A 10-year-old girl, Eliana Mann, is like a "new child" after having a massive tumour that weighed more than 3kg removed.
Her mother said Eliana, from north London, had been transformed by the operation at Great Ormond Street Hospital in London.
Surgeons said the growth had reached the size of a small football.
The tumour was benign, but it would have been deadly had it continued to grow.
In the summer of 2009, Eliana was not growing properly. Her feet had not grown for a couple of years, she was not eating properly and was tired all the time.
Her father, Paul, thought her stomach felt quite hard. Her doctor agreed and Eliana was sent to hospital for testing.
The results showed she had a massive benign tumour growing in her abdomen.
As it was so large, doctors first tried to shrink it with chemotherapy. However, after a year and a half of therapy the tumour was still growing.
In July 2011, the decision was made to remove the tumour.
Eliana's mother, Tara, said the tumour was "absolutely enormous" and "took up 50% of her abdomen".
She told the BBC: "If she didn't have the tumour removed it would end up just growing and growing and growing and it would kill her just by its size.
"We didn't really have much option - it was a lifesaving operation."
Surgeon Edward Kiely said: "This type of tumour in this location is very rare - fewer than one per year at Great Ormond Street Hospital."
Success was not guaranteed; there was a one in 10 chance of not being able to remove the tumour.
The operation took nine hours. Several pieces of tumour were removed - the largest weighed in at more than 3kg.
Tara said: "By September when Eliana went back to school she was like a new child. She's grown, she's put on loads of weight, she's got loads of energy, she does boxing, netball, she goes swimming."
Eliana still needs scans every three months to see if the tumour returns.
Eliana's story is featured in the BBC2 documentary series Great Ormond Street on Tuesday, May 15 at 2100 BST.