Sally Roberts has exclusively told ITV Daybreak that she is concerned that radiotherapy treatment could be damaging to her son Neon, by affecting his DNA.
She said there are "so many" other treatments that her son could be having instead.
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Prince Harry is in Toronto, the host city of the Invictus Games, ahead of Saturday night's opening ceremony.
The Prime Minister said she expected an implementation period of "around two years" as she called for a "creative" approach to talks.
The first signs of autumn are appearing as the equinox marks the start of the season.