What is glioblastoma? McCain’s brain tumor is an aggressive type of cancer

The type of brain tumor Sen. John McCain had removed Friday is an aggressive one known as a glioblastoma, a highly malignant form of cancer that spreads quickly due to its association with a large network of blood vessels in the brain.

The tumor was associated with a small blood clot above the Arizona Republican's left eye, which doctors removed Friday during a minimally invasive procedure, according to a statement issued by the senator's office Wednesday evening.

RELATED: Reaction to Senator John McCain's brain tumor diagnosis

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Reaction to Senator John McCain's brain tumor diagnosis
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Reaction to Senator John McCain's brain tumor diagnosis
Statement regarding my father @SenJohnMcCain: https://t.co/SMte9Hkwkq
@MeghanMcCain Her words are extraordinary. https://t.co/nikKbSxTLE
Melania and I send our thoughts and prayers to Senator McCain, Cindy, and their entire family. https://t.co/SO4XYgnyug
John McCain is an American hero & one of the bravest fighters I've ever known. Cancer doesn't know what it's up against. Give it hell, John.
Karen & I are praying for @SenJohnMcCain. Cancer picked on the wrong guy. John McCain is a fighter & he'll win this fight too. God bless!
John McCain is as tough as they come. Thinking of John, Cindy, their wonderful children, & their whole family tonight.
.@SenJohnMcCain is a hero to our Conference and a hero to our country. https://t.co/Ld2lQH1mMn
I love @SenJohnMcCain. Unbeatable, unbreakable. He's Teddy Roosevelt's "man in the arena" even when we're on opposi… https://t.co/rkLNj5ToVL
Praying for my friend @SenJohnMcCain, one of the toughest people I know.
John and I have been friends for 40 years. He's gotten through so much difficulty with so much grace. He is strong - and he will beat this.
.@SenJohnMcCain is an American hero. I pray for him and his family. I look forward to seeing him soon.
.@SenJohnMcCain is a hero, a patriot and a fighter. I am privileged to call him a friend. Paul & I pray for his recovery and his family.
.@SenJohnMcCain has always been a warrior. It's who he is. All of us, not as Republicans or Democrats, but as Ameri… https://t.co/wIJTj8tdY7
My friend @SenJohnMcCain is a dogged ole S.O.B. Sharp as hell and tougher than a $2 steak. I look forward to catching up with him soon.
Heidi and I are lifting up @SenJohnMcCain in prayer. He's an American hero who fought, bled, and suffered for our Nation. A true patriot.
As he’s shown his entire life, don’t bet against John McCain. Best wishes to him for a swift recovery.
If there is anyone who knows how to fight against the challenges life throws at us, it's @SenJohnMcCain. Prayers for a speedy recovery.
John McCain is a hero, one of the most respected senators and a friend. The hopes and prayers of the nation are with him and his family.
Hang in there @SenJohnMcCain. You're tough! You can beat this. Fight, fight, fight! I am proud to call you my friend.
Just spoke to @SenJohnMcCain. Tough diagnosis, but even tougher man.
Thinking about a hero, my Chairman, my friend John McCain. Stay strong!
John McCain is one tough fighter - we know he'll face this diagnosis with courage and strength. Our family... https://t.co/XXTUltNGey
Praying for @SenJohnMcCain and his family tonight. You won't meet any stronger and braver than him.
If anyone can survive brain cancer it's @SenJohnMcCain - NH loves you. #hero #fighter #survivor #nhpolitics
J.McCain will be around for a long while. I've always known he'd outlive me. I wrote the eulogy he'll give at my funeral. It's very touching
My thoughts and prayers are with @SenJohnMcCain tonight, his wife @cindymccain and the entire family. Join me in saying a prayer for them.
Hatch statement on @SenJohnMcCain. https://t.co/yG7vi0IGfi
Praying for Senator McCain and his family. https://t.co/f4tcusDWKl
Thinking of Sen McCain and his family tonight. Was so honored to visit the infamous "Hanoi Hilton" with him a few w… https://t.co/XdUust5lBT
John McCain is an American hero and a relentless fighter. I wish him Godspeed in this fight and look fwd to welcoming him back to work soon.
.@SenJohnMcCain our prayers for your recovery and comfort to your family. I have been on @FightBrainTumor board for 14 yrs. We fight on. . .
So sad about @SenJohnMcCain, Lion of the Senate. He's a man of incredible strength & courage-qualities I hope will once again see him thru.
Very sad to hear that @SenJohnMcCain has been diagnosed w/ brain cancer. He's a fighter & a true patriot. Praying that he gets better soon.
Keep @SenJohnMcCain, Cindy, and his family in your thoughts and prayers. He's a fighter, and we hope to see him back in the Senate soon.
My thoughts and prayers are with you @SenJohnMcCain, a true American hero and dedicated public servant.
So sad to hear the heartbreaking news about my friend @SenJohnMcCain. My prayers are w/ the McCain family and w/ John, a true fighter.
I love John McCain. I've traveled with him, I've been tongue lashed by him, I've worked with him, and I wish I could be with him tonight.
Hey John! We can't wait to welcome you to the club. You got this! ❤️@SenJohnMcCain #cancersurvivors
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Scans completed since Friday indicate that doctors were able to remove all the tumor tissue they could see in the 80-year-old senator's brain. "The tissue of concern was completely resected by imaging criteria," the statement read. The procedure and surgery were performed by doctors at the Mayo Clinic Hospital in Phoenix.

"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 statement said.

Here's what more we know about the type of cancer McCain was diagnosed with.

What is a glioblastoma?

A brain tumor is a mass of abnormal cells growing in the brain. They arise from star-shaped cells that make up the supportive tissue of the brain, also known as astrocytes or, more commonly, glioma.

In rare cases, brain and spinal cord cancers are found to run in families, but in most instances, people diagnosed with brain tumors have no family history of the disease, according to the American Cancer Society.

The ACS estimates that about 24,000 malignant tumors are diagnosed each year — about 3 in 10 brain tumors are glioblastomas.

Cancerous tumors of the brain and spinal cord differ from other tumors in the body. For one, glioblastomas don't spread to other organs.

Although the statement from McCain's office stated that doctors believe they removed all of the tumor tissue, microscopic pieces that look like a sea urchin's tentacles could remain and spread to other parts of the brain, according to NBC News medical correspondent Dr. John Torres. Because gliomas have often spread deep into the brain by the time of diagnosis, it is difficult for the cancerous tissue to be completely removed.

McCain has a history of melanoma, a deadly skin cancer that can spread to the brain. The cause of his brain tumor has not been determined, but a 2014 study published in the Annals of Epidemiology found that incidences of gliomas were greater among melanoma cases than in people who had never been diagnosed with skin cancer.

Are there symptoms?

With any kind of brain mass, patients typically complain of symptoms such as double vision, forgetfulness or headaches, Torres said. These nonspecific symptoms make it nearly impossible to diagnose based on symptoms alone. Furthermore, symptoms may not persist. For example, headaches associated with a brain tumor may be more painful in the morning and improve during the day.

Related: McCain's Questions at Comey Hearing Left Many Confused

Some people are unaware they have a brain tumor until experiencing a seizure, according to the American Cancer Society. In 2009, Sen. Edward M. Kennedy died at age 77 from a malignant brain tumor after first suffering a seizure.

It's possible the brain tumor was linked to McCain's uncharacteristically incoherent line of questioning during congressional testimony last month by former FBI Director James Comey.

What are the next steps?

McCain's office had previously said he would be recovering from the blood clot procedure for at least the next week. Following this period of rest, treatment to eradicate any cancer cells that might be present typically ensues.

Treatment options might include a combination of chemotherapy drugs to attack the malignant cells and radiation on a daily basis, Dr. Torres said. It's possible McCain could return to the Senate while undergoing treatment, and the senator's statement said his Mayo Clinic care team will determine when, or if, he can get back to work.

What is the prognosis?

Unfortunately, "the prognosis for a glioblastoma is not very good," Torres said. The median survival rates range from 14 months to 3 years.

"The most important prognostic indicators are some of the molecular alterations in the tumor," said oncologist Dr. Andrew S. Chi, co-director of the Brain Tumor Center at New York University's Langone Medical Center, who is not involved in McCain's treatment. "Alterations in two genes in the tumor — IDH and MGMT — are the two most powerful prognostic indicators in gliomas. The status of these two genes will be determined over the next 2 weeks."

The veteran lawmaker, who fought in Vietnam and was a prisoner of war for more than 5 years, is known as a fighter. However, his advanced age could make recovery more difficult, as only about 10 percent of patients live longer than five years following diagnosis.

According to the official statement from McCain's office, "The Senator's doctors say he is recovering from his surgery 'amazingly well' and his underlying health is excellent."

McCain is currently recovering at his home in Arizona.

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