Senator Edward Kennedy, a legendary political figure in the United States, died on Tuesday after a year long battle with malignant glioma, the most common type of brain cancer among adults. Although he had the most up-to-date treatments - surgery at Duke University Medical Center, chemotherapy and radiation - he was not able to survive the disease. Doctors still don't know what causes brain tumors and the National Cancer Institute says the outlook for patients with malignant gliomas is poor.

Senator Kennedy's brain tumor was diagnosed in May 2008 after he had a seizure, a frequent symptom of brain cancer.

At Duke University Medical Center, he had surgery to remove the tumor. His doctors said it was a success and Kennedy was discharged six days later.

Neurosurgeon Dr. Anthony Caputy says success means removing about 90 percent of the tumor. "Because then, the outcome is much better for the other treatments if you get to 90 percent," he said.

Kennedy had what's called a craniotomy, which involves opening the skull to expose the surface of the brain.

Surgeons then determine the exact location of the tumor, often with the help of an MRI or CT (cat) scan.

During part of the procedure, patients can be brought back to consciousness. That helps the doctors avoid cutting in areas that control speech. "The brain will not feel any pain, so you can stimulate while the patient is awake and test the motor function, the speech functions while the patient is awake which allows very accurate determination of where these eloquent areas of the brain are," Dr. Caputy explains.

Doctors still don't know what causes brain tumors. They do know what the risk factors are and Senator Kennedy had them all.  Men are more likely to get brain tumors than women, as are white people and those 70 years or older.

Researchers are trying to discover the cause so they can treat it better. Dr. Markus Bredel at Northwestern University analyzes the genetic makeup of brain tumors, specifically gene mutations within glioblastomas, an even more aggressive form of the disease. But there are hundreds of thousands of genes in the tumors' genome.

"The difficult question is which of those many, many genes are actually important in the disease process and which are just simply bystanders to the process," Dr. Bredel said.

Senator Kennedy's outcome was expected. And American flags are now at half staff in his honor. The National Cancer Institute says brain tumors rank among the highest causes of cancer deaths.

Doctors caution that even with treatment, these aggressive tumors always come back. What's not known is when.