Radiotherapy cures more people of cancer than any drug.
But it has a fundamental problem: once an X-ray beam enters the body, it keeps going.
So while radiotherapists do their utmost to focus the beam on the tumour, surrounding healthy tissue will also receive a dose of radiation.
The latest computerised machines - that track tumours as the patient breathes - minimise the damage to normal cells.
A computer generated image of a proton beam therapy room But they don't stop it altogether - and that's particularly important if the tumour is next to a critical organ or if the patient is a child (they're more susceptible to the effects of radiation).
That's why some doctors are excited about the potential for proton beam treatment.
Protons are tiny particles found at the centre of every atom.
Just like X-rays, they kill cancer cells. But once they hit their target, they stop. They don't carry on through the body, reducing the chances of damage to other tissue.
There are more than 40 proton beam treatment centres around the world, but just one in the UK - a low-energy machine suitable for treating eye tumours.
The NHS refers some patients to the US or Switzerland for treatment - 122 in 2013, 99 of them children. Patients are assessed by a panel of experts, who look at the tumour type, position, severity and so on.
But many more patients could be treated - perhaps 1% of cancer patients - and two proton beam centres are being built in the UK - in London and Manchester - at a cost of £250m.
Ashya with his older brother Brett and his mother Naghemeh From 2018 around 1,500 patients a year could be treated.
So why has the NHS been so slow in the uptake?
According to Cancer Research UK the treatment isn't a "magic bullet".
Although it reduces side effects, there is a lack of good evidence that patients treated with a proton beam do any better than those given advanced X-ray treatment.
Survival rates seem to be about the same, the charity says. But the treatment is too new to know whether children treated with protons have fewer long term side effects.
The NHS will carefully follow up patients to answer some of the unknowns. And there is the possibility of building another treatment centre in Birmingham if there really are advantages for patients.
Where does that leave Ashya King?
We are not privy to his medical notes, so none of us can offer any informed insight into the benefits of proton treatment in his case.
It is a dialogue that should be happening between his doctors and parents. But that relationship appears to have broken down.
If proton beam really isn't the answer for Ashya, doctors have failed to convince his parents.
And one can sympathise with their belief that the latest treatment must be the best.
Ashya's parents are fighting for his life. They will cling to any hope, regardless of the limited scientific evidence.
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