Sen. John McCain John Sidney McCainCindy McCain endorses Biden: He's only candidate 'who stands up for our values' Biden says Cindy McCain will endorse him Biden's six best bets in 2016 Trump states MORE (R-Ariz.) says in a recently released excerpt from his upcoming memoir that he isn't sure how much time he has left to live.

McCain, 81, was diagnosed with glioblastoma, an aggressive form of brain cancer, last year.

“I don’t know how much longer I’ll be here,” McCain says in his book “The Restless Wave: Good Times, Just Causes, Great Fights, and other Appreciations.” The excerpt was provided to NPR on Thursday.

ADVERTISEMENT

Advances in oncology could uncover new cancer treatments that may extend his life, McCain says, noting that he could live another five years.

“Maybe I’ll be gone before you hear this,” he says. “My predicament is, well, rather unpredictable. But I’m prepared for either contingency or at least I’m getting prepared.”

Part of that plan, he says, includes talking to "my fellow Americans a little more."

McCain acknowledges in his book that this will be his last term in the Senate, according to other excerpts released last week.

“I’m freer than colleagues who will face the voters again. I can speak my mind without fearing the consequences much. And I can vote my conscience without worry,” he writes.

But the memoir isn't entirely apolitical — the senator also takes aim at President Trump Donald John TrumpOmar fires back at Trump over rally remarks: 'This is my country' Pelosi: Trump hurrying to fill SCOTUS seat so he can repeal ObamaCare Trump mocks Biden appearance, mask use ahead of first debate MORE. In it, McCain writes that the president seems to care more about “the appearance of toughness” than American values, chiding Trump's coziness with Russian President Vladimir Putin, in particular.

“The Restless Wave: Good Times, Just Causes, Great Fights, and other Appreciations.” will be released on May 22.