Sarah Crawford, from Uddingston, South Lanarkshire, was warned that the growth in her heart wall could prove fatal if she did not have it removed.
The 30-year-old had no idea there was anything wrong until she started experiencing dizzy spells, chest pains and breathlessness.
After a visit to her GP in April last year, Ms Crawford, a solicitor, was referred to specialists at Clydebank's Golden Jubilee Hospital who diagnosed the rare condition myocardial fibroma.
There are thought to be less than 150 reported cases in the world and consultants had to take advice from a clinic in the US before carrying out a risky procedure to remove the benign tumour.
Kenneth MacArthur, a consultant cardiothoracic surgeon at the Golden Jubilee, said: "In 20 years performing cardiac surgery, I have never seen a tumour as large as Sarah's in a case like this. Due to its size and position, surgery was a very risky option as there was a very real chance of causing damage to the surrounding heart, which would have made it impossible to bring her off of the artificial heart and lung machine."
Ms Crawford underwent open heart surgery on July 22 last year and five days in intensive care followed. She is still attending cardiac rehabilitation sessions.
On Tuesday, the lawyer celebrated her 30th birthday with family and friends and is looking forward to returning to her job in Glasgow.
She said: "It was all very scary. My family and I were relieved that the tumour wasn't malignant but there was still fear and shock when we were told about it."