To ensure that the cancer is being treated (and that we are missing tissues that do not need radiotherapy), we need to do checks every day before a patient receives radiotherapy. The reason is that patients can never be set up in precisiely the same position every day, and even if this was possible, organs and cancers move around inside the body. Two great examples are breast and prostate cancer.
The prostate moves every day, often by up to a centimetre or more. This is as a result of variations in the filling of the bladder and rectum, which cannot be controlled despite attempts to standardize the amount of usrine in the bladder or size of the rectum each day.
A great example is shown below. This is a cone beam image taken in a patient of Prof Shakespeare who was having curative prostate radiotherapy. It shows that the prostate moved by about half a centimetre compared to what it was on the day of simulation. This means that if the patient had been treated without the cone beam, 5mm of the target (i.e. the prostate cancer) would have been completely missed, and an extra 5mm of rectum would have been unneccesarily treated (possibly causing worse side-effects).
By doing the cone beam, we are able to check where the prostate is, and if necessary move the radiation treatment field to make sure it is actually being treated. Thus doing daily checks of where the prostate is would seem to be a necessity. However many patients do not have daily imaging, and those few who do have daily imaging usually do not have cone beam, BAT ultrasound or fiducial markers (which are the main ways to check where the prostate is). Patients who just have plain x-rays (port films or EPIs) on the linac are not having the prostate position checked. These simple methods only check that the bones are in the right place. But the prostate moves independent of bones, and thus checking the bones is really not a great method to use. Cone beam radiotherapy should be used with CT-simulation (and for prostate, brain and head and neck cancers, CT-MRI planning), as well as 3D conformal radiotherapy planning.
Patients having breast, rectal, lung (and really all types of cancers) being treated with radiotherapy also benefit from daily imaging, as patients are rarly set-up in precisely the same way every day. For many of these tumours, a plain x-ray is sufficient to check the tumour position every day. These plain x-rays are called EPIs (electronic portal images) or port films. They are ideally done every single day, not just once a week or once only. After all, who knows what is being treated on the days they are not done?
