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ATLANTA (CBSMiami/AP) — Former President Jimmy Carter held a press conference Thursday morning at the Carter Center in Atlanta to discuss his recent cancer diagnosis and revealed he will immediately begin radiation treatment.

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Carter, appearing in a dark blazer, red tie and jeans and surrounded by friends and family, said at first he thought the cancer was confined to his liver and that the operation earlier this month had completely removed it, “so I was quite relieved.”

But that same afternoon, an MRI showed it was on his brain.

“I just thought I had a few weeks left, but I was surprisingly at ease. I’ve had a wonderful life,” the 90-year-old Carter said.

These were his first public remarks since his cancer diagnosis. He underwent surgery on August 3 to remove the small mass on his liver. He said about one-tenth of his liver was removed.

Now he says four spots of melanoma were found on his brain and he will have his first radiation treatment Thursday afternoon. Carter lost his father, brother, and two sisters to pancreatic cancer. His mother also had the disease. Carter said no cancer has been found on his pancreas so far. He says treatment will be his top priority.

He says he will cut back “fairly dramatically” on work at his Carter Center in Atlanta.

Carter says he is not feeling despair or anger over his health. He says he feels good, with only slight pain.

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“I’m perfectly at ease with whatever comes. I’m ready for anything. I’m looking forward to a new adventure,” he said, appearing upbeat and making jokes as he openly talked about his cancer.

Carter said President George W. Bush and Bush’s father called him Wednesday, and he has received well-wishes from President Barack Obama, Bill and Hillary Clinton and Secretary of State John Kerry.

That’s “the first time they’ve called me in a long time,” Carter said to laughter.

The one-time peanut farmer and Georgia Governor burst into the national spotlight in the aftermath of Watergate and Vietnam.

After a single-term as President, Jimmy Carter went on have one of the longest and most active retirements of any President.

“The experiences that I’ve had since then have been the most gratifying, and the most challenging and the most adventurous and the most unpredictable in my life.”

Carter is 90-years-old. He was the nation’s 39th President, defeating Gerald Ford in 1976. Ronald Reagan succeeded him in 1980 after a landslide victory. Carter rebuilt his career as a humanitarian, founding the Carter Center in 1982 to focus on global health care and democracy. He won the Nobel Peace Prize in 2002.

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