A High Court judge will decide on Saturday whether a seven-year-old boy whose mother ran away with him to stop him having cancer treatment can undergo radiotherapy treatment following surgery on a brain tumour.
Mr Justice Bodey heard on Friday that Neon Roberts' mother Sally did not want doctors to begin radiotherapy treatment.
Neon Roberts went missing with his mother on Monday
Ms Roberts, 37, of Brighton, East Sussex, told a High Court hearing in London that she was not a "bonkers mother" and feared that radiotherapy would cause Neon long-term harm.
But doctors said Neon might die within months without radiotherapy treatment and urged the judge to give them the go-ahead.
Mr Justice Bodey said he would announce his decision on Saturday.
He said hospitals where Neon received treatment and medics involved should not be identified in reports.
Neon's father Ben, who lives in London and is separated from Ms Roberts, had agreed to radiotherapy but was "apprehensive", the court heard.
Mr Justice Bodey said Neon's illness was the "stuff of every parent's nightmare".
He said he would balance what radiotherapy treatment would achieve against the "downsides".
The case hit the headlines earlier this week when Ms Roberts, who was then living in Tiverton, Devon, disappeared with Neon.
Both were found safe after a judge ordered a search.
Ms Roberts, a New Zealander, apologised for vanishing and told Mr Justice Bodey that she had panicked.
She told the court that she feared radiotherapy would reduce Neon's intelligence quotient (IQ), shorten his life, put him at risk of having strokes and make him infertile.
"I want him to have the best quality of life and I feel radiotherapy could damage, greatly affect his future," Ms Roberts told the judge.
"I have been told that if he doesn't have radiotherapy treatment he will die. I have been saying, 'show me the studies'."
She added: "Radiation damages DNA for life."
Ms Roberts said she would agree to Neon being given chemotherapy because damage caused could be "overcome".
Robin Tolson QC, for Ms Roberts, said the judge should rule that Neon should not receive radiotherapy.
"This is not an area where doctors should dictate but where parents should decide," Mr Tolson told the judge.
"The reality is parents are not getting a choice, because if they don't agree with the treatment the police will be knocking on their door."
He added: "It is time the medical profession thought again. Is it the case that in five years time, doctors will be looking back and wondering why, in the light of developments, they used this treatment, which is so very damaging?"
A doctor involved in Neon's care told the court that Ms Roberts' comments were very sensible and accepted that there could be side effects.
But he said without radiotherapy the little boy could die within a few months.
"I think it is a balance. I don't think it's a fine balance," said the doctor.
"There is a distinct disadvantage in terms of the overall survival.
"There are side effects that occur but we must not underestimate the quality of life of patients who have these side effects."
He added: "The vast majority of parents will have concerns whether to make the decision but go with the treatment recommended."
Mr Roberts was not at the hearing because he was with Neon - but he wrote to tell the judge that he had agreed to radiotherapy because it seemed to be the "best course".
"Obviously I am concerned about side effects and slightly apprehensive about radiotherapy," said Mr Roberts' message.
"If Sally produces sufficient evidence that radiotherapy is not a necessity then I am happy to support her."