BOSTON — A cancerous brain tumor caused the seizure Sen. Edward M. Kennedy suffered over the weekend, doctors said Tuesday in a grim diagnosis for one of American politics' most enduring figures.
The Massachusetts Democrat has a malignant glioma in the left parietal-lobe, according to doctors at Massachusetts General Hospital, where Kennedy, 76, has been undergoing tests since Saturday after having a seizure at his Cape Cod home.
The usual course of treatment includes combinations of radiation and chemotherapy, but Kennedy's treatment will be decided after more tests.
"He has had no further seizures, remains in good overall condition, and is up and walking around the hospital," said a joint statement issued by Dr. Lee Schwamm, vice chairman of the Department of Neurology at Massachusetts General Hospital and Dr. Larry Ronan, Kennedy's primary care physician.
The doctors said Kennedy will remain in the hospital "for the next couple of days according to routine protocol."
"He remains in good spirits and full of energy," they said.
Son by his side
Kennedy's wife and children have been with him each day since he was hospitalized. Senator Kennedy's son, Rep. Patrick Kennedy, D-R.I., plans to stay at the hospital for the time being.
"Obviously it's tough news for any son to hear," said spokeswoman Robin Costello. "He's comforted by the fact that his dad is such a fighter, and if anyone can get through something as challenging as this, it would be his father. So he's optimistic, he's hopeful, but obviously he's concerned."
Video: Obama: Diagnosis 'heartbreaking' President Bush was notified by his staff of Kennedy's diagnosis at 1:20 p.m.
"He said he was deeply saddened and would keep Sen. Kennedy in his prayers," spokeswoman Dana Perino said.
Malignant gliomas are a type of brain cancer diagnosed in about 9,000 Americans a year — and the most common type among adults. It's a starting diagnosis: How well patients fare depends on what specific tumor type is determined by further testing.
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Average survival can range from less than a year for very advanced and aggressive types — such as glioblastomas — or to about five years for different types that are slower growing.
Surgery can be an option for some types, especially to reduce symptoms as a tumor enlarges and puts pressure on the rest of the brain. Many gliomas infiltrate normal brain tissue instead of forming a solid mass, making it hard to remove much of the tumor.
In a sadly ironic twist, just two weeks ago Kennedy called for a new “war on cancer,” saying at a Senate hearing that he planned to introduce legislation to encourage more coordination of cancer research, prevention and treatment.
“Cancer is still the second-highest cause of death in America,” Kennedy said at the hearing. “Clearly, we need a new way forward in battling this frightening disease. We must build on what the nation has already accomplished and launch a new war on cancer for the 21st century.”
Senators bow heads in prayer
Senate Democratic and Republican leaders both interrupted their parties' regularly scheduled party luncheons to announce the news about Kennedy. Republicans bowed their heads and said a prayer. Sen. John Kerry, D-Mass., told colleagues that Kennedy is optimistic.
Video: Brian Williams: Kennedy is nucleus of modern family "I'm having a hard time remembering a day in my 34 years here I've felt this sadly," said Sen. Patrick Leahy, D-Vt.
"I'm really sad. He's the one politician who brings tears to my eyes when he speaks," former Sen. Bob Kerrey, D-Neb., said when told in a Senate hallway about Kennedy's condition.
New York Sen. Charles Schumer said, "We pray for him and know he will be back because he is a fighter, and we — and America — need him so."
Democratic Senator Robert Byrd of West Virginia broke down in tears on the Senate floor and said "Thank God for you, Ted."
"I am so deeply saddened I have lost the words," Sen. John Warner, R-Va., said in a Senate hallway. Warner said he and Kennedy had been friends for 40 years. Both served on the Senate Armed Services Committee together.
"We hope and pray that they will be able to treat it and that he will experience a full recovery," said presumptive Republican presidential nominee Sen. John McCain on his "Straight Talk Express."
"I have described Ted Kennedy as the last lion in the Senate," said McCain. "And I have held that view because he remains the single most effective member of the Senate."
Democratic presidential candidate Sen. Hillary Clinton offered her own well wishes to Kennedy, saying his "...courage and resolve are unmatched, and they have made him one of the greatest legislators in Senate history."
Her rival for the nomination, Sen. Barack Obama, called Kennedy a "fighter for his entire life," and said, "I have no doubt that he will fight as hard as he can to get through this."
And in a statement issued by the White House, President Bush also expressed his concern for the ailing Senator.
"Ted Kennedy is a man of tremendous courage, remarkable strength, and powerful spirit...We join our fellow Americans in praying for his full recovery."
The 'liberal lion'
The second-longest serving member of the Senate and a dominant figure in national Democratic Party politics, Kennedy was elected in 1962, filling out the term won by his brother John
Video: McCaskill on senator His eldest brother, Joseph, was killed in a World War II airplane crash. President John Kennedy was assassinated in 1963 and his brother Robert was assassinated in 1968.
Kennedy is active for his age, maintaining an aggressive schedule on Capitol Hill and across Massachusetts. He has made several campaign appearances for the Illinois senator in February, and most recently another in April.
He is the senior senator from Massachusetts, the Senate's second-longest serving member, and is not up for election again until 2012.
Were he to resign or die in office, state law requires a special election for the seat no sooner than 145 days and no later than 160 days after the vacancy occurs.
The law was changed in 2004, when Sen. John Kerry of Massachusetts became the Democratic presidential nominee and Republican Mitt Romney was the state's governor. Prior to the change, the governor would have appointed a replacement who would have served until the next general election.
That would created the opportunity to install a fellow Republican in office, something lawmakers in the vastly Democratic state wanted to avoid.
Among the potential candidates for a Senate vacancy would be Democrats Martha Coakley, the state's attorney general; Rep. Edward J. Markey, former U.S. Rep. Joseph P. Kennedy II and Kennedy's wife, Vicki.
Among the potential Republican candidates could be Romney or former Lt. Gov. Kerry Healey.
NBC News correspondents and The Associated Press contributed to this report.