LISBON, Portugal -- Chernobyl has made medical history, accounting for the largest group of human cancers associated with a known cause on a known date, ECCO 11, the European Cancer Conference heard in Lisbon today.
Nearly 2,000 cases of thyroid cancer have been linked to the world's worst nuclear accident which occurred in Ukraine on April 26, 1986, and the number is still rising, according to some of the world's most prestigious cancer researchers.
Professor Dillwyn Williams, of The Strangeways Research Laboratory, Cambridge University, told the meeting, "Four years after the accident, an excess of thyroid cancers was noted among children who had been exposed to fall-out from the disaster. That increase has continued and new cases are still being seen in those who were children at the time of the accident."
Dr. Elaine Ron, of the U.S. National Cancer Institute, in Bethesda, Maryland, explained, "Following external radiation exposure, the elevated risk of thyroid cancer appears to continue throughout life, but there is some indication that the risk may be highest 15 to 19 years after exposure."
External radiation is the only well established cause of cancer of the thyroid gland. People under 20 are at a increased risk of thyroid cancer after exposure to isotopes of iodine.
Professor Williams said, "Exposure to isotopes of iodine gives the thyroid over a 1,000 times the average dose to the rest of the body. The particular sensitivity of children to thyoid cancer after radiation exposure can be linked to a combination of a higher thyroid dose and the biology of thyroid growth which falls to a very low level in adult life. Few of the patients with thyroid cancer have died, but help is still needed."
The United Nations marked the 15th anniversary of the disaster with an appeal for aid for the victims of radiation. According to one report, five million people in the former Soviet Union were exposed to radiation or other health hazards by the Chernobyl catastrophe.
On April 25, 1986, the reactor crew at Chernobyl-4 disabled automatic shutdown mechanisms before an attempted test of the unit the next day. The test was intended to determine how long turbines would spin and supply power following a loss of main electrical power supply.
During the test, as flow of coolant water to the reactor was reduced, power output increased. When the operator moved to shut down the reactor from an unstable condition arising from previous errors, a power surge took place.
The nuclear fuel elements ruptured, and the resulting explosive force of steam lifted off the cover plate of the reactor, releasing radioactivity into the atmosphere. A second explosion threw out fragments of burning fuel and graphite from the reactpr core and allowed air to rush in, causing the graphite moderator to burst into flames.
The graphite burned for nine days, causing the main release of radioactivity into the environment.
Although only 31 people died in the immediate aftermath of the accident, hundreds of thousands were reported to have abandoned entire cities and settlements within the 30 kilometre (20 mile) zone of extreme contamination.
Dr. Williams said, "The effects of Chernobyl differed very greatly from those after the atomic bomb explosions. In Japan, the exposure was very largely to whole body radiation from gamma rays and neutrons. After Chernobyl the exposure was to isotopes in fall-out, and apart from the inert gas xenon, the largest components were radioactive isotopes of iodine."
Post Chernobyl cancer risks are not restricted to the thyroid gland, the meeting was told. Victor Chizhikov, of the Cancer Research Center, Kashirskoye, Moscow, reported that a study of former 43 Chernobyl clean-up workers had shown them to be at a significantly increased risk of lung cancer. All of the 36 smokers and seven non-smokers in the study group had evidence of inhaled radioactive dust in their lungs. They were compared to a control group of 21 smokers and 23 non-smokers who had never been exposed to radiation.
ECCO, the European Cancer Conference, is one of the world's major multidisciplinary cancer conferences, providing a platform for interaction and exchange between experimental and clinical oncologists and cancer nurses.
The conference is organized every two years by the Federation of European Cancer Societies for and on behalf of its six member societies.