The parents of five-year-old Ashya King said they took their son out of a hospital in Southampton to seek proton beam treatment - a procedure not available under the NHS.
The proton beam therapy is a highly-targeted type of radiotherapy with a lower risk of damaging the surrounding tissue and causing side effects.
It works by sending charges into cancer cells, with doses of radiation aimed directly at the tumour.
The therapy is used to treat hard-to-reach cancers located deep within the body and close to critical organs and body structures, especially in children and young adults.
Patients needing this type of radiotherapy in the UK are sent for treatment in the US, according to Cancer Research UK.
There are several proton beam therapy centres in Europe. Ashya King's family said they would seek the treatment in the Czech Republic.
Two £250 million NHS proton beam centres are currently being built in the UK.
The facilities in London and Manchester will offer the therapy to up to 1,500 cancer patients per year starting 2018.