Story highlights McCain said he wasn't trying to paint a "rosy picture"

He said he wanted to be remembered for his service to the US

Washington (CNN) Sen. John McCain acknowledged the severity of his cancer prognosis Sunday on CNN's "State of the Union," saying in his first national interview since receiving the diagnosis that it is the latest test in a lifetime of tough fights.

"I'm facing a challenge, but I've faced other challenges," McCain said. "And I'm very confident about getting through this one as well."

The Arizona Republican, who was diagnosed with brain cancer in July, said he will receive a magnetic resonance imaging test on Monday and expressed optimism about his condition.

"I'm fine," McCain told anchor Jake Tapper. "The prognosis is pretty good. Look, this is a very vicious form of cancer that I'm facing, but all the results so far are excellent."

McCain returned to Washington as the Senate came back from its August recess last week, and he said he planned to focus in the coming week on a key defense bill.

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