ATLANTA — 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, Ga., 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.