Her mum, Cheryl, was told her daughter had just a 10 per cent chance of survival. Ashleigh's condition was so serious that she was christened in hospital surrounded by her family.
While battling the rare cancer in her body, brave Ashleigh's only wish was to have a dad to call her own before it was too late - so as she lay in hospital, her step dad Damian officially adopted her as his own daughter.
Astonishingly, Ashleigh is well enough to return to school, though it will be four years before she is officially in remission.
Her proud father Damian Parks, 28, says: "We always said that one day I would adopt Ashleigh but her being so ill made us get the ball rolling. I am so proud to be her father officially, and not just because she has been so brave, but because she is a wonderful girl and any father would be proud to call her his daughter.
I was always there for her anyway, but now whatever the future holds she knows she has a dad to turn to.
Ashleigh was just 16 when her mother Cheryl Watson, 36, first noticed she was unwell in October 2010.
"I know all teenagers like to sleep but Ashleigh was exhausted all the time. She would fall asleep straight after school. She used to love shopping and ice skating with her friends but she could barely keep her eyes open to finish her tea."
Worried Cheryl took her daughter to see their GP but was told Ashleigh was suffering from nothing more than typical teenage laziness.
"They just said I was a typical teenager and teenagers needed lots of sleep and not to worry," explains Ashleigh.
Unconvinced, the family returned twice with the same concern only to be told the same thing and Ashleigh was prescribed iron tablets as they said she could be anaemic.
It was only in March 2011, six months after first visiting her GP, that Ashleigh fell down the stairs in a sleepy daze and her mum rushed her to hospital that doctors discovered the awful truth.
An MRI scan showed a large tumour at the base of her brain and Ashleigh's mother was told she had a very rare form of childhood cancer called ATRT.
Doctors explained it was one the fastest growing with a high mortality rate in children - the main symptom being tiredness and lethargy.
"I was shaking when they told me she only had a 10 per cent chance of survival," says Cheryl. "For months we'd been told she was just a typical teenager and then we're told she might not pull through."
Cheryl went to her daughter's bedside at the Royal Doncaster Infirmary: "I told her how poorly she was but she already knew. I couldn't help breaking down as I held her."
Cheryl's partner Damian was at home in Doncaster looking after her two children by previous relationships, Marcus, 13, and Jessica, nine.
"I called him to tell him and he went into shock," says Cheryl. "My other two children still saw their fathers but Ashleigh's had never been in her life and she thought of Damian as her dad because he'd brought her up since she was nine. He already thought of her as his daughter and I could hear the terror in his voice."
Two days later Ashleigh was taken to theatre for surgery to remove the tumour, but surgeons had to abandon the operation because it was too close to her brain stem. Devastatingly, tests showed three more tumours the size of peas dotted down her spinal cord. They were also too dangerous to remove so Ashleigh would need to start intensive chemotherapy and radiotherapy.
Her condition was so serious Cheryl was advised to get her daughter baptised by the hospital chaplain. The ceremony took place in May last year.
Her weight plummeted from seven stone to four stone and she was tube fed. Cheryl and Damian spent every day at her bedside. Damian even gave up his job as a plasterer to be with her every day.
When her hair fell out devoted Damian shaved his off too to make her feel better. Damian also started fund raising and took Ashleigh out in her wheelchair to keep her spirits up. It was from her hospital bed shortly after she was baptised that Ashleigh asked Damian to become her dad properly.
"We'd always said I would adopt her one day but we thought we had all the time in the world," says proud dad Damien.
Only now it seemed like a race against time. I was already her dad but Ashleigh wanted to make it official. The next day we contacted social services and told them we needed their help quickly.
They agreed to rush it through as quickly as possible and a few weeks on all the forms arrived. The family went through several rounds of interviews before they received a date for the court hearing earlier this year.
Unfortunately Ashleigh was too sick to attend but Damian and Cheryl rushed straight to her hospital bedside once it was made official in March this year.
Over the next few months, Ashleigh fought hard against the cancer and her family were thrilled when doctors said recently she was finally showing signs of responding to treatment against all the odds.
It will be another four years until she is in remission but Ashleigh's just pleased her hair has started growing back. "She is so strong and we are so proud," says Cheryl. "She's always been a daddy's girl but now it's official. We want other parents to know the signs of this awful cancer because early diagnosis is crucial.
"We were told she was just being a typical teenager when she was desperately ill with cancer. We could have lost her. Thank God we didn't."
What an incredible tale of family strength. Good luck with the rest of your recovery, Ashleigh!
Words and photos: Jeremy Durkin
See Ashleigh's amazing recovery and meet the rest of the family in our gallery below
More on Parentdish: Inspiring teenagers beating the odds
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