Initially, three-year-old Caoimhe Neeson, from Glasgow, was turned down for proton therapy, a targeted form of radiotherapy available in the US which her mum believes is her best chance of survival.
The decision was made by a panel of experts in England, based on a recommendation by specialists at Yorkhill Hospital.
Doctors believed the cancer had spread to Caoimhe's spine and the treatment would be of no benefit to her.
However, Caoimhe's mum Joe-Anne fought for a second opinion, including flying to Nottingham to discuss her daughter's scans with a cancer specialist.
Both that doctor and a specialist from the US backed the move and after re-assessing the submission the Proton Therapy UK Panel has now approved NHS funding for the £250,000 treatment.
The family, from Sandyhills in Glasgow's East End, are expected to fly out to Jackson-ville, Florida, in January for the treatment.
It follows a mammoth fundraising drive by Joe-Anne to raise the £250,000 needed for the treatment. So far, she has raised £17,000.
The 29-year-old said: "I feel so relieved, it's like an enormous weight has been lifted. But I feel angry it was such a hard fight to get to this situation.
"An American specialist and another doctor from Nottingham looked at the scans and the notes and disagreed with doctors in Glasgow, who said she wouldn't benefit from proton therapy.
"The US doctor stated that the cancer has not spread to her spine.
"The new evidence was seen by the proton therapy board and we found out on Monday they will fund the treatment."
Caoimhe was diagnosed with a fast-growing, malig-nant brain tumour three months after becoming unwell.
Doctors told Joe-Anne there was a only a 40% chance she would survive to her fifth birthday.
The youngster is currently being treated with chemotherapy at Glasgow's Sick Kids' Hospital, which has stabilised the cancer but it is probable it will return, and more aggressively.
Days after her cancer was diagnosed, Caoimhe was on the operating table, however, surgeons were unable to remove all of the tumour, because of its size and location.
After the surgery Caoimhe, who has an 11-year-old brother Corrie, was left unable to talk and walk but has amazed medics and her family by regaining her mobility and her speech.
Joe-Anne and best friend Jaclyn Smith, 28, launched a fundraising campaign to send Caoimhe to the US for proton therapy, which delivers a more precise dose of radiation without killing healthy tissue surrounding the tumour and is particularly beneficial for children who are still growing.
Conventional treatment can lead to deafness, reduced IQ and secondary cancers.
The UK government announced in April that proton therapy will soon be offered in London and Manchester, with the capacity to help 1500 patients a year.
Almost 150 UK patients have been funded for the treatment overseas.
Joe-Anne said: "Proton radiation is targeted – it's like the difference between a shotgun blast or a sniper."
The family will use the £17,000 already raised to help fund flights and accomm-odation for the three-month trip to the US.
Joe-Anne has lodged a formal complaint with NHS Greater Glasgow and Clyde over Caoimhe's care.
She also believes her diagnosis was delayed and claims that her little girl was originally treated for a bowel obstruction.
Experts say early detection of brain tumours can have a significant bearing on the outcome of treatment. The health board said it was looking into Joe-Anne's complaint.
A spokeswoman said: "Following further clinical assessment we re-applied to the Proton Therapy UK Panel who have approved funding for this patient to receive proton therapy in Florida."