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John McCain Has Been Diagnosed With Glioblastoma. What Is That?

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Senator John McCain, the Republican from Arizona who ran for the presidency against Barack Obama in 2008, has been diagnosed with brain cancer, his office said July 19.

The veteran senator, 80, was undergoing surgery to have a blood clot removed from above his left eye on Friday, when doctors discovered a brain tumor called glioblastoma. Here’s what you need to know about the aggressive tumor:

What is glioblastoma?

Glioblastoma can occur in many areas of the brain and is highly invasive within the central nervous system, because the cells reproduce very quickly. It is a type of glioma, a brain or spinal tumor that arises from glial cells. Glioblastoma makes up about half of all malignant (cancerous) brain tumors and affects around five people out of 100,000 per year.

How are they caused?

That’s little understood, although genetic or environmental factors are not believed to contribute greatly to this kind of tumor. Glioblastoma is thought to “arise spontaneously,” Dr Elizabeth Stoll, a research fellow at the U.K.’s University of Newcastle’s Institute of Neuroscience, told TIME in a telephone interview. “It really can hit anybody,” she added.

Like many other cancers, age is a factor in glioma incidents and malignancy. “It’s thought that accumulation of mutations in cells over time can contribute to causing spontaneous tumors,” Dr Stell said. “Like many cancers, you’re more likely to get it when you’re older. Glioblastoma can occur at any age, although the number of incidents can increase with age.”

What are the symptoms?

The symptoms of glioblastoma are usually caused by increased pressure in the brain, according to the American Brain Tumor Association (ABTA). These symptoms are generally neurological, including seizures, headaches, dizziness and nausea.

Some patients may develop other symptoms, such as feeling weak on one side of their body, difficulties with memory and/or speech and visual changes, the ABTA says.

What is the typical prognosis?

The median survival rate is about 14 to 14.5 months (up from eight to 12 months in recent years), and with current treatment about 5% of patients make it to five years or more, Dr Stoll told TIME. “It’s a very difficult diagnosis,” she added.

What treatment is available?

There are three main treatments available: surgical resection (in other words, having as much of the brain tumor removed as possible via a roughly half-day surgical procedure), chemotherapy and radiotherapy, which can be very targeted to the brain, reducing radiation exposure to other parts of the brain and the body.

“There are therapies inching forward and much more experimental therapies under clinical and pre-clinical investigation,” Dr Stoll said. “There are ongoing clinical trials and research in labs trying to help identify new drugs. We’re hoping to develop new options to help these patients.”

McCain’s office said in a statement that the senator and his family are reviewing further treatment options with his Mayo Clinic care team and that his “treatment options may include a combination of chemotherapy and radiation.”

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Write to Kate Samuelson at kate.samuelson@time.com