A "very poor prognosis."

That's what Sen. John McCain said, in an interview with "60 Minutes", doctors gave him as he began his battle against an aggressive form of brain cancer.

"They said that it's very serious. That the prognosis is very, very serious," McCain told "60 Minutes" correspondent Lesley Stahl in an interview that aired Sunday. "Some say three percent, some say 14 percent. It's a very poor prognosis."

It was a Friday in July and McCain was headed to his cabin near Sedona when he first learned he had a blood cot behind his left eye. Doctors called him and said he had to come in, he told Stahl.

"I said, 'Hey, today is Friday. I'll just come in on Monday.' And she said, 'No, you have to come now. It's very serious,'" McCain said.

The 81-year-old Republican senator went into surgery to remove the clot. Lab results would later confirmed he had glioblastoma -- brain cancer.

"I just said, 'I understand. Now we're gonna do what we can, get the best doctors we can find and do the best we can,'" McCain said. "And, at the same time, celebrate with gratitude a life well lived."

Stahl turned to McCain's wife, Cindy, "was he that tough?" she asked.

"Yes. He is that tough," Cindy McCain responded.

Just days after his diagnosis, Sen. McCain returned to Capitol Hill against, according to "60 Minutes," doctors' advice.

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"I got very choked up," McCain said. "All of them coming over and giving me a hug. It was deeply moving. I had never seen anything like that."

He would cast the deciding vote that killed the GOP's attempt at a so-called "skinny repeal" of Obamacare. Months later, McCain announced he would not vote for another attempt to repeal and replace the Affordable Care Act.

They were decisions that expanded the gap in what has been a troubled relationship with President Donald Trump.

"I'd be glad to converse with him. But I also understand that we're very different people. Different upbringing. Different life experiences," McCain said. "He is in the business of making money and he has been successful both in television as well as Miss America and others. I was raised in a military family. I was raised in the concept and belief that duty, honor, country is the-- is the lodestar for the behavior that we have to exhibit every single day."

Near the end of the interview, McCain said his diagnosis hasn't changed who he is.

"You just have to understand that it's not that you're leaving. It's that you-- that you stayed," he told Stahl. "I am so grateful. Every night when I go to sleep, I am just filled with gratitude."

He also said he never felt panic.

"You've been around a long time, old man. You've had a great life. You've had a great experience," McCain said.

"I want, when I leave that the ceremony is at the Naval Academy. And we just have a couple of people that stand up and say, 'This guy, he served his country.'"

You can watch the entire "60 Minutes" interview here.