Sen. Edward M. Kennedy was awake for much of the targeted brain surgery he underwent Monday at Duke University Medical Center.

The 76-year-old Democrat was diagnosed last month with a malignant glioma, an especially lethal type of brain tumor. Kennedy's operation started at 9 a.m. Monday in Durham, N.C., by Dr. Allan Friedman.

The surgery lasted about three-and-a-half hours and was successful, Friedman said. Kennedy will also undergo chemotherapy and radiation.

"I am pleased to report that Senator Kennedy's surgery was successful and accomplished our goals," Friedman said in a prepared statement. "Sen. Kennedy was awake during the resection, and should therefore experience no permenant neurological effects from the surgery."

Kennedy was unconscious for the first part of the surgery.

“Once they do the painful part of the surgery – removing the bone flap, they will wake him up and put electrodes on the surface of the brain,” said Dr. Lynne Taylor, a neuro-oncologist at Virginia Mason Medical Center in Seattle, Wash., who has not treated Kennedy. “They will make him speak and point out different objects to find out where the expressive and receptive language center is. They want to mark that in real time in the patient’s brain and work around that.”

After Kennedy can identify objects and answer the doctor’s questions, doctors will begin to remove the tumor, which will also be performed while he is awake, but under sedation, Taylor told FOXNews.com.

Targeted brain surgery is a delicate balance — removing as much tumor as possible improves cancer control, but there's also the risk of harming healthy brain tissue that lets patients walk and talk.

"The surgeon usually does as much as possible within the bounds of safety. We do not want to do neurological damage in an effort to remove as much of the tumor as possible," said Dr. Mark Gilbert, a brain tumor expert at the University of Texas M.D. Anderson Cancer Center in Houston. Gilbert spoke in Chicago from a conference attended by 30,000 cancer specialists.

Taylor said it is hard to predict what the surgery’s benefits to Kennedy will be because the senator’s camp is being careful not to give out too much information on his condition. The risks should be minimal if it is performed correctly.

“If done properly, there shouldn’t be much bleeding,” said Taylor, who is a fellow with the American Academy of Neurology. “The most common issue is that you can’t get out as much of the tumor as you had hoped. The physical characteristics of the tumor could be that it is stuck to blood vessels or the tumor infiltrates to eloquent cortex, which is the part of the brain that affects your speech or is important for vital brain function.”

More than 18,000 primary malignant brain tumors are diagnosed each year in the United States; about 9,000 of those are malignant gliomas, according to the National Cancer Institute.

In general, half of all patients die within a year.

These tumors are the second-most common cause of cancer deaths in the 15 to 44 age group. Median survival for patients with moderately severe malignant gliomas is three to five years.

The Associated Press contributed to this story.

Click here to watch a video about the surgery.