BOSTON, Massachusetts (CNN) -- U.S. Sen. Edward M. Kennedy has a malignant brain tumor, doctors treating him at Massachusetts General Hospital said Tuesday.

Sen. Edward Kennedy suffered a seizure Saturday in Hyannisport, Massachusetts.

Kennedy, 76, was hospitalized Saturday morning after suffering a seizure at his family's compound at Hyannisport, Massachusetts.

"Preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma in the left parietal lobe," according to a statement from the doctors treating the senator.

Malignant glioma is the most common primary brain tumor, accounting for more than half of the 18,000 primary malignant brain tumors diagnosed each year in the United States, according to the National Cancer Institute.

White House spokeswoman Dana Perino said President Bush was "deeply saddened" by the news and would keep the senator in his prayers.

Kennedy had surgery in October to clear his carotid artery in hopes of preventing a stroke.

In recent days, the powerful Democrat had appeared to be in fine health. On Friday, he took part in a ribbon-cutting ceremony for a new maritime learning center in New Bedford, Massachusetts.

"Decisions regarding the best course of treatment for Sen. Kennedy will be determined after further testing and analysis," the doctors continued.

"Sen. Kennedy will remain at Massachusetts General Hospital for the next couple of days according to routine protocol. He remains in good spirits and full of energy."

Kennedy has represented Massachusetts in the Senate since 1962. He is one of only six senators in U.S. history to serve more than 40 years. He is known as a liberal champion of social issues such as health care, family leave and the minimum wage.

Senate Democrats and Republicans were in their weekly closed policy lunches in the Capitol when the news of Kennedy's diagnosis broke.

Majority Leader Harry Reid of Nevada announced the news to his Democratic colleagues. The wind was taken out of the room, said one senator. There was "stunned silence," said another