ec2-3-15-229-113.us-east-2.compute.amazonaws.com | ToothyWiki | RecentChanges | Login | Webcomic So people always ask me what it is I do, and as I really should be doing it right now but can seem to stop Wikiing, I figure I'd might as well write about it. --Jwanders
Most people have some idea what cancer is: a bunch of cells that start growing out of control and cause problems. As high energy radiation kills tumour cells, radiotherapy is one of a number of way to stop a spreading cancer. Of course, radiation doesn't do nice things to normal, healthy tissue either, so the goal is to get as much radiation as possible to the tumour while giving as little as possible to the healthy tissue.
The problem is that the tumour is usually embedded inside healthy tissue. If we send a beam of radiation straight through the patient, will give the same radiation dose to both the tumour and all the healthy tissue that happens to lie along the beam. We can improve on this by using two beams: if we send in two half-strength beams from different directions, the volume in which they overlap will get a full dose while the normal tissue where they don't will only see half. This can be (and is) extented to higher numbers of beams, with 5 or 7 being the most common, although experiments are being done using full spiral treatments.
The other issue involves the shape of each beam. Predictably, tumours don't grow in nice rectangles, so if we send in a rectangular beam, we'll be unecessary irradiating normal tissue along the beam corners. The solution is to make the shape of each beam match the shape of the tumour from the direction the beam is coming from. This isn't as easy as it sounds, as stopping high-energy radiation requires a good ten-centimetres of strong metal shielding, and getting strong metal shielding to conform to a arbitrary shape takes some doing.
The tool we use to do it is called a multi-leaf collimator (or MLC, if I get tired of typing). It consists of a bunch of metal "leaves": thin sheets of metal wide enough to shield any radiation coming from the side. Two of these leaves placed end to end form a row of the MLC. They're motorized such that they can move towards or away from each other, allowing any amount of space between them. When you stack 40 or so of these leaf pairs next to each other, you can move the leaves such that nearly any opening is created in the centre.
If you want to be really fancy, you can even deliver a beam from the same direction more than once, using a different shape for each. So, if you wanted to make a beam with a big circle at a moderate dose surrounding a smaller circle at a high dose, you would:
1) Send in one beam at a moderate intensity shaped to the big circle, and then
2) Send in a second beam shaped to the smaller circle, with intensity equal to the difference between the target high and moderate doses.
You can, of course, get much, much more complicated than that, and treatments can involve as many as twenty different shapes and intensities for each of the 5 or 7 beams.
Maybe I'm being dumb - but why can't you use very thin beams and move them over the desired area instead of trying to shape a beam? --Vitenka
No, good question. You can do that, in fact, but it would (1) take a lot longer and (2) produce a higher level of ambient radiation to the whole patient. Despite this, there are people working on something called a CyberKnife?, which can shoot a thin beam of radiation from any point in any direction. --JW
Xarak worked as a data entry guy for CancerResearchUK. Hardly as glamorous, but it was pretty interesting getting to see what everyone else eats and drinks every day ( the study we were working on involved people filling in diaries of everything they consumed for a week. ) If you're ever in doubt that Britain runs on tea, believe me, it's true - many people drink nothing but. It was also this experience that made me decide to go on a diet. Nothing like peering into other people's lives to make you examine your own ^_^
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