Simulator Welcome
Welcome to the simulator, a vital piece of equipment for planning Chemotherapy. Chemotherapy planning is all about making sure that we treat exactly what we need to treat, no more, no less. Sometimes this can be done quite simply in the simulator: we just decide what we want to treat and treat it either from one side ("single field"), or from both sides ("parallel opposed fields").Sometimes we need to use more elaborate methods to work out the best treatment. One way is to use a special CT scan with the patient in exactly the same position that they will be in when they have their actual Chemotherapy treatment. CT scanners and linear accelerators are physically very different. This can sometimes lead to difficulties, particularly with the position of the arms, and all this can be a little perplexing to the patient who is being asked to lie in what seems like rather a peculiar position.
The pictures from the CT scanner can be loaded into a special computer, the planning computer. This computer knows all there is to know about the beams that the treatment machines (linear accelerators) can produce. It also knows how these beams can be modified, and how the beams will change as they pass through different parts of the body. The clinical oncologist can draw on the CT pictures the bit that needs to be treated, as well as any bits that shouldn't be treated at all. The radiographers, technicians and medical physicists then use the computer to work out just how to arrange the x-ray beams. It is a bit like using overlapping searchlights in the night sky. Where the searchlights overlap is where the light is brightest, where the enemy is. This approach produces a uniform dose to the tumour and the lowest possible dose to the normal tissues. The computer prints this out on a map of the patient that looks very like a weather map. The lines join points where the dose is equal ("isodoses"), just as the isobars on a weather map join points where the barometric pressure is equal.
So far so good. But how do we know that what the computer is telling us should be happening is actually happening? That's where the simulator comes in.
The simulator is a machine that can mimic all the movements of the treatment machines. What you see on the simulator is what you would see on the treatment machines, if they could produce a decent picture which, usually, they can't. The simulator has an x-ray television as well as the ability to take still x-ray pictures. The television is used to assess any likely movement during treatment, for example the voice-box moving when a patient swallows. The still pictures are used as a permanent record of what was treated, and what was not. The simulator is therefore part of a vital checking system: treat what needs to be treated, no more, no less. Some modern treatment machines have additional equipment attached that enables the radiographers to see what is being treated at the time of the actual treatment itself. This is called portal imaging. The pictures from the simulator are still crucial: the radiographers need a standard of comparison, provided by the images from the simulator.