Radioactivity and medicine

A lifetime of exposure to high levels of radioactivity eventually killed Marie Curie. Her notebooks are still too radioactive to handle safely. But radioactivity is used routinely and very effectively in medicine to cure disease. So how does that work? Chris Smith was joined by cancer specialist Tom Roques, from the Norfolk and Norwich University Hospital...

Tom - So I guess you could think of radioactivity as lots of tiny bullets of energy which can go through the body's tissues. And they can damage the DNA, the life code inside cells. If they damage them a lot, that cell can die, and we use that in medicine with radiotherapy. But if the damage is more minor then you can get mutations in the DNA and those mutations over time can add up and cause cancer.

Chris - And are any particular parts of the body vulnerable? Or is there any part of the body susceptible to this happening?

Tom - So the parts of the body that are more susceptible to damage from radiation are the parts where the cells are growing quickly. So again, with the radiotherapy that we use, the tissues that tend to have more side effects from radiation are things like your mouth and your gut, where the cells are growing and dividing all the time; rather than parts like your brain, which tend to be fairly stable.

Chris - And so it's the fact that cells which are dividing are susceptible to being damaged by the radiation: that's why you can actually turn this around and say, rather than worrying about it causing cancer, if someone's already got cancer, you can use the vulnerability of fast growing cells to hit them with a dose of radiation and kill them?

Tom - We use the fact that cancer cells tend to be growing more quickly than normal cells. So in normal cells, the DNA is packaged away very safely and tightly inside the nucleus of the cell, so it can't be hit by these bullets of energy that the radiotherapy gives. Whereas cancer cells, their DNA is outside being divided and grown, and that makes them more sensitive to the radiation.

Chris - When you use radiation therapeutically in this way, what sort of radioactivity do you use? What's the source?

Tom - So I guess that's the other difference between now and the radiation that the Curies were using. They were using naturally occurring radiation, which you can't turn on and off. Now in hospitals we use a machine called a linear accelerator, which is about two million pounds worth of high-technology kit with a generator of particles that hits a tungsten target, and its that that produces the energy of radiotherapy; but you can turn it on and off. So when the machine is off, it's perfectly safe to go into the room, and when it's on, only the person in the line of the beam is actually affected by the radiation.

Chris - So you zap a person's cancer, if you know where it is, with a dose of radiotherapy to destroy the cells, hopefully. But why doesn't that radiation then damage more healthy tissue and cause the person to have more new cancers?

Tom - So it does damage that healthy tissue, but to a lesser extent. And over the years, doctors have learned ways of minimising the damage to the normal parts of the body, while maximising the damage to the cancer. So for example, most of the time when we're trying to cure cancer, we give radiotherapy in lots of little repeated doses over a number of weeks, with the hope that the normal cells are better at repairing damage to their DNA and so can recover overnight, if you like, whereas the cancer cells tend to die more easily.

Chris - Is the same true therefore when we use a degree of radiation exposure to do imaging? I'm thinking things like X-rays and CT scans, we're using a dose that's useful to us in terms of imaging the person but not sufficiently large that we're going to cause a big increase in risk of them actually getting a cancer.

Tom - Yeah, so the energy that we use for imaging is much less than for radiotherapy because you're not trying to kill any of the cells or to damage them at all. So the energies that we use are much less, but every dose of radiation we gave - whether that's an X-ray or a CT scan - does give some radiation, which in theory could cause damage at a very low level in cells. So doctors always try to minimise the dose of radiation that we need and not put people through scans that they don't need.

Chris - And how else can we actually use radiation in order to image? Because obviously people will be familiar with things like a chest X-ray, or going in a CT scanner, the thing that looks a bit like a doughnut; but what other measures and mechanisms have doctors like you got at your disposal?

Tom - So I guess the most exciting change in the last 10 years or so has been things like PET scans. So a CT scan will give you a 3D picture of the inside of your body in different shades of grey, but it doesn't tell you what the tissues inside the body are doing. We use the fat that cells that are growing and dividing need sugars like glucose in order to grow, and if you give someone some glucose and make it radioactive, you can detect where that glucose is going, and show up in different colours where areas of cancer are. So that enables us to find cancer where we couldn't otherwise see it.