Sen. John McCain (R-Ariz.) has been diagnosed with an aggressive and lethal cancer called glioblastoma. Here's what you need to know about it. (Monica Akhtar,Daron Taylor/The Washington Post)

Glioblastoma, the cancer with which Sen. John McCain (R-Ariz.) has been diagnosed, is a highly lethal malignancy that killed Sen. Edward Kennedy (D-Mass.) and Beau Biden, the son of former vice president Joseph Biden. Here is what you need to know:

What is glioblastoma?

Glioblastoma is an aggressive cancer that is the most common of all malignant brain tumors. About 12,400 new cases are expected in 2017, according to the American Brain Tumor Association. A glioblastoma is, by definition, a grade IV astrocytoma, a kind of tumor that arises from the star-shaped cells that make up the supportive tissue of the brain, according to the association.

Typically, said Matthias Holdhoff, associate professor of oncology at Sidney Kimmel Comprehensive Cancer Center at Johns Hopkins University, “the tumors are considered not curable.”

About 23,000 adults, more of them men than women, are diagnosed with various types of primary brain cancers a year, according to Cancer.net, a website of the American Society of Clinical Oncology. They are more common in older adults — McCain is 80 — than in younger people. Unlike most other cancers, brain tumors do not spread to other parts of the body. They kill by interfering with normal body function, depending on their location.

What are some symptoms of a glioblastoma?

Depending on the location of the tumor, a patient can have seizures, headaches, blurred vision and confusion.

“If it affects the portion of the brain that controls your strength, you could have weakness on one half of the body, speech problems, sometimes double vision, the inability to understand or express yourself, even cognitive problems or understanding what you’re listening to, or the lack of insight,” said Deepa Subramaniam, director of the brain tumor center at Georgetown University’s Lombardi Comprehensive Cancer Center.

Often, patients and their families recognize symptoms only in retrospect. “If you’re a family member or a patient, there’s always a lot of 20-20 hindsight and looking backwards,” noted Jeffrey Weinberg, professor of neurosurgery at MD Anderson Cancer Center in Houston.

Indeed, looking back, McCain's tangled questioning last month of former FBI director James B. Comey — which attracted widespread attention, given the high-profile congressional hearing in which it occurred — seems to hold more potent meaning. McCain laughed off his performance afterward, saying that “maybe going forward I shouldn’t stay up late watching the Diamondbacks' night games.”

Sen. John McCain (R-Ariz.) has been diagnosed with a brain tumor. (Victoria Walker/The Washington Post)

How is the cancer treated?

Surgery is performed to remove as much of the tumor as possible. But microscopic “infiltrating tumor cells” generally invade healthy brain tissue and are responsible for recurrence, said Weinberg.

To target those cancer cells, patients typically receive six weeks of oral chemotherapy and radiation. Sometimes that course of treatment is reduced to three weeks for older people. After that, most patients get chemotherapy several days a month for an additional six months.

The median survival time following treatment is about 12 to 16 months, experts said. But that varies considerably: “It’s not just a matter of the cancer but where it is and what it’s doing to the patient,” said Frederick Smith, a Chevy Chase, Md., oncologist.

Up to 30 percent of patients live past the two-year mark, and 10 percent live more than five years. Subramaniam said some patients even live more than 15 to 20 years, though she added, “Those are clearly the exceptions.”

Age can affect how long a person survives; in general, being young is better. Other key factors include how well a person was functioning before being diagnosed and the molecular characteristics of the tumor.

What about immunotherapy and other treatments? 

New treatments that unleash the immune system against malignancies can help patients with several kinds of cancer, including metastatic melanoma and lung and bladder cancers. But while there are many clinical trials testing immunotherapy for glioblastoma, so far the studies haven’t shown a strong survival benefit, experts say.

Another approach involves a device that delivers alternating electrical currents to the scalp. Some medical centers are already using it, but Georgetown is still evaluating the treatment, according to Subramaniam. A patient, whose head is shaved, must wear the cap-like device for 18 hours at a stretch. “If you’re a frail patient, it may be hard,” she said. “That’s why there is hesitation to widespread adoption of this technology.”

What happened to Kennedy and Biden? 

Kennedy disclosed his diagnosis in May 2008, three days after suffering a seizure. He underwent more than three hours of surgery shortly after that at Duke University. He was 77 when he died in August 2009.

Biden, the former vice president's oldest son and the attorney general for the state of Delaware, was diagnosed in 2013 and had surgery, radiation and chemotherapy before going back to work. He died in May 2015 at the age of 46.