The Kiwi mother of a seven-year-old British boy with cancer says she's doing the right thing launching another appeal to try to prevent him from having life-saving cancer treatment.

New Zealand woman Sally Roberts, 37, told a British television program her fear of not becoming a grandmother was a factor in her decision to take further court action, just days before her son, Neon, is due to begin radiotherapy.

Last month, High Court judge David Bodey rejected Ms Roberts' arguments that radiotherapy could cause Neon devastating harm, ruling the boy should undergo treatment on a cancerous brain tumour.

Justice Bodey said Ms Roberts' judgment had "gone awry".

However, Ms Roberts said she wouldn't even consider an appeal if she didn't think it was the best thing for Neon.

"It should be my decision. I am the one that is going to be caring for him," she told ITV's Daybreak show.

The treatment would damage Neon's DNA, affect his growth and leave him infertile, Ms Roberts said.

"(Having) no grandchildren is a big factor."

She said she wanted the opportunity to have alternative treatment options considered.

Ms Roberts' estranged husband, Ben Roberts, has consented to radiotherapy treatment and currently has custody of Neon.