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Sen. John McCain's Brain Cancer: What Are Glioblastomas?
Senator John McCain (R-Arizona)
Credit: Win McNamee/Getty

Sen. John McCain was diagnosed with a type of brain tumor called a glioblastoma, his office confirmed yesterday (July 19).

McCain had surgery on July 14 to remove a blood clot from the area above his left eye. After the procedure, pathologists analyzed the tissue around the clot, and found that the senator had a "primary brain tumor known as a glioblastoma," according to a statement from McCain's office.

But what are glioblastomas, and how serious are they? [5 Facts About Brain Cancer]

Glioblastomas are the most common type of cancerous brain tumor, according to the American Cancer Society (ACS). They can occur in people of any age but tend to happen more often in older adults, the Mayo Clinic says. McCain is 80.

The tumors arise from a type of cell in the brain called an astrocyte, according to the American Brain Tumor Association (ABTA). Astrocytes are star-shaped cells that help support the nerve cells in the brain. "Astrocytomas," or tumors that arise from astrocytes, make up about 20 percent of brain tumors, the ACS says. Glioblastomas represent about two-thirds of all astrocytomas.

Glioblastomas are "highly malignant," the ABTA says. This is because astrocytes reproduce very quickly. In addition, many blood vessels are connected to the astrocytes, supplying them, and the tumor, with the blood they need to grow.

A "primary" glioblastoma means the tumor formed very quickly, according to the ABTA. "Secondary" glioblastomas, on the other hand, may have started as less aggressive tumors that eventually became more aggressive.

Symptoms of glioblastomas are often caused by the tumor's rapid growth, which can put extra pressure on certain parts of the brain, according to the ABTA. Symptoms can include headache, nausea, vomiting and drowsiness, and, depending on where in the brain the tumor is located, also may cause problems with memory and speech, and visual changes.  

Treatment for glioblastomas includes surgery to remove the tumor from the brain, according to the Mayo Clinic. But because this type of cancer penetrates into normal, healthy brain tissue, removing the entire tumor isn't possible. Therefore, patients need other therapies, such as radiation and chemotherapy, to target the remaining tumor cells.

Glioblastomas can be "very difficult to treat and a cure is often not possible," the Mayo Clinic says. Rather, "treatments may slow [the] progression of the cancer and reduce signs and symptoms."

People with aggressive glioblastomas survive for a median of 15 months after they are diagnosed, the ABTA says. ("Median" means that an equal number of people survive for a longer amount of time and an equal number of people survive for a shorter amount of time.) The two-year survival rate is 30 percent.

Originally published on Live Science.