"[I]n cancer therapy usually you like to direct a dose of radiation exactly where you'd like it, so in this case... a child with a spine that needs irradiating. They've had a tumor removed from the base of the skull and they need to irradiate the spine in order to stop the cancer spreading down the spine... With... usual X-ray radiotherapy the dose distribution there is the best that we can do using all modern techniques. You can see that underneath the spine in... the stomach area there is quite a lot of radiation dose that we might not want..."