The UK's first high-energy proton beam therapy centre for treatment of life-threatening cancers is due to open in Wales.
The private clinic in Newport is expected to start using the treatment next week.
But the parents of Freya Bevan, five, from Neath, who was treated in the United States, said it was frustrating it had taken so long to arrive.
Her mother said it had given them their "little girl back".
The Rutherford Cancer Centre, operated by Proton Partners International in the Coedkernew area of city, has been granted regulatory approval for the treatment by Health Inspectorate Wales.
Treatment is likely to cost about £60,000 a patient, but it is not yet clear whether Welsh NHS patients will be treated in Newport and discussions are continuing between the company and the Welsh NHS.
Proton beam is a highly targeted form of radiotherapy that can treat hard to reach cancers with a reduced risk of damaging surrounding tissues.
It is more expensive than traditional radiotherapy but until now cancer patients who wanted the treatment would have had to travel abroad.
Three years ago, Freya underwent proton beam therapy in Oklahoma City for a rare brain tumour.
Her parents' bid for funding for the treatment was turned down by the NHS and her community helped raise the £110,000 needed for the treatment abroad.
Freya, now five, has recovered and is attending school.
Her mother, Katherine Bevan, said: "There were dark days, when we didn't know what the future would hold," she said.
"We sometimes forget and you can pretend it never happened and lead a normal life, it's all we ever wanted."
But her father, John Paul, said he was frustrated it had taken so long to arrive in the UK.
"It angers me that we are so far behind. We've had these in America for more than 20 years. It's not something new," he said.
A low energy form of proton beam therapy is available at the Clatterbridge Cancer Centre on the Wirral where it is used to treat rare cancers of the eye.
But the new facility in Newport, helped by a £10m investment from the Welsh Government, can deliver much higher doses to treat a broader range of cancers.
In England, a proton beam centre at the Christie Hospital in Manchester is expected to open later this year, followed by another in London.
Until now the NHS has funded a limited number of patients to be treated abroad. When the two NHS centres in England are up and running, the overseas funded programme will be wound down.
"What we need to do is have this grown up conversation nationally to say proton beam therapy is here," said Mike Moran, chief executive officer of Proton Partners International.
"The genie is out of the bottle, we need to use it effectively, use it properly and give patients access to this valuable treatment."
Dr Martin Rolles, a consultant clinical oncologist in Swansea, said the arrival of the technology in the UK was "exciting".
He said "Clinical trials are ongoing to gather evidence about the benefit of the treatment.
"At the moment it is too expensive to use generally," he said.