The parents of a six-year-old Perth boy with brain cancer have won another court battle against his doctors to stop him having chemotherapy and radiation treatment.

Oshin Kiszko was diagnosed with a rare brain tumour in late 2015.

He underwent surgery in December last year, and his medical team recommended follow-up chemotherapy and radiotherapy treatment.

But his parents Angela Kiszko and Adrian Strachan refused their permission for the recommended treatment, preferring to pursue alternative therapies, amid concerns over serious side effects of the radiation.

The doctors took Oshin's parents to the Family Court, which in March this year ruled Oshin should undergo chemotherapy.

Tests done in February and March showed that Oshin's disease was progressing.

The conclusion drawn by the doctors was that a palliative approach to Oshin's care could not be supported and that the proposed conventional treatment involving both chemotherapy and radiotherapy should commence urgently.

They estimated at the time that the recommended treatment would give Oshin a 50 to 60 per cent prospect of being alive in five years which, if achieved, would be regarded as a "cure", giving Oshin a good chance of a long life.

Oshin's parents remained firmly opposed to the recommended treatment.

The matter returned to court in May because the doctors again wanted him to have radiation treatment, however the case was adjourned after the parents gave an undertaking they would let their son continue to have chemotherapy.

Today, the court ruled Oshin be allowed to only receive palliative care.

A statement from the Family Courts said that "by August 2016, specialist medical opinion confirmed that the delay in therapy had substantially reduced Oshin's chances of a cure, which are now remote".

Case is solely about Oshin's best interests: judge

In his judgement, Justice Richard O'Brien said his decision was determined by what was in the best interests of Oshin.

"This case is not about parental rights, nor is it about the perceived power of the medical profession. It is not about whether, in a general sense, quality of life is more important than duration of life, nor is it about the relative merits of traditional medicine and alternative or complementary therapies," he ruled.

"It is not about overarching considerations as to the right to life, or the right to a peaceful death, nor is it about a philosophical consideration of the best interests of children generally.

"This case is solely about Oshin Kiszko, and how to determine what is in his best interests as a unique individual child in the specific circumstances which he now faces."

Justice O'Brien said Oshin's relationship with this parents weighed heavily in his decision.

"I am deeply concerned that any perpetuation of the conflict over Oshin's treatment will continue to diminish the ability of his parents to focus their energies solely on the provision of that support and love directly to him when he needs it most," he said.

Justice Richard O'Brien emphasised that his decision did not set a precedent for other cases involving medical treatment of a child.