FACTBOX: Gliomas are hard to treat

WASHINGTON (Reuters) - Sen. Edward Kennedy, a Massachusetts Democrat who outlived two assassinated brothers to become an elder statesman, has a brain tumor known as a glioma, his doctors said on Tuesday.

Here are a few facts about gliomas:

- Gliomas arise from brain cells known as glial cells, which are different from the better-known neurons.

- They can be high-grade, which are more dangerous, or low-grade, which are often slow-growing and require little or no treatment.

- The American Cancer Society estimates that 21,000 Americans a year are diagnosed with brain tumors. About 40 percent of these are gliomas.

- Brain tumors can be surgically removed but they are often difficult to reach for treatment. Some chemotherapy drugs can reach into the brain to attack tumor cells. Radiation therapy is also used to treat glioma.

- There are several types of glioma, including astrocytomas, ependymomas, glioblastoma multiforme, ologodendrogliomas and mixed gliomas. They are named according to the specific type of brain cell affected.

(Reporting by Maggie Fox; editing by Will Dunham and Mohammad Zargham)