One of the fascinating things about cancer is that it touches every aspect of biology. Of course, most will know that it’s caused by mutations – changes in the material that carries our genetic code. But many influences play on the genetic keyboard of DNA and those that are part of the world around us are a very mixed bunch. In Betrayed by Nature I split them into two: those we can do something about and the rest. The latter includes radiation from the ground… it’s all around us, we’ve evolved bathed in it and, apart from not living where the levels of radon are particularly high, there’s nothing we can do about it – so just forget it.
At the other end of the spectrum, so to speak, comes sunshine. We’ve evolved with that too – indeed we wouldn’t be here without it. Aside from driving photosynthesis in plants, humans use the radiant energy of the sun to make vitamin D (sometimes called the “sunshine vitamin”). Vitamin D deficiency is one cause of the childhood bone defect rickets, a condition that has reappeared in the UK in recent years because some kids are seeing less of the sun. So for humans catching the rays is desirable but we teeter along a sunny tightrope between what we need and what may ultimately be fatal. The risk comes from the ultra violet component of sunlight – radiation that has sufficient energy to damage DNA directly, making it a mutagen that can cause cancer. The cancer in question is, of course, melanoma that develops from abnormal moles on the skin. The global incidence of melanoma is increasing and, in the UK, about 90% of cases are estimated to be linked to exposure to ultra violet light. To most folk this means sunshine but those so inclined can walk the tightrope horizontally by using sunbeds (incredibly, in 1999 Cancer Research UK found that a quarter of men and a third of women questioned said they’d used a gizmo of this sort in the previous six months).
Which goes to show that human beings seem unable to resist the pursuit of the unattainable. The fair skinned think it cool to be darker whilst pharmaceutical giants are apparently making pots from selling creams to Indian ladies on the pitch that they will lighten their skin!
With a sigh for humanity let us pass from risks we take for no reason other than vanity or stupidity to those we may feel obliged to take as the lesser of two awkward options. There’s almost no chance that anyone reading this hasn’t had an X-ray of some sort. We have them to give our dentist a precise guide to the cause of our agony, rather than have him solve the problem by a series of trial and error excavations, or to tell our orthopaedic surgeon how best to go about piecing together the results of our latest stress-test on the human frame. We know X-rays are bad for us – they’re even more energetic than ultra violet radiation, so they’re a super-mutagen. Waves of cancer you might say.
So the issue here is one of choice. It’s a bit like a general anaesthetic: they do tend to make you throw up and about one in every 100,000 is fatal but, confronted with surgery, which would you vote for: a whiff of halothane or the offer of a slug of whisky and a rag to bite on? Computed tomography (CT) is an alternative application of X-rays but, instead of a single shot giving a two-dimensional image, CT acquires a large number of such images, taken as the radiation beam moves through the body, to give a 3-D picture. This can represent whole organs, and it has become an immensely powerful diagnostic tool since its introduction in the early 1970s. However, there’s no advance without anguish, and the additional information provided by a CT scan requires much more radiation than a traditional X-ray (typically 10 millisievert (mSv) compared with about 0.04 mSv for a chest X-ray). As our annual dose of “unavoidable” natural radiation is about 3 mSv it’s probably safe to say that these medical exposures are not a serious hazard – although babies in the womb are particularly sensitive to radiation. Even so, there are estimates that about 1% of USA cancers are due to CT scans, although there is no evidence that doses below 100 mSv induce tumours in animals.
A new study has enlarged the picture by finding that CT scans of children under 15 may increase the risk of leukemia and brain cancer. Three-fold increases were estimated for acute lymphoblastic leukaemia as a result of five to ten scans and for brain tumours by two or three scans. This sounds somewhat scary but it’s worth noting that these diseases are very rare in children. In the UK the incidence in under-20 year olds is just over four per 100,000 of leukemia – slightly less for brain or central nervous system cancers.
So the evidence indicates a small increase in an already low level of risk. As ever in life, therefore, it’s a matter of balance. The sensible advice for children (and everyone else for that matter) is not to have CT scans unless they are likely to provide critical clinical information that cannot be obtained by other means, for example, ultrasound or conventional X-rays.
Pearce, M.S., Salotti, J.A., Little, M.P., McHugh, K., Lee, C., Kim, K.P., Howe, N.L., Ronckers, C.M., Rajaraman, P., Craft, A.W., Parker, L. and de González, A.B. (2012). Radiation exposure from CT scans in childhood and subsequent risk of leukaemia and brain tumours: a retrospective cohort study. The Lancet 380, 499–505.