According to officials, about 20,000 people in the U.S. are diagnosed with glioblastoma every year.
Doctors at Yale University's Smilow Cancer Center said they see it often and that the first step is surgery.
“The nature of this disease the entire tumor can never be completely removed because there is always an infiltrating component with microscopic spread of cells," said Dr. Jennifer Moliterno, Smilow Cancer Center.
Moliterno said that can make it a difficult tumor to treat.
Officials said Arizona Sen. John McCain already went through surgery. Chemotherapy and radiation are likely the next steps.
There are also clinical trials.
McCain is 80 years old.
The American Cancer Society puts the five year survival rate for patients over the age of 55 at 4 percent.
"It has a way of changing its mutations and outsmarting chemotherapy and the drugs," Moliterno said, "And then it recurs and it's just kind of a matter of time before it recurs, not when or if it will recur."
Lawmakers on both sides of the aisle have been sending McCain their well wishes and said they know he'll put up a good fight.
"John McCain is a fighter and a friend," said Sen. Richard Blumenthal. "If anyone can beat this cancer he'll do it. A true American hero."
Sen. Chris Murphy called the news heartbreaking.
"I've traveled the world with Sen. John McCain," he said. "I learned a lot from him. First and foremost, there is no one tougher."
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