Doctors told 19-year-old Daniel Tamburro that if they had not discovered the growth in his heart - which was forcing it 3cm to the right - it would have exploded inside him.
Daniel, from Halesowen, West Midlands, thought he was suffering from chronic indigestion, but was rushed to hospital after collapsing at work.
Doctors performed an X-ray which showed a large shadow on his left lung. Medics were left in shock when a CT scan revealed a grapefruit-sized cyst pressing against his heart. It was growing so fast it was forcing his major organs across to the opposite side of his body.
He underwent a six hour operation at New Cross Hospital in Wolverhampton, West Midlands, in June to remove the 15cm by 12cm growth, and three months later was back at work as a production assistant.
"I was just at work and sat down but every time I breathed in it hurt. It was like indigestion," explained Daniel.
"The boss said go to hospital to get it checked. I expected to be back by lunch but ended up being off for three months.
"I was really worried after the scan, I didn't know anything about it. They said it was really rare and they had never seen anything like it. It had moved my heart 3cm to the right.
"I was in theatre for six hours in the end. They cut me open from the back and they had to crack a few ribs. They managed to cut it out and reconnect a vein that it was connected to. I was in hospital for six days after and had three months off work.
"I couldn't sit up properly and even now I can't lift heavy stuff. It was terrifying. I can't believe something so big was growing inside me and I didn't know anything about it. Doctors told me it was like dodging a seven inch bullet. I'm surprised to be alive today.
"I'm very grateful to the hospital for operating so quickly. My parents were really worried about it and my girlfriend Danielle hated it because the operation was on her 18th birthday.
"She knew how important it was though and as soon as I came back around she was fine. I feel very lucky to be here today. It's amazing my life was saved thanks to a bit of indigestion."