He was diagnosed with colon cancer at 31 in prison, and his survival odds weren't great. Now 59, he says spiraling is pointless.

• Nick Leeson's unauthorized trades led to Barings Bank's collapse, a major banking scandal in 1995.
• Leeson battled colon cancer during his imprisonment, undergoing surgery and chemotherapy treatment.
• Post-cancer, Leeson emphasizes that mindset and regular checkups are key to a full life post-cancer.

In 1995, Nick Leeson became synonymous with one of the most significant banking collapses in British history. His unauthorized trades resulted in over a billion dollars (827 million pounds) in losses and brought down Barings Bank, a 233-year-old merchant bank. The fallout led to his arrest, extradition, and imprisonment in Singapore.

What unfolded during his sentence drew less public attention.

While serving time at Tanah Merah prison, Leeson was diagnosed with colon cancer. The symptoms began with dizziness.

Eventually, the tumor in his colon grew so large that it collapsed one of his lungs, and he underwent immediate surgery in 1998. The doctors removed a mass nearly the size of a softball.

"I was given a 60% chance of living for five years," he told Business Insider. Leeson was released from prison in 1999.

Nearly 30 years later, Leeson has undergone at least 10 colonoscopies since the surgery as preventive follow-ups. He's had no recurrence of colon cancer, and says that living a full life after cancer is about having the right mindset.

"You become very aware that there are things in your life you can influence and things in your life that you can't influence," he said. "If you are diagnosed, you can't influence the fact that you've been diagnosed and that you have it. What you can influence is how you move forward from that point."

The biggest warning sign he ignored

In prison, Leeson's early symptoms didn't immediately register as something serious. He says that he would lie on the floor most of the day and when he stood up, "I'd get very dizzy."

He was 30 when the dizzy spells started. Looking back now, he says it was a sign that something was wrong.

"It shouldn't happen to anybody at that sort of age, so that was probably the biggest warning sign that I was ignoring at the time," he said.

He later experienced changes in his stool patterns and sharp, stabbing stomach pain before losing consciousness after his lung collapsed, and he was taken for surgery.

While still incarcerated, Leeson underwent chemotherapy: five days on, followed by three weeks off, for six months. What helped him get through it were two books a friend sent him: "What You Need to Know About Cancer of the Colon and Rectum" and "Chicken Soup for the Surviving Soul."

"Some of them aren't the most encouraging stories," Leeson said of "Chick Soup." "While it's difficult to read, whilst you're experiencing some of these things yourself, it does have a certain amount of uplift, and for me it had a positive message."

Living a full life after cancer

Leeson says colon cancer did not fundamentally change his outlook on life. Rather, it reinforced how he approaches setbacks. "It's happened, right? There's no point of getting into that spiral of self-pity or depression or negativity."

He says self-pity has its place, but only briefly. "You can feel sorry for yourself, I don't think there's any issue with that, but it doesn't really push you forward," he said. "The less that you can wallow in the grief, in the self-pity, the better."

He also stresses being proactive with medical care — but that doesn't mean diagnosing yourself online. Instead, he maintains an open line of communication with his specialist and gets regular checkups. "If I'm worried about something, I phone him. I don't Google and try to work out what's going on. I think that can be dangerous," he said.

At the same time, Leeson hasn't radically changed his lifestyle. "I eat as badly as I did 30 years ago," he said. "The oncologist said to me I shouldn't be eating so much red meat, ice cream, and cake. I eat all of them in abundance."

Doctors who treat colon cancer have told Business Insider that it's best to avoid sugary drinks, red and other processed meats, and drinking alcohol heavily. Moreover, a recent study found that pescatarians had a lower risk of colorectal cancer compared to red meat eaters.

Leeson balances his diet with exercise, attending strength and conditioning classes three to four times a week and spinning occasionally.

"If you can get yourself back to a situation where you're still exercising, you're still getting out and about, and you're enjoying your life as you used to before, I think psychologically that makes everything a lot easier," he said.