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The type of brain tumor that doctors removed from Sen. John McCain on 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.

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 we know about this type of cancer:

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.

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 three 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.

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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?

“Glioblastoma is a bleak diagnosis but in comparison to when Ted Kennedy was diagnosed, our outlook is much more hopeful due to tremendous advances in research,” said neurosurgeon Dr. Steven Kalkanis, chairman of the department of neurosurgery at the Henry Ford Cancer Institute in Detroit. "The median survival rates range from 16 to 18 months, but I have seen patients live for 10 years with new therapy."

New and promising clinical trials, such as being conducted by biotechnology company Tocagen and the Optune device, use low dose electricity and viruses to inhibit cell growth to improve survival for patients with glioblastomas. But Kalkanis notes that “survival is directly linked to how much tumor is removed.”

A new drug recently approved by the Food and Drug Administration, 5-Amino-Levulinic Acid (5-ALA) or Gliolan, is a substance that patients drink before surgery which lights up tumor cells in a magenta or violet color making tumor visualization and removal much easier for surgeons.

Whether McCain is a candidate for these new therapies or clinical trials remains to be seen.

The veteran lawmaker, who fought in Vietnam and was a prisoner of war for more than five 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.