US Senator John McCain, a former Vietnam War POW and Republican Party presidential candidate, has been diagnosed with an aggressive and malignant brain tumor.

Doctors at the world-renowned Mayo Clinic had performed surgery last week to remove a five centimeter blood clot on the 80-year old's brain, and discovered the tumor.  "Subsequent tissue pathology revealed that a primary brain tumor known as a glioblastoma was associated with the blood clot," the Mayo Clinic said in the statement.  It continued, "The Senator and his family are reviewing further treatment options with his Mayo Clinic care team.  Treatment options may include a combination of chemotherapy and radiation."  McCain has an extensive history of skin cancer.

US Senator John McCain (R-Arizona)

Glioblastoma is an aggressive type of brain cancer; "The problem with it is it has arms that spread out from the tumor center itself," said Dr. John Torres as quoted by NBC News.  The Mayo Clinic says doctors got most of it, but Torres says, "It's very difficult to do that".  The prognosis for this kind of cancer is generally poor.  Legendary Democratic Senator Edward M. Kennedy was diagnosed with it in 2008 and survived fewer than 15 months after that.

Former President Barack Obama famously beat McCain in the 2008 presidential election, but pointedly showed respect to his rival before, during, and after the campaign.  "John McCain is an American hero and one of the bravest fighters I've ever known," Mr. Obama tweeted, "Cancer doesn't know what it's up against.  Give it hell, John."

George H.W. Bush also shared support:  "The Hanoi Hilton couldn't break John McCain's spirit many years ago, so Barbara and I know - with confidence - he and his family will meet this latest battle in his singular life of service with courage and determination," he said in a statement.

McCain's best friend on Capitol Hill, South Carolina Senator Lindsey Graham, said McCain told him he "wants to get back and get to work". 

John McCain was a Navy pilot who was shot down and held as a prisoner of war by the North Vietnamese from October 1967 until March 1973.  McCain's captors tortured him and held him in solitary confinement.  Because his father was a high ranking military official, the North Vietnamese offered to release him early, but McCain refused release until those who had been at the prison longer than he were let go.

McCain After Returning From Vietnam

McCain was first elected to the House of Representatives in 1982 and moved up to the Senate in 1987.  All along the way, he stubbornly cut his own path that frequently strayed from his own Republican Party line.  He criticized the Reagan administration for sending US Marines on a peacekeeping mission to Beirut, Lebanon; shortly after that 305 people including 247 of the Marines were killed in the 1983 Beirut Barracks Bombing.  But he supported most other US military misadventures, leading to criticism that he never met a war he didn't like. 

More recently, McCain has been a fierce critic of Donald Trump, while supporting and defending the national security team propping up the White House.