The 80-year-old lawmaker has glioblastoma, an aggressive cancer, according to doctors at the Mayo Clinic in Phoenix, where McCain had a blood clot removed from above his left eye.
US Senator John McCain, the 2008 Republican presidential nominee known for political independence during more than three decades in the Senate, has been diagnosed with an aggressive form of brain cancer, his office said on Wednesday.
The 80-year-old lawmaker and former Navy pilot, who was re-elected to a sixth Senate term in November, has been recovering at home in Arizona since undergoing surgery at the Mayo Clinic in Phoenix last Friday to remove a blood clot from above his left eye.
Tissue analysis since that procedure revealed that a brain tumour known as a glioblastoma was associated with the clot, his office said.
Glioblastoma is considered a grade IV tumour, the most malignant of gliomas. It can be very aggressive and spread into other parts of the brain quickly, according to the MD Anderson Cancer Centre in Texas.
McCain's doctors said he was recovering from his surgery "amazingly well" and that his underlying health was excellent.
His treatment options include a combination of chemotherapy and radiation, his office said.
"The news of my father's illness has affected every one of us in the McCain family," McCain's daughter Meghan McCain said in a statement on Twitter.
"My grandmother, mother, brothers, sister, and I have all endured the shock of the news, and now we live with the anxiety about what comes next."
She said the family was familiar with the experience of battling cancer since her father had fought it previously, and that through all of this, McCain himself was the "most confident and calm" of all of them.
McCain has had non-invasive melanomas removed at least three times. He also overcame injuries suffered as prisoner of war in Vietnam, where he endured beatings and torture by his North Vietnamese captors.
McCain's office said the senator "is in good spirits as he continues to recover at home with his family in Arizona," and that he will consult with his care team about when he might be able to return to work in Washington.
The news triggered a flood of messages of support from across the political spectrum for McCain, who chairs the Senate Armed Services Committee and is in his sixth term in the upper chamber of Congress.
Messages of support
"Senator John McCain has always been a fighter. Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family," President Donald Trump said in a statement. "Get well soon."
Trump was accused of crossing the line with his criticism of McCain shortly after the billionaire businessman announced his presidential bid in 2015, disparaging the senator as "not a war hero."
"I like people who weren't captured," Trump said, in remarks that foretold the nastiness of the 2016 race.
But Trump is well aware of the importance of McCain's vote on health care, with the latest effort to repeal and replace the Affordable Care Act hanging by a thread.
"Plus, we need his vote," Trump said Monday after wishing McCain well after his blood clot surgery.
"John McCain is an American hero & one of the bravest fighters I've ever known," former president Barack Obama, who defeated McCain after a hard-fought 2008 race, wrote on Twitter. "Cancer doesn't know what it's up against. Give it hell, John."
Senate Majority Leader Mitch McConnell quickly issued a statement saying the entire chamber was praying for their colleague and hoped to see him back soon.
"He has never shied from a fight, and I know that he will face this challenge with the same extraordinary courage that has characterised his life."