ATLANTA (AP) — Former President Jimmy Carter says he will step back from his humanitarian work and surround himself with his family as he undergoes three months of treatment for melanoma cancer.

This weekend, relatives will gather in his tiny hometown of Plains, Georgia, to celebrate his wife's 88th birthday. He plans to teach Sunday School at his church, as he often does. And on Oct. 1, Carter will turn 91.

The former president was relentlessly upbeat Thursday, making jokes and flashing his wide smile during an open and honest 45-minute news conference about his cancer diagnosis and treatment. He said he was "ready for a new adventure" and felt his life's work was not done.

"Within the bounds of my physical and mental capability I'll continue to do it," Carter said. "But I'm going to have to give the treatment regimen top priority."

His spirits only seemed to dampen when he expressed doubt about being able to participate in a home-building mission in Nepal this November with Habitat for Humanity. The trip would have been the 33rd for Carter and his wife, Rosalynn. Instead, he said, family members may have to take his place.

Carter received targeted radiation therapy Thursday. It was aimed at four small tumors in his brain.

Earlier in the week, he received an injection of a newly approved drug to help his immune system seek out and destroy cancer cells that may develop anywhere else in his body. He will have that treatment three more times at three-week intervals, and there could be other radiation treatments, if needed.

Doctors also removed a small tumor from his liver on Aug. 3.

He said Thursday that he felt little pain or weakness and had slept well after receiving his first injection of pembrolizumab, which was approved by the FDA for melanoma patients last fall.

Jason Carter, his grandson, said the extended Carter family already had plans to gather in Plains for Christmas this year. In the meantime, Jason Carter said he expects his grandfather to spend time with his wife and do a lot of fishing.

"For the longest time, I think a lot of people have looked at him as someone who's just going to go on forever," Jason Carter said. "There's this old gospel song we were just talking about that says 'I'm going to stay on the battlefield,' and that's always the way that he's approached his life."

Carter said as much as he's able, he will continue lecturing at Emory University, raising money for the Carter Center's $600 million foundation, and meeting with experts on guinea worm and other diseases the center is working to eradicate.

"We understand that he has to focus on his treatment, and that's what we want him to do," the center's CEO Mary Ann Peters said. "The best thing we can do is continue to do our job so that when we report to him, we're not creating any problems."

Carter's team at Emory Healthcare includes Dr. Walter Curran, Jr., who runs Emory's Winship Cancer Institute. Treatments for melanoma have improved tremendously recently, and Carter's prospects are good even at the age of 90, Curran said. But he cautioned against the idea that Carter can be "cured."

"We're not looking for a cure in patients who have a disease like melanoma that has spread," Curran said. "The goal is control and to have a good quality of life."

Doctors told Carter they had completely removed cancer from his liver during surgery on Aug. 3, but an MRI exam that same afternoon showed the spots on his brain. Carter said he went home that night thinking he had only a few weeks to live, but found himself feeling "surprisingly at ease."

"I've had a wonderful life," Carter reflected. "I've had thousands of friends, I've had an exciting, adventurous and gratifying existence. So I was surprisingly at ease, much more so than my wife was."

Carter didn't discuss his long-term prognosis.

He said the path toward his cancer diagnosis began in late May, when he departed an election monitoring trip to Guyana early because of a bad cold. Doctors found a spot on his liver during a follow-up exam, and recommended its removal after a scan. But he wanted to complete a book tour before the surgery, and only told others about the diagnosis once it was certain.

More tests have failed to determine where his melanoma began or how it may have spread, but Curran said not knowing where it started won't hinder the treatment, and Carter said the cancer still could show up in other places.

Carter served in submarines in the Navy and spent years as a peanut farmer before running for office, becoming a state senator and Georgia governor. His "plainspoken" nature helped Democrats retake the White House in 1976. On Thursday, he said he remains proud of what he accomplished as president, but more gratified by the humanitarian work he's done since, which earned him a Nobel Peace Prize in 2002.

"When he first got the news that there was melanoma in his brain, he was worried that he wasn't going to get to finish the book he was reading," Jason Carter said. "But now he is clearly, having spoken with the doctors, he understands that there's a period of time and he's going to be able to go catch some fish, and hopefully catch more of his grandkids' baseball games."

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