Ashya King will start proton beam therapy in the Czech Republic today (Monday) to treat a brain tumour.
The five-year-old arrived at the Proton Therapy Centre (PTC) in Prague earlier for the first of 30 planned sessions.
He has been having tests at the clinic over the past week since arriving from Spain.
Ashya's parents Brett and Nagmeh King sparked an international police hunt after they took Ashya out of Southampton General Hospital without doctors' permission so they could take him abroad for the specialist treatment which is not available on the NHS.
The Proton Therapy Centre released a schedule for the first day of the boy's treatment for a brain tumour.
Radiation oncologist Dr Barbora Ondrova said: "There is a 70 to 80 per cent survival rate for the condition such as Ashya has and there is now every reason to hope that he will make a full recovery."
Ashya's mum and dad faced a legal battle to get their child to Prague after removing Ashya from hospital on August 28, with a High Court judge approving the move after they had been released from police custody in Spain.
Ashya now faces 30 visits to the Therapy Centre, with the first 13 focussing on the brain and spine, to ensure the tumour he had removed has not spread, and the remaining 17 to irradiate where the brain tumour once was.
In a statement on its website the Centre outlined the difference between proton beam therapy and conventional radiotherapy.
It said: "Proton therapy can be much better modulated and precisely focused into the tumour volume. In case of irradiation of medulloblastoma in Ashya, it is necessary to apply the highest possible dose into the former location but protect the tissue around as much as possible.
"In Ashya's case, those are vital structures of the heart, lung, liver and intestines."
Proton therapy would decrease the dose to other parts of the body, including the heart, liver and intestines and trachea by up to 80 per cent.
The PTC said: "This means decreasing the risk of developing cardiovascular disorders, breathing disorders or metabolic disorders on the first place and better protection of the swallowing functions and normal food intake."