In 2014, Chris Borland’s dream had come true. Football was what he loved, what he says he was “born into.” After a standout career at the University of Wisconsin, he’d been drafted into the NFL by the San Francisco 49ers. Borland was given the No. 50 jersey, a four-year contract worth close to $3 million and a signing bonus of more than $600,000.

Less than a year later, Borland was out of the game — by choice. Prompted by concerns over the long-term risks to the brain that can come from football, he chose to retire midway through his rookie season. Borland’s decision shocked the NFL establishment and led ESPN to call him “the most dangerous man in football.”

Never before had such a promising young NFL star voluntarily walked away from football so publicly because of worries over head injuries. At a time when the league was emphasizing its efforts to boost player safety, Borland’s decision marked a turning point.

In the above video, Borland explains his decision to FRONTLINE. “The idea that just the basis of the game, repetitive hits, could bring on a cascade of issues later in life, that was, it changed the game for me,” he says. “I couldn’t really justify playing for money and I think what I wanted to achieve put me at too great a risk so I just decided on another profession.”

An encore presentation of FRONTLINE’s investigation into the NFL’s concussion crisis, League of Denial, airs on many PBS stations, Tues., Dec. 22, at 10 p.m. EST. Check local listings here. The film can also be streamed anytime on the FRONTLINE website.