A woman diagnosed with breast cancer said she was "in disbelief" after her private health insurer refused to pay for the treatment she wants.
Jan Johnson from Swansea will now pay £66,000 for proton beam therapy despite her Bupa policy costing £18,000 a year.
Bupa said it was "unproved" treatment not covered by her policy and it based its decision on medical advice.
A leading treatment centre in Wales said it would not give Mrs Johnson the therapy if it was not right for her.
Mrs Johnson is the first breast cancer patient in the UK to receive the treatment - patients previously had to travel abroad to access the treatment.
High energy proton beam therapy is more targeted than conventional radiotherapy so reduces the risk of side effects and damage to surrounding healthy tissue.
Ms Johnson, who chose the therapy after doing her own research, said: "I was just so disappointed and I was in disbelief that actually you're paying £18,000 per year to Bupa and I've always been a healthy person and never ever actually made any serious claims with them.
"And this is critical illness. I don't want to be saying to Bupa 'I have cancer, I want the best radiotherapy'. I wish I was never ever making that phone call to them."
Ms Johnson said she was lucky her family had come together and helped raise enough money for the treatment.
In a letter to her explaining its decision, Bupa said the treatment was "unproved".
Bupa does fund high energy proton beam therapy for other types of cancer, and would pay for conventional radiotherapy for breast cancer.
Dr Tim Woodman, Bupa UK medical director, said it only recommends proton beam therapy with a "specific group of tumour types".
"We cover treatment that is evidence based and puts the health and wellbeing of the individual customer first," Dr Woodman said.
Mike Moran, chief executive at the privately run Rutherford Cancer Centre in Newport, said: "We'll always be ethical in our planning so we'll do a dual plan - normal radiation and a proton plan.
"Our medical advice and the clinical evidence clearly shows that proton beam therapy is a better treatment for Jan."
The treatment is currently unavailable on the NHS for breast cancer patients.