The family of a five-year-old boy with leukaemia said he had received specialist treatment in Singapore over Christmas that could save his life.
Oscar Saxelby-Lee, from Worcester, had stem-cell treatment in the UK, but tests showed cancer was still present.
He flew to Asia with his parents for CAR-T therapy, with the help of £500,000 raised to pay for it through crowdfunding.
His mother, Olivia Saxelby, said they would know in a week if it had worked.
Mrs Saxelby said the therapy, which is not available to Oscar on the NHS, is specifically developed for individual patients and involves reprogramming their immune system cells, which are then used to target the cancer.
She said it had been an "emotional ride" since her son received the treatment on Christmas Eve.
"It feels surreal, but it also feels like nothing has changed because we just haven't seen a difference yet and we don't know what's happening. It's really tough," she said.
"It's gone in. It was as quick as five minutes and now it's waiting.
"It's done and there's no going back. It was such a big step to take."
"It was such a worry taking him away from everything we'd ever known. Every team that was behind him were doing everything possible for him, but it just wasn't enough," she added.
"So having to make that decision was the hardest thing in our lives to do."
The family jetted to Singapore in mid-November and could be there for up to six months.
The CAR-T therapy is the same treatment that Zac Oliver, from Shropshire, underwent for his leukaemia.
Oscar had been treated at Birmingham Children's Hospital for acute lymphoblastic leukaemia since December 2018.
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