Proton beam therapy, a new cancer-zapping therapy for which a British couple took their child to the Czech Republic amid an international police hunt and media storm can be highly effective, experts say, but is only suitable for rare tumours.
The parents of British five-year-old Ashya King, who has a brain tumour, ignored medical advice, removed him from hospital and left the country at the end of August, saying they wanted to take him to a private clinic in Prague for proton beam therapy - a type of radiotherapy not currently available in Britain.
The couple were detained in Spain after an international manhunt that drew condemnation in British media, separated from their sick son and then released days later by a Spanish judge.
After a long legal battle, a High Court judge in Britain later approved the move to take Ashya to Prague for proton beam therapy.
On 23 March 2015 Sky News reported that the therapy had worked and that Ahya was cured.
Read: Ashya King arrives in Prague for Proton Beam Therapy
PBT not suited for medulloblastoma
Experts caution, however, that the treatment - a more targeted way of destroying cancerous cells than conventional radiation using proton beams - only offers extra benefits in a small minority of cancer cases.
They declined to comment directly on Ashya King's case but said the type of brain cancer he has - medulloblastoma - is not usually suited to proton beam treatment.
"Dose for dose, proton beam therapy is actually no more effective on the cancer cells than conventional radiotherapy - so it isn't a magic bullet in that sense," said Adrian Crellin, a consultant clinical oncologist and the government's national clinical lead in proton beam therapy.
"What it does do, however, is allow us to take advantage of a different set of properties of the proton, and that - particularly for something like some children's cancers - gives the opportunity to reduce some of the later side effects."
The treatment uses beams of protons rather than X-rays or photons and targets them at the cancerous cells to kill them.
The protons are directed at a tumour more precisely than X-rays and unlike in conventional radiotherapy, the proton beams stop once they hit the target rather than continuing through the body - making them less likely to harm healthy tissue.
Simon Jolly, a lecturer in accelerator physics at University College London (UCL), said these key features of the proton beam make it highly suited to some hard-to-reach tumours, or tumours growing very close to other key organs that could be badly affected by radiation, such as the brain stem or spinal cord.
"What you're trying to do is deliver dose to the cells that you want to kill... and do it in a targeted way," Jolly told reporters at a briefing for reporters given by experts on proton therapy.
"The key advantage with the proton is that it goes in and then stops. And it dumps must of its energy, doing most of its damage, at the end of its path. So not only are you doing less damage on the way in, but it also means that if there are sensitive areas on the far side of the tumour, you will not damage them."
Read: Should Ashya King have PBT?
Treatment should not be targeted
But proton beam therapy is not recommended for medulloblastoma because the treatment should not be targeted to the tumour only.
"There are some tumours that require radiotherapy to the whole brain and whole spinal cord," said Yen-Ching Chang, a specialist in paediatric radiotherapy at UCL.
In cases of medulloblastoma, she added, "we're unable to spare any part of the brain (from radiation)...so the reduction of long-term side-effects is less."
Plans for PBT in Britain
The government's department of health says that from April 2018, proton beam therapy will be offered to up to 1,500 cancer patients at two major hospitals - UCL in London and the Christie cancer hospital in the northern hub city of Manchester.
Until then, British cancer patients who doctors believe would benefit from the specialist treatment will continue to be sent overseas - either to the United States or Switzerland - for treatment paid for by the taxpayer-funded National Health Service (NHS), the experts said.
Crellin said that in 2013, almost 100 children and 24 adults in Britain were approved for NHS funding to have proton beam treatment abroad.
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