Sen. Edward Kennedy’s (D-Mass.) seizure Saturday morning was apparently caused by a brain tumor, his doctors announced today.

In a statement, neurologist Lee Schwamm and Kennedy’s primary care physician Larry Ronan said “preliminary results from a biopsy of the brain identified the cause of the seizure as a malignant glioma in the left parietal lobe. The usual course of treatment includes combinations of various forms of radiation and chemotherapy. Decisions regarding the best course of treatment for Senator Kennedy will be determined after further testing and analysis. Senator Kennedy will remain at Massachusetts General Hospital for the next couple of days according to routine protocol.”

Still, the doctors at Massachusetts General Hospital said Kennedy has had no further seizures and “remains in good overall condition, and is up and walking around the hospital.”

Senate Majority Leader Harry Reid (D-Nev.) has informed the Democratic Conference.

The news was also announced to the Republican Conference, and a moment of silence was observed.

Sen. Robert Byrd (D-W. Va.) gave an emotional speech praying for Kennedy’s recovery on the Senate floor Tuesday afternoon.

“Ted, Ted, my dear friend, I love you and I miss you,” said Byrd, who was crying.

“Thank God for you Ted, thank God for you.”

Senate Majority Whip Dick Durbin (D-Ill.) said in a statement: “Ted Kennedy has spent his life caring for those in need. Now it’s time for those who love Ted and his family to care for them and join in prayer to give them strength.”

According to the Mayo Clinic, treatment for a glioma — and survival odds — depend on the tumor type, size and location, and the patient’s age and overall health. Gliomas can be complex, and a variety of techniques and procedures are needed to treat them.

According to the National Cancer Institute, malignant gliomas are the most common primary brain tumor, accounting for more than half of the more than 18,000 primary malignant brain tumors diagnosed each year in the United States. These tumors are the second-most common cause of cancer death in the 15 to 44 age group.

The outlook for patients with malignant gliomas is poor, said the institute’s Web site. Median survival for patients with moderately severe (grade III) malignant gliomas is three to five years. For patients with the most severe, aggressive form of malignant glioma (grade IV glioma or glioblastoma multiforme), median survival is less than a year.

It is unclear exactly what kind of tumor Kennedy has been diagnosed with.

First elected in 1962, Kennedy is the senior Senator from Massachusetts and the chairman of the Health, Education, Labor and Pensions Committee. He is the brother of the late President John F. Kennedy and Sen. Robert F. Kennedy.

Kennedy has long been one of the most powerful Senate Democrats and a key player in brokering deals on many vexing issues. His liberal politics have made him a national hero to some and a lightning rod to others.

Like his brothers, Kennedy also sought the presidency, in 1980. After his unsuccessful bid, he returned to the Senate, where he has made a career focusing on domestic matters such as health care, civil rights and education. For instance, he negotiated with President Bush to craft the No Child Left Behind Act, a sweeping education reform law that has been praised and panned.

Legislating aside, Kennedy has made his mark on the campaign trail as well. Kennedy dropped jaws earlier this year when he endorsed Sen. Barack Obama (D-Ill.) over Sen. Hillary Rodham Clinton (D-N.Y.) for president. Kennedy’s support was viewed as a major blow to Clinton and a boost to Obama, who at the time had not yet attracted the backing of the Democratic establishment.