CT Simulation

When you receive radiotherapy, treatment is aimed at the cancer and tries to completely exclude normal tissues. Thus it is very important to map out the tumour precisely before radiotherapy starts. This "mapping out" process is called "simulation".

Simulation usually occurs between 1 and 3 weeks prior to radiotherapy, and involves marking on the skin, pinhead sized tattoos placed on the skin, and taking of x-rays, CT scans, MRIs and PET scans to make sure we have identified both the cancer (which we want to treat) and the normal tissues (which we want to miss).

There is little doubt that for many patients, the best simulation is by using CT scans (i.e. "CT-simulation"). "Conventional simulation" is an old technique that uses plain x-rays to target the tumour and view normal tissues. Conventional simulation is unable to localize many types of cancer adequately: for example it is completely useless in seeing prostate, brain, head and neck and rectal cancers or lymph nodes, and is very poor at viewing lung cancer, and most normal tissues we are trying to avoid. CT-simulation on the other hand can see most of these tumours and normal tissues very well, and thus is really the ideal for most patients. Anyone about to have radiotherapy should ask whether they will receive CT-simulation before their treatment. You can see what a CT-simulator looks like here. It is very different to a conventional simulator as shown here.

Some patients need different types of simulation to ensure ideal treatment. For example, patients with non-small cell lung cancer (NSCLC) should have (and be offered) a PET scan as well as a CT as part of their simulation, and patients with prostate cancer should have (and be offered) both MRI and CT scans as part of their simulation.

Just as important as simulation, is doing imaging every day immediately before or during radiotherapy. Click here for more information.

Below is an image that shows what would happen to a patient with breast cancer if they do not have CT-simulation.

As can be seen, a significant amount of heart and lung would be treated. The exact amounts can only be seen with CT-simulation (which is what this image was taken from). In addition CT-simulation is the only way to know whether parts of the breast itself are receiving too much radiation. By doing CT-simulation we are better able to shield out heart and lung, and modify the radiation treatment to deliver an even dose of radiation through the breast. It is thought by many radiation oncologists that ensuring the dose is even will decrease the chance of breast side-effects from radiotherapy, such as swelling and pain. Another excellent way to minimize heart and lung radiation is prone breast radiotherapy.