ATLANTA (AP) — 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.