A cancer stricken seven-year-old "will die if he does not receive the radiation therapy treatment his mother desperately opposes", the High Court has been told.
Sally Roberts, 37, is fighting NHS plans for her son, Neon, to undergo radiotherapy following the removal of a life-threatening brain tumour in October.
Mrs Roberts, originally from New Zealand, says she is taking a "principled" stand against treatment because of the risks which it could pose in the long-term. She sparked a brief manhunt this week when she disappeared with Neon from where they had been staying in Tiverton, Devon. A High Court appeal helped locate her in East Grinstead, Sussex.
His dad, IT consultant Ben Roberts, from whom Mrs Roberts is separated, supports the NHS view as to how Neon should be treated.
Without it, the boy's doctors say, the medulloblastoma tumour will recur and Neon, who is a twin, will die, Mr Justice Bodey was told at the High Court today.
NHS barrister Eleanor Grey QC said radiotherapy has been the norm for medulloblastoma patients since the 1950s and before that "all patients died".
"Alternatives such as omitting radiotherapy and using chemotherapy only are associated with significantly poorer survival rates," she told the court in written arguments.
"So, if a balancing exercise assessing the proposed treatment is conducted, on the positive side of the scales is the fact that prompt treatment, involving radiotherapy and chemotherapy, leads to a high chance of recovery.
"Since with no additional therapy medullablastoma will recur, the alternative is death."
Giving evidence, Mrs Roberts said her big fear was Neon being left with a lowered IQ after the treatment and she only wanted the best for her son.
"I want him to have the best quality of life and I fear that radiotherapy could damage and greatly affect his future," she told the judge.
"He has an incredible sense of humour. He is a great artist.
"He has been made out that he is sick and this is not true. He is very well. He is running around. He is not a sick boy.
"I feel that, if you go ahead with the radiotherapy, we are depriving him of his talents."
Of the manhunt she sparked, she told the court: "I was forced into that situation. I was given no choice. I thought if I brought him to the hospital they would not let me take him home and proceed with the treatment."
The mother's barrister, Robin Tolson QC, said Mrs Roberts is taking a "principled" stand against the therapy and wants Neon to have chemotherapy alone.
In recent weeks, she has been investigating natural remedies and options abroad, which might not present the same risks of long-term effects that radiotherapy brings.
"Our main submission is that the mother's position in this litigation, as opposed of course to her being in hiding, is principled, reasonable and in the best interests of Neon," Mr Tolson said.
He pointed to evidence of risks of significantly lowered IQs in those who undergo radiotherapy and chemotherapy, as opposed to chemo alone.
"Such a significant loss of intellectual functioning is a real impairment to quality of life, particularly in the world Neon is likely to inhabit as an adult," he said in court documents.
The judge banned identification of any of the hospitals, NHS trusts or doctors involved in Neon's case.
One cancer specialist, referred to as Dr H, told the court that, while there can be serious side-effects to the therapy, things can be done to mitigate.
Children can get extra help at school and some even go on to be academically successful, while hormone replacement can limit growth stunting, he said.
"We really feel very much in a dilemma", he told the court.
The hearing continues.
