The drug, which costs $8612 a month, is listed on the Pharmaceutical Benefits Scheme, but it is not subsidised for the likes of Ann Bantoft’s son, Brad Williams, because it has only been approved for melanoma patients and his tumour is growing in his brain.

"I would have to sell everything I own to be able to afford it," Ms Bantoft said.

"We just don't have that kind of money.

"The frustrating thing is that Brad has just turned 21. It's not like he's a little old man who's had a long life - he's got his whole life ahead of him.

"He is adamant that he doesn't want us to face financial ruin. We just want our son."

Precision medicine - whereby genetic sequencing allows therapies to be tailored to the individual - promises a new dawn in the treatment of rare and fatal cancers.

But strict regulations for access to PBS medicines makes them unaffordable.

The Pharmaceutical Benefits Advisory Committee approves medicines for government subsidies once the manufacturer can prove they are effective and that they are cost-effective.

There will never be enough patients to run a clinical trial to establish the cost-effectiveness of using a specific drug on a rare cancer, which renders the PBS requirements an insurmountable barrier.

The potential for the drug trametinib to treat Mr Williams arose when his oncologist, Rob Zielinski, sent a sample from the tumour to a German research group for molecular analysis and a mutation was identified that is found in other tumours.

Dr Zielinski said trametinib had produced dramatic results in advanced melanoma patients with this mutation, and drug companies needed to think about cost-share schemes for patients who were not eligible for PBS subsidised medication.

"There's an increasing number of patients who are getting molecular profiling done on their tumours and they throw up these abnormalities that there's an existing drug for but no PBS funding for it," Dr Zielinski said.

"Most companies won't give it for free because they require phase two clinical data, but you're probably never going to get the phase two data because there are so few of these patients.

"The drug may completely not work and I'm not suggesting that it's a life-saving drug, but for a 21-year-old bloke you want to exhaust all possibilities."

Novartis, which manufactures trametinib under the brand name Mekinist, has a compassionate access scheme but its head of government relations, Peter Murphy, said it was becoming more challenging to provide in the current landscape.

Decisions on who could access the scheme were made on an individual basis, based on the evidence and discussions with the treating doctor, he said.

But industry was working with the government to ensure the reimbursement scheme better reflected the way cancer was now diagnosed and treated.

Mr Williams’ tumour has not been affected by radiation or chemotherapy and is too perilous for surgery. He has been given anywhere from six months to two years to live without a radical improvement in his condition.

"I'm frustrated," he said. "It's either pay all this money for something that might not even work, or just live out my days. I don't want to die."