ATLANTA » Former President Jimmy Carter announced Thursday that his cancer showed up in four small spots on his brain and he will immediately begin radiation treatment, saying he is "at ease with whatever comes."

"I’m ready for anything and looking forward to a new adventure," said Carter, appearing upbeat and making jokes as he openly talked about his cancer during a news conference.

So far, the pain has been "very slight" and Carter said he hasn’t felt any weakness or debility. Still, he will dramatically cut back on his work with the Carter Center and will give the treatment regimen his "top priority." His first radiation treatment was set for Thursday afternoon.

Carter, 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. He thought an operation Aug. 3 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. "It’s in God’s hands. I’ll be prepared for anything that comes."

He didn’t give any prognosis, but spoke about receiving three months of treatments and cast doubt on the possibility of traveling to Nepal in November to build houses for Habitat for Humanity, a Georgia-based organization he has worked with for decades. He said other family members may have to represent him there.

A small cancerous mass was removed Aug. 3 along with about a tenth of his liver and doctors believe they got rid of all the cancer there, Carter said.

It’s still not clear exactly where the cancer originated, although with melanoma, he’s told that 98 percent of the time it develops first in the skin. He also said that the rest of his body will be scanned repeatedly for months to come and that more cancers may show up elsewhere. The cancer spots on his brain are about 2 millimeters in size.

His father, brother and two sisters died of pancreatic cancer. His mother also had the disease. Carter, who had been tested for pancreatic cancer, said no cancer has been found there so far.

What the former president has, he said, is melanoma, and experts say his lifelong activities may have increased his risk for skin cancer. He lives in the South, is fair-skinned and freckled, and through Habitat for Humanity and travel, has spent a lot of time outdoors, noted Dr. Anna Pavlick, co-director of the melanoma program at NYU’s Laura & Isaac Perlmutter Cancer Center.

Carter said the radiation will focus on the tumors in his brain, and he has already begun receiving a drug to boost his immune system.

Dr. Patrick Hwu, a melanoma expert at the University of Texas MD Anderson Cancer Center, said the key immune system cells needed to attack the tumor can get into the brain, so the treatment gives Carter a fighting chance.

"Every patient is going to be different," he said.

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

"It was the first time they’ve called me in a long time," Carter said to laughter.

Carter’s health has been closely watched this year. He cut short an election monitoring trip to Guyana in May. A spokeswoman said he did not feel well and Carter later said he had a bad cold.

Carter was the nation’s 39th president, advancing as a virtual unknown on the national stage to defeat President Gerald Ford in 1976. But several foreign policy crises, in particular the Iran hostage crisis, crushed his bid for re-election and Ronald Reagan swept into the White House.

He said Thursday he still regretted not being able to rescue the hostages.

The native of tiny Plains, Georgia, rebuilt his career as a humanitarian guiding the center focused on global issues. Carter earned a Nobel Peace Prize in 2002, helped defuse nuclear tensions in the Koreas and helped avert a U.S. invasion of Haiti.

He and his wife, Rosalynn, still make regular appearances at events in Atlanta and travel overseas. When the couple is in Plains, Carter frequently teaches a Sunday School class before services at Maranatha Baptist Church. He plans to teach this weekend as scheduled.

"No matter where we are in the world, we’re always looking forward to getting home to Plains," Carter said.

He and his wife have thought for many years about cutting back their work at the Carter Center, which he established in 1982 to promote health care and democracy.

"We thought about this when I was 80. We thought about it again when I was 85; we thought about it again when I was 90. So this is a propitious time I think for us to carry out our long-delayed plans."

AP Chief Medical Writer Marilynn Marchione in Milwaukee contributed to this report.