Margaret Boyce revealed her horror when she found a tumour on her son’s back when he was just a child.
Brendan, from Milford, was rushed to hospital and had the cancerous lump removed from his spine.
Two months later he was given the all-clear and never looked back.
Mum Margaret told Highland Radio she found it hard to believe her son was competing in the 50k walk around the streets of London last weekend.
“I was just thinking back how far he’d come and I was very proud of him,” she said.
Brendan said he never dwelled on his life-saving operation as a young boy.
“I didn’t know too much about it. I don’t dwell on that at all. It happens to a lot of people and you just get on with it,” he said.
Brendan smashed his own personal best time by two minutes to finish in 29th place and will now concentrate on the next Olympic Games in 2016 in Brazil.
“My plan is to improve and by time Rio comes around I should be more competitive at the front of the field,” he said.