Glioblastoma multiforme -- the type of tumor that U.S. Sen. John McCain, R–Ariz., has been diagnosed with -- is an aggressive form of brain cancer that is rare and typically occurs in adults.

According to the American Brain Tumor Association, there are about 12,390 new cases a year in the U.S.

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McCain had undergone surgery last week to remove a blood clot from above his left eye. Doctors at the Mayo Clinic in Phoenix said the clot was an indicator that the tumor started to grow.

Patients with this aggressive form of cancer face a grim survival rate. About 4 percent of patients over age 55 live for five years.

McCain’s doctors managed to remove all of the tumor that was visible on the brain scans, but surgery is hardly ever enough. Glioblastomas plant microscopic roots that go deeper into the brain tissue, making the tumors aggressive and hard to target.

Standard treatment following surgery includes a combination of chemotherapy and radiation, which can take weeks to months. Even among those who respond well to initial treatment, the cancer can come back, often within 12 to 24 months.

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McCain is a long-term survivor of melanoma but doctors classified this new cancer as a "primary tumor," meaning it's not related to his former malignancies.

The Associated Press contributed to this report