Rosalie Barnes told the BBC that her 13-year-old son Alex received the treatment in the United States eight years ago.
His family raised the money themselves after doctors in England told them proton therapy was expensive and unproven.
The NHS have since decided to back the therapy, building two therapy centres costing £250 million - but they're not ready yet.
Ms Barnes told BBC Inside Out Yorkshire and Lincolnshire: "I was told that in 2013/14 our new proton centres would be open in this country and I was absolutely thrilled. But here we are in 2017 and no sign of any proton centres yet.
"What's upset me is how many families have lost children because they didn't have proton therapy. How many adults have died because they didn't have it?"
Patients who want proton therapy can sometimes receive treatment from the NHS, if the health service pays for them to go abroad.
Bradley Marshall, 17, from Bridlington, was given NHS funding to go to the United States for therapy when he was 10.
His mother, Dawn Marshall, said: "For the parents whose children are refused the treatment, it is soul-destroying.
"They must be feeling really vulnerable."
Since 2008, 950 have qualified for treatment, costing around £100,000 each.
The first proton therapy treatment centre in the UK is due to open in 2018 at Christie's Hospital in Manchester.
BBC Inside Out Yorkshire & Lincolnshire airs on Monday January 16 at 7.30pm.