Question: how do x-rays harm the body to increase your chances of getting cancer?

Susan Cartwright answered on 20 Jun 2015:

X-rays are a class of “ionising radiation” – that is, they carry enough energy to knock electrons out of atoms. Doing this can damage the long molecules that living systems use, including DNA. Studies have shown that exposure to X-rays causes various kinds of damage to DNA, including damaging the four “bases” A, C, G and T that actually spell out the genes, causing cross-links between the DNA strands that shouldn’t be there, and breaking one or both of the strands of the DNA helix.

Damage to DNA is a fact of life, and cells have evolved mechanisms to try to repair damaged DNA. This is not too hard if only one strand is affected. However, if both strands have broken, the task becomes more difficult, and if there is more than one chromosome with both strands broken, there is a real chance that they will be put back together incorrectly, perhaps with the tail of one chromosome attached to the head of another. In general, the processes that fix double-strand breaks in mammals like us aren’t very good: they often make mistakes, which can have dangerous consequences.

Radiation damage to DNA appears to be pretty random: it doesn’t target specific areas or hit some genes more often than others. So, even if the damage is not repaired correctly, it may not cause cancer. There is also evidence that some radiation-induced cancers are not caused directly by the initial DNA damage, but are later consequences of the overall damage to the cell, leading to problems with its metabolism and behaviour from which it never quite recovers, and which cause increased likelihood of cancer-causing mutations further down the line.

In short, high doses of X-rays definitely do lead to increased cancer risk, and the main pathway to this seems to be DNA damage, expecially badly repaired double-strand breaks, but the actual mechanisms by which this damage leads to cancer are complicated, and there are several different ways it can happen.

Of course, the cancer risks associated with X-rays are well known, and the exposures you get when you need an X-ray in hospital are calculated with that in mind. Hospital X-rays do much more good than harm – a tiny increase in your lifetime cancer risk is much to be preferred to lifetime disability caused by not diagnosing and treating a broken bone!

Note that at high doses, X-rays don’t merely damage cells – they kill them. This, paradoxically, although X-rays can cause cancer, they are also used in cancer treatment: cancer radiotherapy is high doses of high-energy X-rays, directed specifically at the cancer, with the aim of killing it while minimising (though not eradicating) the risks to surrounding tissue. Patients who have undergone radiotherapy do have an increased risk of a different, unrelated, cancer arising in the part of the body that was exposed to the X-rays – but, since the original cancer would have killed them if it hadn’t been treated, most patients accept this as a risk worth taking.

Interestingly (well, I think it’s interesting, anyway), not everything is as susceptible to radiation damage as we are. The bacterium Deinococcus radiodurans (sometimes nicknamed Conan the Bacterium!) is an incredibly tough organism, capable of surviving without significant damage a radiation dose ONE THOUSAND TIMES the lethal dose for humans. D. radiodurans has backup copies of its genome and super-efficient DNA repair processes – it’s particularly good at repairing the double-strand breaks that cause problems for our repair mechanisms. Biologists are pretty confident that D. radiodurans did not evolve these mechanisms specifically to deal with radiation – there isn’t much call for radiation resistance in the natural world – but instead to deal with DNA damage caused by drying out (desiccation) – it lives in soil, and is at risk of being dried out in its natural environment. There is another, more recently discovered, bacterium (technically, an archaean, but they are very similar) called Thermococcus gammatolerans which is equally radiation resistant: it lives in the very hot water around deep-sea volcanic vents, and probably evolved its radiation resistance as a side-effect of surviving high temperatures (its preferred operating temperature is 88 C!).

So, even if we had a massive nuclear war and made the whole of the Earth’s surface radioactive, some living things would survive!