Ashya King's life was saved because he was given treatment not available for him on the NHS, his parents have alleged.
The five-year-old's family have told of his "miracle" recovery, as the centre where he was treated declared him cancer-free, The Sun reported.
Ashya's mother Naghmeh, who alongside her husband Brett sparked an international manhunt last summer by removing the little boy from hospital in Southampton without medical consent, described the news as incredible.
Ashya King is transported to the Proton Therapy Centre where start his proton treatment
"If we had left Ashya with the NHS in Britain, he would not be with us today. He was too weak and would not have survived," she told the paper.
Ashya was finally allowed to undergo treatment at the Proton Therapy Centre (PTC) in Prague for brain cancer after a long legal battle fought by his parents. The Sun quoted a report from the PTC which stated that the oncology department "could speculate that Proton Therapy received could be sufficient to sterilise sites of possible future relapses of the tumour and chemotherapy could deteriorate the quality of life of Ashya".
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Ashya's father Brett said his son's condition now justifies their actions in taking him from Southampton General Hospital last August, to Spain where they have a holiday home.
He said: "We have saved his life", adding that they would do the same thing again if they felt they had to.
The Kings were arrested in Spain and spent several nights in prison away from their son, before being released.
A High Court judge approved the move to take Ashya to Prague for proton therapy, which the PTC said is more effective than the radiotherapy Ashya was being offered on the NHS.
It limits the collateral damage of radiation to other vital organs, such as the heart and liver in Ashya's case. This would lead to less severe long-term side-effects including heart and breathing problems.
The therapy was not available for him on the NHS, although the health service later agreed to fund Ashya's treatment.
The family, who have previously spoken of their apprehension over returning to the UK for fear social services would get involved, are staying in Marbella where Ashya will continue his recovery.