The Queensland State of Origin hero known universally as "the King" sought medical advice after becoming concerned by regular bouts of memory loss.
A form of dementia, CTE is a result of repeated concussions and head knocks.
Lewis admits he suffered many of those in a decorated career that netted eight State of Origin player-of-the-match awards and led to the league Immortal being cast in a bronze statue outside Suncorp Stadium.
But what the 63-year-old and his doctor saw when they checked his latest scans shocked them both.
"I was expecting to see this just tiny little mark," Lewis told ABC Radio Brisbane.
"It was enormously different. I just looked at it and could not believe the difference.
"She (Lewis's doctor) just said, 'I don't think I've seen one like this'."
The only conclusive test for CTE is conducted through post-mortem brain tissue analysis, but Lewis's neurologist Rowena Mobbs said she was "90 per cent certain" based on the scans.
"You could interpret it as guesswork, but it's educated guesswork by a specialist in dementia," Dr Mobbs told 60 Minutes.
"It's devastating. I cried that night on my way home from work.
"It's hard to see these players go through it. They're people I've admired and, and loved growing up, so the last thing I want to do is diagnose them with dementia."
Dementia Australia chief executive Maree McCabe thanked Lewis for sharing his story and stressed the condition was preventable by managing head injuries and concussion by following "evidence-based, best practice guidelines".
McCabe hoped Lewis's story would create greater awareness of a condition that is also a risk for family violence survivors and war veterans who experience traumatic brain injury.
Brisbane hooker Billy Walters, son of Lewis's former teammate and Broncos coach Kevin, felt the NRL's efforts to prevent CTE were appropriate.
"It is pretty sad to hear someone as big as Wally going through something like that but it is good that he has shown that you shouldn't be afraid to come out if you have these issues," he said.
"If you get a head knock you can't really hide these days in the NRL.
"The whole coaching staff, medical teams and even players these days know how serious it is.
"We are all on board to follow the protocols and do what we have to do to get back playing."
National Dementia Helpline
1800 100 500 or dementia.org.au.
