rationale for a linear extrapolation that very greatly overestimates
the effects of low doses and low dose rates. Exposures of a few mSv/yr cannot be
accumulated, especially for those lower than 0.02 mSv/yr, delivered to a large
number of individuals (as done with the use of collective doses) to estimate the
risk of excess cancers (15). The
Academy of Medicine, joining the position of other large international
institutions, strongly states that such calculations have no scientific
validity, notably to evaluate the risks associated with low dose or dose rate
radiation such as in the case of the fallout from Chernobyl outside the USSR.
The UNSCEAR 2000 report and the controversy with the OCHA
The
Chernobyl catastrophe has caused to this day about 2,000 cancers of the thyroid
in children, essentially by exposure to iodine-131 and to the short-lived iodine
isotopes. The delivered doses to
the thyroid were in the order of 1 Gy and 3 Gy on average in the most exposed
regions (16). This carcinogenic
effect is therefore in keeping with our current knowledge. No increase in thyroid cancers was
observed outside of the USSR, for example in Poland or other bordering
countries. In
2000 UNSCEAR concluded that there is an absence of excess in leukemia and in
cancers other than thyroid cancer in the population around Chernobyl; it did not
find a relationship between the exposures to radiation and congenital
malformations in these populations (1).
This conclusion was questioned in 2001 by the OCHA, the humanitarian
organization of the UN, but the OCHA publication was refuted in a response by
the UNSCEAR committee, that alone has the medical and scientific competence to
speak in the name the UN and WHO on this subject (17). A conference was therefore held in Kiev
in June 2001 with WHO, OCHA, UNSCEAR, ICRP and the IAEA, and the conclusions
have been published (17). These
conclusions find that health conditions are alarming because of the general
deterioration of health and social conditions, notably in Belarus, but do not
contradict the UNSCEAR conclusions.
In fact, this deterioration is probably caused by the living conditions
of the relocated populations, as well as psycho-sociological factors. Different questions have been raised
that do appear to necessitate research on the epidemiology of the conditions of
the catastrophe consisting of multiple possible factors that altered the health
of these populations: this is the recommendation of the Kiev conference.
It is possible to reduce human exposure to ionizing
radiation, in particular of medical origin, but this necessitates means.
Radiological
examinations represent, by far, the principal cause of irradiation of human
origin (effective dose about 1 mSv/yr in France). The recent directive of the European
Union introduces two important notions to this problem: optimization (to reduce
as much as possible the dose per examination) and justification (to evaluate the
benefit and the risk of each examination, and to not practice it unless it is
useful). These principles
necessitate therefore the evaluation of effective doses received by the patient
examined and the associated risks. According to the examinations and the
techniques used, the effective doses vary from a fraction of a mSv to several
tens of mSv (examination by CT scan or interventional radiology) and the risks
vary widely according to age. An
over-evaluation of risks could deprive a child of a useful examination;
inversely, an under-evaluation could favor the multiplication of medical X-ray
examinations that are not useful.
The Academy counsels therefore: 1) to focus on the study and evaluation
of examinations from which the potential risks are the largest: CT scans of
young subjects, multiple radiological examinations of premature infants, and
interventional radiology, 2) to promote techniques likely to reduce or to
eliminate irradiation without harming the quality of clinical information and to
encourage technical and fundamental research in this area, 3) to conduct
epidemiological studies on groups of patients, notably children, having received
the highest doses from radiological examinations, 4) to favor initial and
continuing training of clinicians in matters of radiation protection.
It
is unacceptable that, while irradiation of medical origin represents in France
95% of irradiation added to natural background irradiation, so little money is
devoted to its reduction, whereas radiation protection in industry is well
funded. It is necessary to define health priorities with regard
to radioactive waste.
Outside
of this context some recommendations can be made concerning the problem of
radioactive waste and health. It
appears essential to support epidemiological efforts concerning the populations
exposed naturally to a high level of radiation, as well as populations of the
ex-Soviet Union massively exposed to radioactive waste and other pollution. In the framework of studies dealing with
potential health effects of nuclear waste management, the isotopes that should
be considered in priority should not be selected according to the collective
dose that would result, but according to the individual dose potentials, since
the calculated collective doses from low individual doses to a few microSieverts
do not have any health significance.
An important national effort should be undertaken, as was done within the
framework of the programs of the U.S. DOE, on the biological mechanisms involved
in the cellular response to doses below 100 mSv, in particular regarding effects
on DNA repair, cell signaling, and the hereditary transmission in DNA sequence
encoding of parental DNA modified by irradiation.
Recommendations
The
French Academy of Medicine: 1
recommends increasing efforts in radiation protection in the area of
radiological examinations, on the one hand to reduce received doses from certain
types of examinations (CT scans of children, interventional radiology, lung
X-ray examinations of premature infants, etc
) and on the other hand, to allow
radiology departments, notably in radio-pediatrics, to benefit from a staff well
trained in dosimetry and capable of ensuring the quality control of equipment,
in a way similar to that previously done with mammography in breast cancer
screening. It also recommends
reducing patient exposure through increased clinical and technical research in
this area and improved training. 2
recommends an effort in fundamental research: on the biological mechanisms
activated by the repair of the DNA damage after low doses up to 100 mSv; and on
the effects of these doses on the exchanges of intra- and inter-cellular
molecular signals. 3
denounces the utilization of the linear no-threshold (LNT) relation to
estimate the effect of doses lower than a few mSv (equivalent to variations of
natural radiation in France) and of doses hundreds of times lower, such those
caused by radioactive waste, or 20 times lower, such as those resulting in
France from radioactive fallout from the Chernobyl accident. In agreement with many international
institutions, the Academy denounces the improper use of the concept of the
collective dose to this end, since these procedures are without any scientific
validity, even if they appear to be convenient for administrative purposes.
4
subscribes to the conclusions of the 2000 Report of the Scientific Committee
of the United Nations (UNSCEAR) concerning the analysis of health consequences
of the Chernobyl accident, and denounces the propagation of allegations
purporting an excess in cancers other than that of the thyroid, and an excess of
congenital malformations. 5
recommends the introduction of the ADIR (Annual Dose of Incorporated
Radioactivity, being equivalent to 0.2 mSv, resulting from homogeneous exposure
of the human body to natural potassium-40 and carbon-14), as this dose
equivalent is almost constant whatever the size of the individual and the
geographic region. 6
The Academy of Medicine, in accordance with its statement given October
3rd 2000, continues to recommend maintaining, without modification,
the European directive concerning regulatory limits (to 100 mSv/5yr). To substitute dose limits of 20 mSv/yr
would reduce the flexibility of the European norm, without offering any health
advantage, and would harm medical radiology departments by making the
development of new techniques more difficult. Glossary
Bq
or becquerel, the radioactivity characterized by one disintegration per
second. In the human body, 10.000
Bq of the natural sources represent 1 ADRI that is equivalent by convention to a
dose equivalent of 0.2 mSv. Gy
or gray, the absorbed dose corresponding to 1 joule per kg.
Sv
or sievert, the unit of equivalent dose obtained from the product of the dose
absorbed by the weighting factor
-for radiation quality (1: for X, beta and gamma radiations
20 for
alpha radiation). The effective
dose also expressed in Sv is the product of the dose equivalent by the weighting
factor for organs (0.05 for the thyroid
1 for the entire
body). IAEA:
International Agency of Atomic Energy ADIR:
Annual Dose of Incorporated Radioactivity, recommendation G. Charpak.
DOE:
Department of Energy, US ICRP:
International Commission on Radiation Protection NCRP:
National Council on Radiation Protection and Measurements (USA)
OCHA:
Office for the Co-ordination of Humanitarian Affairs WHO:
World Health Organization UNSCEAR:
United Nations Scientific Committee on the Effects of Atomic Radiation
References
1)
UNSCEAR:
Sources and effects of ionizing radiation, Continuation to the general assembly
with annexes, United Nations 2000. 2)
Feinendegen
L, Pollycove M, Biologic response to low measure of ionizing radiation:
detriment versus hormesis, J Nuclear Medicine, 42, 7, 17N-27N and 26N 37N,
2001. 3)
BEIR
V: Committee on the Biological Effects of Ionizing Radiation. Health effects of
exposure to low levels of ionizing radiations. National US Academy of Sciences,
National Research Council, Washington 1990. 4)
Nair
MK, Nambi KS, Amma NS, Gangadharan P, Jayalekshmi P, Jayadevan S, Cherian V,
Reghuram KN Population study in the high natural background radiation area of
Kerala, India. Radiat Res. 152, 145-148S, 1999 5)
Jaikrishnan
J'S and al, Genetic monitoring of the human population from high-level natural
radiation areas of Kerala on the southwest coast of India. Prevalence of
congenital malformations in newborns. Radiat Res 152, 149-153S, 1999.
6)
Cheryan
VD et al. Genetic monitoring of the human population from high level natural
radiation areas of Kerala on the southwest coast of India incidence of numerical
structural and chromosomal aberrations in the lymphocytes of newborns. Radiat
Res. 152, 154-158S, 1999. 7)
Tao
Z J Radiat Res (Tokyo) 41 Suppl:31-4, 2000. 8)
Wei
LX, Sugahara T. High background radiation area in china. J Rad. Research (Tokyo)
41, Suppl. 1-76, 2000). 9)
National
Council on Radiation Protection and Measurements Evaluation of the linear
non-threshold model for ionizing radiation NCRP-136, Bethesda 2001.
10)
Academy
of Sciences Problems associated with the effects of ionizing radiations at low
doses. Report 34, Oct 1995. 11)
Tanooka
H. Threshold
dose-response in radiation carcinogenesis: an approach from chronic
alpha-irradiation experiments and a review of non-tumour doses.
Int. J Radiat. Biol., 77, 541-551, 2001 12)
IARC
2000 Monographs on the evaluation of carcinogenic risks to humans, vol. 75,
Ionizing radiation - IARC, Lyon, France 13)
Academy
of Sciences Symposium Carcinogenic Risks due to ionizing radiation Report
Academy of Sciences, Series III, 322, 81-256, 1999 14)
Berrington
HAS. Darby Sc, Weiss HA, Doll R. 100 years of observation on British
radiologists mortality from cancer and other causes 1897-1997. British Journal
of Radiology, 74, 507-519, 2001 15)
Symposium
Warrenton: Bridging radiation policy and science (K.L. Mossman et al. ed) 2000
16)
IAEA,
executive summary Belarus, Ukrainian and Russian 2001: Health effects of the
Tchernobyl accident. 17)
Holm
LE (UNSCEAR Chairman) Chernobyl effects. Lancet, 356, 344, 2000
18)
European
Directive 97/43 on radiological examinations, 1997 |