The parents of a six-year-old Perth boy with brain cancer have won a battle with doctors to stop him having radiotherapy, with Western Australia's Family Court ruling he does not have to have the treatment.

Angela Kiszko and Adrian Strachan have said they do not want their son Oshin to suffer from the side-effects of chemotherapy and radiotherapy.

The court ruled in March that he undergo chemotherapy for a malignant brain tumour but today Judge Stephen Thackray said he would not order Oshin to have radiotherapy if he continued to have chemotherapy.

A Princess Margaret Hospital (PMH) doctor took legal action after the parents refused to allow the treatment.

A doctor, whose name is suppressed, earlier told the court the chemotherapy would not be effective on its own, and radiotherapy was "the only viable curative strategy".

He said Oshin would have a 30 to 40 per cent chance of survival in five years' time if he received both chemotherapy and radiation, and outlined the long-term side effects of radiotherapy in young children, including stunted growth, hearing problems and cognitive deficits.

"We will not cure Oshin without radiation," he told the court on Monday.

But Justice Thackray said there were conflicting medical opinions on whether radiotherapy was the best course of treatment for Oshin.

He said the boy's chance of survival were now "reduced significantly from what they might have been" and he wanted to spare Oshin further psychological distress.

"In arriving at my decision, I have also taken into account the conduct of the parents and their passionate and highly public objection to Oshin undergoing radiotherapy," he said.

"Their behaviour gives cause for concern about their ability to control their emotions around this topic in the presence of Oshin.

"If Oshin were to have 'forced' radiotherapy, I fear he would be exposed to his parents' hostility and bitterness, potentially causing him even more psychological trauma."

Parents motivated by love: judge

The judge said Oshin's parents had "done what they thought was right".

"Although it was they who chose to expose themselves to the glare of an, at times, unforgiving public, there is no reason to consider they were ever motivated by anything other than genuinely held beliefs and love for their son," he said.

"I wish them well in their journey with Oshin in the difficult days and months ahead."

Judge Thackray adjourned PMH's application to enforce radiation therapy, instead of dismissing it, in case further treatment was deemed necessary.

"This will provide opportunity for the hospital to bring the matter back before the court in the event that they consider any other intervention is required for Oshin's best interests," he said in his judgement.

The judge said the decision was a "social, moral or ethical issue" involving whether "greater emphasis should be placed on life itself or the quality of the life", and whether he or Oshin's parents were best placed to make such a decision.

He said he was not aware of any other case where an independent assessor had supported the parents' views, but where medical treatment was being forced on the child regardless.

"In saying this, I wish to make it plain that I admire and respect the viewpoint of the PMH doctors who have an entirely valid alternate view of life and morality," he said.

He said it was "outrageous" the doctors had been subjected to a social media backlash "for performing the very task that our society expects them to perform - that is, doing their best for a young and vulnerable child entrusted by his parents to their care in December 2015".