EURATOM RESEARCH The unexplored territory of low-dose radiation
Radiation – the word immediately evokes the dreadful fallout of nuclear bombs or the aftermath of the Chernobyl disaster. Yet radiation is also a useful weapon in the therapeutic arsenal, for medical imaging as well as combating cancer. More subtly, it is also present in our everyday environment that is home to an increasing variety of ionising radiation of natural or technological origin – but which is not considered a danger as it is far below the acceptable safety levels. The real long-term effects of this background radiation are nevertheless a mystery, which is why they are currently being investigated by the Risc-Rad project.
According to the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the natural rays from the Sun and the Earth transmit about 2.4 miliSieverts(1) (mSv) to each individual every year. Human activities expose us to an additional radiation dose, especially the techniques used in non-invasive medical imaging (radiography, CT scanners) that are becoming increasingly common in the industrialised countries. UNSCEAR estimates that this accounts for an average dose per individual of 1.2 mSv/year.
The present impact of Chernobyl, atmospheric nuclear tests and electricity generation by nuclear plants account for very minute proportions, just 0.002, 0.005 and 0.002 mSv respectively. Therefore, this combined radiation remains very low compared with the 100 mSv threshold below which no carcinogenic effects have been established.
Environment, occupation, genetics The biological effects of frequent exposure to low-dose radiation is nevertheless unknown territory as it is not possible to measure the direct effects on health. This is because they are too low to be distinguished among the percentage of cancers present among the population in any event. On the other hand, some populations are more exposed than others, depending on where they live – some granite rocks have a natural concentration in radioactive substances that is up to 100 times the average – or their occupation. Workers in the nuclear industry, laboratory researchers, airline navigators, astronauts, medical staff and other professions are all exposed to higher doses of radiation. Scientists have also noted that the body’s sensitivity to radiation varies according to the individual, as does sensitivity to the sun’s ultraviolet rays. These differences could come from a particular configuration of certain genes that result in a predisposition to radiation-induced cancers.
The hypotheses formulated by scientists come up against one obstacle: our lack of understanding of the connection between the immediate effects of radiation – damage to the DNA strands – and the long-term effects – the development of cancers – that is a long and complex process. This missing link in our knowledge is the subject of research by the Risc-Rad European project.
Laure Sabatier, the Project Coordinator, believes that "an in-depth understanding of the mechanisms of radio-induced cancerogenesis is needed before you can identify individual variation in the body’s response to radiation”. While this variation plays a marginal role in the case of high radiation doses, it could be determining for the response to low-dose radiation and needs to be taken into account in quantifying the associated risk. This approach could ultimately compensate for the limits of epidemiological studies on which present radioprotection norms are based.
Margot Tirmarche, of the French Institut de Radioprotection et de Sûreté Nucléaire, has participated in a wide-ranging epidemiological study carried out by the International Cancer Centre among workers in the nuclear industry, the results of which were published in the British Medical Journal in June. She explains that “the results of this study, carried out on a population for which we know the precise doses received, show a relationship of proportionality between the accumulation of the doses and the increased risk. Genetic factors no doubt come into play when one studies the dose/effect distribution in an epidemiological study, but they are mixed with exposure to other toxic agents. At the present time, molecular biology specialists are not able to tell which genetic factors must be isolated.”
The challenge for Risc-Rad is to make up for this deficiency and identify the candidate genes that play a role in susceptibility to developing a radio-induced cancer, and to test their validity on animal models. “Once we have found these genes, we can then search in the population for corresponding polymorphisms, that is, different variants of these genes,” explains Laure Sabatier.
The disciplinary ‘melting pot’ To achieve this ambitious objective, scientists with different specialities and from different cultures must work together: Spanish radiologists, Italian cancerologists, German modellers, etc. The project management thus becomes vital to its success, as does the scientific quality of the teams. “As an Integrated Project, Risc-Rad is distinctive for its size and diversity of subjects studied. It must also be flexible. Its objectives and strategy are discussed every year when defining the new implementation plan,” stresses the Project Coordinator. The challenge for the Risc-Rad management is to associate progress in cancer research with science’s participation in the social issue of individual risk linked to low doses.
(1) The Sievert, a unit that expresses the effects of radiation on the living organism, makes it possible to calculate the effective dose received by an individual by taking into account the difference in aggressivity between different radiation types and the radiosensitivity of organs.
The Risc-Rad project
Risc-Rad (Radiosensitivity of Individuals and Susceptibility to Cancer Induced by Ionising Radiations) was launched in January 2004 for a four-year period. With EU financing of €10 million under the Sixth Framework Programme’s Euratom activities, 33 laboratories from 11 European countries ...
Even at low doses, ionising radiation, in the form of rays (X-rays or gamma rays) or particles (alpha and beta), is penetrating radiation that is able to remove electrons from the atoms or molecules present in the environment through which it travels and thereby transform them into electrically charged ...
Risc-Rad (Radiosensitivity of Individuals and Susceptibility to Cancer Induced by Ionising Radiations) was launched in January 2004 for a four-year period. With EU financing of €10 million under the Sixth Framework Programme’s Euratom activities, 33 laboratories from 11 European countries are participating in the project, with a total of 380 researchers helping to understand radiation-induced cancerogenis. Six research themes are being studied: the immediate effects of radiation at the molecular level, repair mechanisms, the transmission of radio-induced damage, cancerogenesis, genetic factors for susceptibility to radio-induced cancers, and mathematical modellings of the quantification of the risk linked to low-dose radiation.
Even at low doses, ionising radiation, in the form of rays (X-rays or gamma rays) or particles (alpha and beta), is penetrating radiation that is able to remove electrons from the atoms or molecules present in the environment through which it travels and thereby transform them into electrically charged ions. It acts on body cells by causing lesions in the DNA, the conveyor of genetic information. As a result, the irradiated cells either die – a property that is useful when treating cancers with radiotherapy – or specific biological mechanisms repair the DNA lesions. These repairs are not exempt from errors, however. In the long term, the transmission of this damage to the descendants of the irradiated cells and the increase in chromosomatic aberrations can lead to the development of cancers. Radiation can therefore be beneficial or harmful, which is why its use requires a precise knowledge of the effects.