At the same time, Sean faced the rare genetic blood disorder fanconi anemia, which can lead to bone marrow failure and dramatically break down the immune system, fueling infection.
Quietly suffering through ongoing leg pain as a result of the procedure to avoid having to be "prodded at by any more doctors", Sean finally admitted to professionals he was in pain.
"We were told the only option was to amputate his leg, which as a mother was very stressing," Mrs Prendeville said.
"Sean was the one who accepted it first and told his sisters and I that it was what it was, it had to be done."
During his rotationplasty in June this year, performed by Auckland surgeon Dr Mike Hanlon, with assistance from Starship Hospital surgeon Dr Andrew Graydon, the midsection of Sean's leg was removed to just below the knee.
In an intricate process that took 12 hours, surgeons carefully removed tissue from around his blood vessels and nerves, which were left intact, coiled up and placed back within the remaining tissue, to avoid long-term phantom pain.
Surgeons then rotated his lower leg to 180 degrees, so his foot was facing backwards, before attaching the larger bone of his lower-leg to his thigh bone using surgical plates and screws.
As a result, Sean's ankle joint now functions as his knee joint, with the backward-facing foot a natural fit for a prosthetic leg.
As part of the extremely detailed process, surgeons calculated how tall Sean is predicted to grow and made his right leg slightly longer than the left, to ensure both legs correspond by the time he stops growing.
Currently undergoing physiotherapy, as well as working to re-train his mind to 'rewire' how he views the role of his foot, Sean said he chose rotationplasty over full-leg amputation because below-knee amputations are a lot less restrictive.
"Sean is an outdoor kid who loves tramping, hunting, running and cycling, and before he was diagnosed he was into vaulting," Mrs Prendeville said.
"Now he's created a to-do list, and one of his goals is to scuba dive.
"He's so incredibly brave and has defied the odds, which had doctors telling us in 2011 he would die.
"He doesn't like it when I say it, but he really is a miracle."
In a journey that has been hard for Sean, his mother, and two sisters Isabella and Annalisa, the family now face the beginning of a long fundraising effort to raise $20,000 for an additional prosthetic leg and recovery costs.
"A prosthetic costs a minimum of $15,000, and while the one Sean has now is good for walking, he'll need a different one for activities such as running and scuba diving."
For the sole mother who, in the first year of Sean's diagnosis found herself with just over $4 in her bank account, the cost of getting her son "fully-functioning" will require "a lot of sausage sizzles".
With an abundance of support flowing in from friends, local churches and the community, Mrs Prendeville said she will continue to "not let anything break me".
"It's about the children, and we're so lucky to have had so much social help and support.
"I've learnt that with every negative comes a positive."
The latest positive in Sean's long and hard battle is a step towards an easier future, which he anticipates with excitement.
"When I first saw photos of what rotationplasty looked like I was quite shocked," said Sean, who has not felt any pain in his leg since the surgery 10 weeks ago.
"But as soon as I woke up from surgery and had no pain, I felt so much joy.
"I'm looking forward to getting outdoors and going hunting and tramping again.
"I'm feeling good."
To help cover the costs of Sean's recovery, visit his givealittle page, or visit the Sean's Journey Battling Bone Cancer page on Facebook.
Donations can also be made to BNZ bank 02-0733-0088038-097.
