Glioblastoma, the type of brain tumor diagnosed in Sen. John McCain, is an aggressive type that is difficult to treat, experts say.

“It’s a very aggressive tumor,” said Dr. Joseph Zabramski, a neurosurgeon at Barrow Neurological Institute in Phoenix. “In general, it is a tumor that has relentless force. You can slow it down but not stop it.”

Michael Berens, deputy director of the Translational Genomics Research Institute in downtown Phoenix, said unlike other cancers, glioblastoma "begins and then spreads within the brain with finger-like projections."

“The tumor cells wander around and create guerilla warfare in the brain,” Berens said.

McCain’s office said Wednesday the tissue of concern was removed during a craniotomy. The senator and his family are reviewing further treatment options with his team at Mayo Clinic.

Berens, who has studied glioblastoma for 30 years, said patients who contract the cancer and undergo surgery, radiation and chemotherapy live on average 16 to 18 months.

Berens added that only about 18,000 Americans are afflicted with glioblastoma.

Zabramski said patients generally take at least a few weeks after a tumor is surgically removed to assess other treatment options. That may include genetic testing of the tumor that can inform doctors which type of treatment may be most effective.

“There are some new (treatment) options out there that have been showing some promise,” Zabramski said.

However, glioblastoma tumors typically are malignant and difficult to treat because they contain many types of cells, according to the American Brain Tumor Association.

Glioblastoma is one of several types of tumors known as glioma that start in the glial — or gluey — cells that form the structure of the brain. Glioma is the most common type of primary brain tumor, meaning tumors that begin and generally stay in the brain.

Glioblastoma is the highest grade of glioma, and its most malignant form. Sen. Edward Kennedy of Massachusetts and Beau Biden, the son of former Vice President Joseph Biden, both died of glioblastoma.

MORE: TGen doctor uses 'Moonshot' to fight cancer that killed Beau Biden

"Obviously, this is a very challenging tumor to overcome," said Nader Sanai, a brain-tumor surgeon and director of neurosurgical oncology at Barrow. "But a small percentage of patients can still do well with it."

Sanai said this form of cancer has been puzzling to doctors.

"This is one of those tumors that have more questions than answers, and why they happen and how best to treat them," he said.

The median survival rate for the most common type of glioblastoma is 14.6 months, according to the American Brain Tumor Association. About 30 percent of patients live two years with glioblastomas.

Depending on the exact circumstances of McCain’s diagnosis and how soon he chooses to begin treatment, Zabramski said McCain might be able to resume his duties in the U.S. Senate.

“It depends on when he starts his therapy and whether he wants to (return),” Zabramski said.

Sanai said most patients return to work within two to six weeks of surgery. But he said immediately after surgery, patients typically have difficulty in concentrating.

Coincidentally, TGen annually presents the John S. McCain Leadership Award, named after the senator, to individuals who have made a significant impact in the fight against cancer.

“When I heard of the diagnosis, my heart sank,” Berens said. “Sen. McCain has been a stalwart for this institution. ... He’s a long-term survivor of cancer, and then this diagnosis pops up. God bless him. He’s a man of great courage and endurance. He has a rough journey ahead of him.”

READ MORE:

Meghan McCain on father, Sen. John McCain: 'He is the toughest person I know'

Roberts: Praying tonight for John McCain

Montini: John McCain is NOT dying