MOST cases of the commonest type of childhood thyroid cancer could be caused by low levels of natural radiation from rocks and cosmic rays, say two European scientists. Their finding strengthens the idea that even the tiniest doses of radiation, including those that come naturally from the environment, can be dangerous.
Some in the US and France remain sceptical of this notion, arguing that low-level radiation is safe, and may even benefit health by stimulating DNA repair mechanisms. But the Scandinavian study has concluded for the first time that hundreds of cases of thyroid cancer among children in Europe could be due to natural radiation. 鈥淥ur results support the position that all radiation exposures, no matter how small, are potentially damaging to health,鈥� says Keith Baverstock, from the World Health Organization鈥檚 European Centre for Environment and Health in Bonn, Germany. He conducted the study with P盲ivi Kurttio from the Radiation and Nuclear Safety Authority in Helsinki, Finland.
Baverstock and Kurttio came to their conclusion after examining data on cases of papillary thyroid cancer, which accounts for two-thirds of thyroid cancer cases in children. Radiation doses from the 1986 accident at the Chernobyl nuclear plant in Ukraine are known to have caused at least 2000 cases of the disease. Using this information, along with data on medical X-rays given to children, the researchers calculated how many thyroid cancers they would expect to be caused by 9 milligrays of natural radiation.
Advertisement
They chose 9 milligrays because it is the average amount of radiation that each European child鈥檚 thyroid is exposed to during the first nine years of their life. The reason under-nines are important is that the researchers can be reasonably sure they have only been exposed to natural radiation. Children this young are rarely given dental X-rays, says Baverstock, and there is no other known cause of childhood thyroid cancer bar ingesting abnormal amounts of iodine.
The dose was enough, they estimated, to cause papillary thyroid cancer in one or two children out of every million each year. That matches the average incidence of the disease seen in children under the age of 15 in Finland, Norway, Sweden and Denmark, countries which have some of the best data on thyroid cancer in the world.
Other radiation specialists agree that Baverstock and Kurttio鈥檚 results should be taken seriously. But they doubt their implication that natural radiation can be a cause of almost every case of papillary thyroid cancer. An earlier study using data from A-bomb survivors by Mark Little, a medical statistician at Imperial College, London, suggests that just 6 to 16 per cent of cases are due to natural radiation, although the other causes are unclear. He says that Baverstock and Kurttio may be overstating the proportion of cases caused by natural radiation, but he acknowledges that there are also uncertainties in his own calculations.
Baverstock and Kurttio are writing up their results for a journal, having presented them to a nuclear industry conference in Oxford last year. Given the hazards of natural radiation, Baverstock argues that it is vital to minimise any additional exposure from medical treatments or the nuclear industry. He is particularly concerned about dental X-rays, which might deliver a significant dose to a child鈥檚 thyroid. 鈥淭he experience after Chernobyl showed that childhood thyroid cancer is a very aggressive disease which could be fatal if not treated promptly,鈥� he says.