NUCLEAR ACCIDENTS AND CANCER
The most extreme example of exposure to artificial radiation came, of course, from the atomic bombs dropped on Hiroshima and Nagasaki in 1945. These caused immediately about 120,000 deaths from the explosions together with the extremely high levels of nuclear radiation released in the form of gamma rays and (fast) neutrons. Although this killed almost everyone within one kilometre of the epicentre, the radiation level subsequently fell so rapidly that within one week it had returned to normal. This dramatic fall may in part account for an unexpected long-term consequence of the atomic bombings, namely that in those who survived because they were more distant from the epicentre but had been transiently exposed to high levels of radiation, the overall incidence of radiation-induced cancers was very small.
Of 48,000 atomic bomb survivors who were exposed to at least 5 mSv, some 5,900 died between 1950 and 2000 as a result of developing solid tumours but of these only 480 (~8%) were attributable to radiation. Furthermore, within the exposed group of survivors, there has not appeared significant evidence for mutations caused by radiation being passed to offspring (that is, for the acquisition of germline mutations). A much higher percentage of cancers of the blood (leukaemias: Fig. 1) was attributable to radiation (~50%) but the number of such cancers was very small (<4% of the total). All of which suggests that although transient exposure to high levels of radiation may cause mutations, the body is quite good at repairing the damage.
1. Leukaemic cells
The worst nuclear accident so far has been the explosion of a reactor at the Chernobyl power plant in 1986, which blew a large amount of radioactive material into the air – 1.1 × 1019 Bq in fact, some 400 times that released by the Japanese bombs. Although 60% fell on Belarus, radioactivity from the cloud was subsequently detected in most countries of Northern Europe and as far away as North America. To this day sheep from some 330 farms in North Wales have to be scanned for radiation before they can be dispatched to market. The main products of nuclear fission released were caesium-137 (half-life 30 years), iodine-131 (half-life 8 days) and strontium- 90 (half-life 29 years). These decay by beta and gamma emissions and thus constituted a health hazard to humans on the receiving end of the fall-out. There’s a degree of irony in this because all have been used in medicine: iodine and strontium isotopes specifically in the treatment of cancer. Iodine is taken up by the thyroid gland (and used to make hormones, e.g. thyroxine): because thyroid cells are the only ones in the body that can absorb iodine, patients with thyroid cancer are given radioactive iodine orally after surgical removal of the gland to kill any residual cells and hence ensure the cancer does not recur (they require hormone tablets thereafter to compensate for the loss of their thyroid). The problem with the aftermath of Chernobyl was the uncontrolled uptake of iodine-131 that caused thyroid tumours to develop in a large number of young children (Figs. 2 and3).
2.The thyroid gland.
3. A tissue section of a normal and an enlarged thyroid gland (a mouse goitre).
The comprehensive reports (Chernobyl Forum) on the accident found that just over 50 people were killed directly and that by 2005, of the ~600,000 people most exposed to fall-out, there had been some 4,000 cancer deaths attributable to radiation, most of these being thyroid tumours. These reports also conclude that there has been no increase in inherited birth defects or in other types of solid cancers as a consequence of the fall-out, a result consistent with the Japanese atomic bomb data. They are also consistent with studies of wildlife in the vicinity of the reactor, specifically comparing populations of voles with those in uncontaminated areas. These showed that a lifetime’s exposure to about 10 msec of radiation a day caused no detectable DNA damage in the animals. The simplest explanation for these somewhat surprising findings is that exposure to low levels of radiation may activate DNA repair mechanisms and thus confer protection.