IONISING RADIATION AND CANCER
Ionising radiation and ultraviolet (UV) radiation are important because they can damage living tissues, most significantly by causing mutations in DNA that can lead to cancer. We have evolved against a background of natural radioactivity that comes from radioactive elements in the earth and rocks, trace amounts of which are also present in food, water, and in radiation from space. On average, about 87% of the ionising radiation that hits humans comes in these unavoidable forms. The rest is artificial radiation of one sort or another, including X-rays used in medicine.
By way of reassurance before confronting the more extreme forms of radiation, it is instructive to spend a moment considering potassium, which is important for generating the electrical signals relayed by nerve cells (an action potential).
Potassium is the major radioactive emitter in our bodies and there are three forms (isotopes) that occur naturally: 39K and 41K (together comprising over 99.9%) and 40K. The first two are stable but 40K is radioactive and has a half-life of 1.3 billion years – so it’s a good job that it only makes up 0.012% of our potassium. We can’t avoid it because it’s in the earth – though we increase the amount we eat by using fertilisers that contain nitrogen, phosphorus and potassium, which adds several thousand curies a year to US soil. This means that the fruit and vegetables considered essential in a good, cancer-protecting diet contain lots of potassium, including the radioactive fraction. Because cows eat grass, if you drink milk you will be downing about 74 Bq in every litre (or 2 × 10−9 Ci).
Radiation exposure can also occur in diagnostic medical procedures – mainly X-rays and computed tomography (CT). In a conventional X-ray the radiation is directed at a part of the body and what passes straight through is collected on a photographic film, which when developed gives a two-dimensional image of the sort familiar to almost anyone who has ever broken a bone. Computed tomography also uses X-rays to acquire two-dimensional images but, from a large number of such images taken as the radiation beam moves through the body, a three-dimensional picture can be pieced together.
The biological damage caused by radiation is measured in units called the sievert (Sv). A typical chest X-ray requires a dose of ~0.04 mSv; for a corresponding CT scan, the figure is 8 mSv (Table 1).
These might be compared with our annual dose of ‘unavoidable’ natural radiation, which is about 3 mSv. So, in general, it is probably safe to say that these typical medical exposures are not going to have harmful effects. Nevertheless, we should bear in mind that some conditions may require multiple scans. Currently, in the USA there are about one million CT scans and worldwide about two billion medical exposures every year.
Radiation is, of course, one form of cancer therapy, the strategy is to target and kill the cells of a tumour. For this, much higher exposures are required, which has driven the development of increasingly sophisticated machines that can target precisely the contours of a tumour.
The first major case of accidental exposure to radiation came in the years following the First World War as a result of the commercial use of radium to make luminous dials for watches and instruments. Women employed by the US Radium Corporation to apply the radium-containing paint were instructed to lick their brushes to maintain their shape (of the brushes, obviously). As the dial painters began to fall ill and indeed die it became clear that the cause lay in their working conditions. Eventually five ‘radium girls’ sued US Radium. The high-profile case focused public attention on the hazards associated with radium, a given that the element had been discovered as long before as 1898. It was, in fact, radium radiation that was to kill one of its discoverers, Marie Curie, in 1934. Radium (226Ra) is a decay product of uranium, has a half-life of 1,602 years, and decays to a number of short-lived products, one of which is the gas radon. Despite all this, and quite amazingly, radium was in the USA in the early 1900s as a treatment for everything from high blood pressure to stomach cancer, and only ceased to be sold in 1931.