Sen. John McCain revealed Wednesday that he has a primary brain tumor. The cancer was discovered during cranial surgery last week to remove a blood clot above his left eye.

In a statement from Mayo Clinic, McCain's doctors described the tumor as a glioblastoma.

The American Brain Tumor Association describes glioblastoma tumors as typically malignant and difficult to treat because they contain many types of cells.

“It’s a very aggressive tumor,” said Dr. Joseph Zabramski, a neurosurgeon at Barrow Neurological Institute in Phoenix not involved in McCain's treatment. “In general, it is a tumor that has relentless force. You can slow it down but not stop it.”

The median survival rate for the most common type of glioblastoma is 14.6 months, according to the association. About 30 percent of patients live two years with glioblastomas.

The 80-year-old McCain, R-Ariz., is reviewing treatment options with his family. Those could include a combination of chemotherapy and radiation, according to the Mayo statement.

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“Scanning done since the procedure (a minimally invasive craniotomy with an eyebrow incision) shows that the tissue of concern was completely resected by imaging criteria," Mayo Clinic said in its statement.

"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. The Senator’s doctors say he is recovering from his surgery ‘amazingly well’ and his underlying health is excellent.”

A written statement from McCain's office reiterated that the six-term senator, 2008 Republican presidential nominee and former prisoner of war in Vietnam is in "good spirits" as he rests at his home in Arizona.

“He is grateful to the doctors and staff at Mayo Clinic for their outstanding care, and is confident that any future treatment will be effective," McCain's office said in the statement. "Further consultations with Senator McCain's Mayo Clinic care team will indicate when he will return to the United States Senate.”

"Thank all of you for the wonderful thoughts," Cindy McCain, the senator's wife, said on social media. "@senjohnmccain is doing well. We as a family will face the next hurdle together. One thing I do know is he is the toughest person I know. He is my hero and I love him with all my heart."

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Michael Berens, deputy director of the Translational Genomics Research Institute in downtown Phoenix, said McCain is fighting a very serious, aggressive cancer.

Berens, who has studied glioblastoma for 30 years, said patients who contract the cancer and undergo surgery, radiation and chemotherapy live on average 16 to 18 months.

T-Gen annually presents the John S. McCain Leadership Award, named after the senator, to individuals who have made a significant impact in the fight against cancer.

"When I heard of the diagnosis, my heart sank,” Berens said. “Sen. McCain has been a stalwart for this institution. ... He’s a long-term survivor of cancer and then this diagnosis pops up. God bless him. He’s a man of great courage and endurance. He has a rough journey ahead of him."

As the grim news broke, McCain's colleagues and even past rivals unleashed a bipartisan outpouring of well wishes, with many acknowledging his reputation for toughness.

"Senator John McCain has always been a fighter," President Donald Trump said in a written statement. "Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. Get well soon."

Former President Barack Obama, who defeated McCain in the 2008 presidential race, called McCain "an American hero & one of the bravest fighters I've ever known" in a post to Twitter.

"Cancer doesn't know what it's up against. Give it hell, John," Obama tweeted.

U.S. Senate Majority Leader Mitch McConnell, R-Ky., called McCain "a hero to our Conference and a hero to our country."

McConnell wrote, "The entire Senate family’s prayers are with John, Cindy and his family, his staff, and the people of Arizona he represents so well. We all look forward to seeing this American hero again soon.”

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On Capitol Hill, D.C., Sen. John Hoeven, R-N.D., was gathered with other GOP senators for a Wednesday night meeting on health care when they got the news about McCain's diagnosis.

Hoeven told reporters they asked Sen. James Lankford, R-Okla., a former religious camp leader and co-chair of the Congressional Prayer Caucus to lead a prayer.

"We prayed, it was very emotional," Hoeven said.

He added that Sen. Lindsey Graham, R-S.C., who is a close friend of McCain's, told the group that McCain "wants to get back and wants to get back to work."

McCain, a retired naval aviator, was first elected to Congress in 1982, succeeding the retiring former U.S. House Minority Leader John Rhodes, R-Ariz. In 1986, McCain ran for and won the Senate seat vacated by the retiring U.S. Sen. Barry Goldwater, R-Ariz., and was re-elected in 1992, 1998, 2004, 2010 and 2016.

He first ran for president in 2000, coming up short in the GOP primaries against then-Texas Gov. George W. Bush. In 2008, McCain secured the GOP nomination, losing to Obama in the general election.

Chuck Coughlin moved to Arizona in 1986 to work as finance director for McCain’s first Senate campaign.

“Never underestimate him,” Coughlin said of his long affiliation with the senator. “The guy has endured more (s--t) than anyone I’ve met in my life in terms of health issues.”

Coughlin, who runs a political-consulting business, said he suspected McCain would stay in the Senate as long as his condition permits. The Senate is a lot of what sustains him, he said.

He noted that both McCain’s father and grandfather died almost immediately after they retired.

McCain's father, the late Navy Adm. John S. "Jack" McCain Jr., died on March 22, 1981, at age 70. His grandfather, Navy Adm. John S. "Slew" McCain, died on Sept. 6, 1945, at age 61, shortly after the end of World War II.

"The minute they lost their mission, they gave up,” Coughlin said.

In the event that McCain steps down or dies in office, Gov. Doug Ducey would appoint his replacement who, according to Arizona law, would serve "until the person elected at the next general election is qualified and assumes office."

The statute stipulates that Ducey, a Republican, would have to appoint someone of the same political party as McCain, also a Republican.

"John McCain is undoubtedly the toughest man in the United States Senate," Ducey said in a statement. "He is an American hero and has served our country like few ever will.

"‪Sen. McCain has set an example for all Americans in the toughest of fights, in the most difficult circumstances. I have no doubt he'll do it again. My prayers and my full support are with him and his family."

Ken Alltucker, Ronald J. Hansen, Craig Harris and Mary Jo Pitzl of The Arizona Republic and Eliza Collins of USA TODAY contributed to this story.

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