(CNN) Sen. John McCain died Saturday , more than a year after he was diagnosed with glioblastoma, an aggressive brain tumor .

On Friday, his family said in a statement that McCain had made the decision to discontinue medical treatment

Unlike other brain tumors that start in the body and spread to the brain, glioblastoma starts in the brain or spinal cord. The tumor arises from star-shaped brain cells known as"astrocytes." The American Brain Tumor Association labels the tumor "highly malignant" and cancerous because of its ability to invade and stay within normal brain tissue.

The senator underwent surgery to remove a blood clot in July 2017 at the Mayo Clinic in Phoenix. Lab results from the surgery confirmed the presence of glioblastoma.

Glioblastoma is the most common of all malignant brain tumors, representing 15.4% of all primary brain tumors, according to the brain tumor group.

This form of tumor killed Sen. Ted Kennedy and Beau Biden, the son of former Vice President Joe Biden.

Glioblastoma symptoms

CNN Chief Medical Correspondent Dr. Sanjay Gupta spoke in 2017 to doctors involved in McCain's care with permission from his family.

Gupta said he learned McCain had felt tired over the months before his diagnosis and had a bout of double vision but blamed it on his intense travel schedule.

Doctors ordered a CAT scan and an MRI scan of McCain's brain that revealed the tumor.

The symptoms of glioblastoma are usually a result of increased pressure on the brain.

The American Brain Tumor Association lists headaches, nausea, vomiting and drowsiness as symptoms for the tumor. Depending on where the tumor is, however, weakness on one side of the body, memory and speech difficulties and visual changes can all be developed as a result.

Photos: The life and career of US Sen. John McCain US Sen. John McCain speaks at a town-hall meeting while campaigning for the presidency in 2008. He was the Republican Party's nominee for that year's election, which he lost to Barack Obama. Hide Caption 1 of 29 Photos: The life and career of US Sen. John McCain McCain sits on a sofa with his sister, Sandy, in a reproduction of a family photo taken around 1938. McCain was born in 1936 to Roberta McCain and John McCain Jr., a Navy admiral. Hide Caption 2 of 29 Photos: The life and career of US Sen. John McCain McCain sits with his grandfather and his father, both of whom were Navy admirals, in this family photo from the 1940s. Hide Caption 3 of 29 Photos: The life and career of US Sen. John McCain McCain, bottom right, poses with his Navy squadron in 1965. Hide Caption 4 of 29 Photos: The life and career of US Sen. John McCain McCain graduated from the US Naval Academy in 1958 and served in the Navy until 1981. Hide Caption 5 of 29 Photos: The life and career of US Sen. John McCain During the Vietnam War, McCain's plane was shot down and captured by North Vietnamese forces. Here, he is pulled out of a lake in Hanoi by North Vietnamese soldiers and civilians in October 1967. McCain broke both arms and his right knee upon ejection and lost consciousness until he hit the water. Upon capture, McCain was beaten, he has said. He was held for five years by the North Vietnamese and tortured. Hide Caption 6 of 29 Photos: The life and career of US Sen. John McCain A Vietnamese doctor examines McCain in 1967. For his service, McCain was awarded the Silver Star, the Bronze star, the Legion of Merit, a Purple Heart and the Distinguished Flying Cross. Hide Caption 7 of 29 Photos: The life and career of US Sen. John McCain McCain, center, and his wife, Cindy, pose with US Rep. John Rhodes after McCain was elected to the House in 1982. McCain has represented Arizona ever since. In 1986, he became a US senator. Hide Caption 8 of 29 Photos: The life and career of US Sen. John McCain McCain talks with people in Hanoi, Vietnam, during the filming of the CBS special, "Honor, Duty and a War Called Vietnam" in 1985. Hide Caption 9 of 29 Photos: The life and career of US Sen. John McCain McCain, left, joins President George H.W. Bush at a news conference about soldiers missing in action during the Vietnam War. Hide Caption 10 of 29 Photos: The life and career of US Sen. John McCain McCain gets a kiss from his wife as they kick off his campaign for the 2000 presidential election. Hide Caption 11 of 29 Photos: The life and career of US Sen. John McCain McCain spends time with his wife and children at their home in Phoenix in 1999. John and Cindy McCain have two daughters, Meghan and Bridget, and two sons, Jack and Jimmy. He also has three children from a previous marriage: Andrew, Douglas and Sidney. Hide Caption 12 of 29 Photos: The life and career of US Sen. John McCain McCain rests in a New Hampshire motel room while on the campaign trail in 1999. Hide Caption 13 of 29 Photos: The life and career of US Sen. John McCain McCain poses with his wife and seven children in 2000. The children, from left, are Andrew, Jimmy, Jack, Bridget, Meghan, Douglas and Sidney. Hide Caption 14 of 29 Photos: The life and career of US Sen. John McCain McCain laughs during an interview with "Tonight Show" host Jay Leno in 2000. Hide Caption 15 of 29 Photos: The life and career of US Sen. John McCain McCain reaches out to supporters during a campaign rally in Portland, Maine, in 2000. He suspended his campaign several days later and eventually endorsed his primary opponent, George W. Bush. Hide Caption 16 of 29 Photos: The life and career of US Sen. John McCain John McCain addresses a shadow convention at the University of Pennsylvania in 2000. McCain was booed when he asked supporters to back George W. Bush for President. Hide Caption 17 of 29 Photos: The life and career of US Sen. John McCain McCain and his wife host George W. Bush and his wife, Laura, at the Arizona's Red Rock Crossing in 2000. Hide Caption 18 of 29 Photos: The life and career of US Sen. John McCain McCain acts in a skit while hosting "Saturday Night Live" in 2002. Hide Caption 19 of 29 Photos: The life and career of US Sen. John McCain McCain talks with US Marines in Iraq as he and other senators stopped at Camp Falluja in 2005. Hide Caption 20 of 29 Photos: The life and career of US Sen. John McCain McCain and fellow US Sen. Hillary Clinton listen to President George W. Bush speak at the National Prayer Breakfast in 2007. Hide Caption 21 of 29 Photos: The life and career of US Sen. John McCain McCain, again running for President, speaks during a campaign rally in New York in 2008. Hide Caption 22 of 29 Photos: The life and career of US Sen. John McCain McCain shakes hands with US Sen. Barack Obama, the Democratic presidential nominee, before their first debate in 2008. Obama defeated McCain in the general election. Hide Caption 23 of 29 Photos: The life and career of US Sen. John McCain McCain and US Sen. Mitch McConnell speak about health care reform in 2009. Hide Caption 24 of 29 Photos: The life and career of US Sen. John McCain McCain listens as former FBI Director James Comey testifies to the Senate Intelligence Committee in June 2017. Hide Caption 25 of 29 Photos: The life and career of US Sen. John McCain McCain returned to the Senate floor in July 2017, less than two weeks after surgeons removed a large blood clot from his brain and diagnosed him with brain cancer. He received a standing ovation on both sides of the aisle. Hide Caption 26 of 29 Photos: The life and career of US Sen. John McCain McCain votes no on the GOP's "skinny repeal" health care bill in July 2017. He was one of three Senate Republicans who voted against the failed bill. Hide Caption 27 of 29 Photos: The life and career of US Sen. John McCain McCain speaks to members of the media while heading to a roll-call vote in October 2017. Hide Caption 28 of 29 Photos: The life and career of US Sen. John McCain "No place I would rather be," Meghan McCain tweeted with this photo of her and her father in March 2018. Hide Caption 29 of 29

Glioblastoma treatment

There is no specific treatment used for glioblastoma, but there are a few approaches doctors can take, Gupta said.

"This is a malignant cancer. What that means in this case is that you operate on this," he said. "It needs to be treated as well with chemotherapy and radiation."

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When a cancer is malignant, this means cells are dividing uncontrollably and can invade nearby tissues. They may also spread to other parts of the body through the blood stream or lymph system.

The brain tumor association labels the prognosis for glioblastoma survival in terms of median survival, the length of time at which an equal number of patients do better and an equal number of patients do worse. Depending on the type of glioblastoma and treatment used, this can range from 14 months to three years.

The association also cites a 2009 study that found 10% of patients with glioblastoma may live five years or longer.

The average survival time for malignant glioblastoma is around 14 months with treatment, Gupta said.