NEW YORK - Former US President Jimmy Carter said Thursday that doctors had found four spots of melanoma cancer on his brain and he will begin radiation treatment right away.

Carter, 90, said it was unclear where the cancer originated and that it was likely to also show up in other places in his body. He plans to cut his nonprofit work “rather dramatically” to undergo treatment, which will include four rounds of radiation therapy every three weeks. In a televised news conference remarkable for its candor, Carter spoke calmly as he talked about his diagnosis from the Carter Center in Atlanta, Georgia, and said he was “surprisingly at ease” when doctors told him that the recently discovered cancer had spread to his brain.

After initially hearing the news, “I just thought I had two weeks left,” Carter said. However, he said, “I’ve had a wonderful life, I’ve had thousands of friends, I’ve had an exciting, adventurous, gratifying existence.

I was pleasantly surprised that I didn’t go into an attitude of despair or anger or anything like that.” Carter did not specifically address his prognosis and his attitude was optimistic Thursday as he occasionally smiled and joked while discussing his treatment.

“I’m perfectly at ease with whatever comes. I’m ready for anything,” Carter said. “I’m looking forward to a new adventure.” Carter said he would cut back his role at the Carter Center and follow the advice of his doctors at Emory University, though he will continue to have some meetings and to teach Sunday school at church. “I’ll do what the doctors recommend for me to extend my life as much as possible,” Carter said, adding that he had already taken medication intravenously on Wednesday. “I don’t anticipate any trouble and pain and suffering.”

The cancer’s spread was discovered after an Aug. 3 surgery in Atlanta to remove a 2.5 cubic centimeter melanoma tumor that had been detected in the 39th president’s liver, which doctors think spread from somewhere else in Carter’s body. He said Thursday about one-tenth of his liver was removed. After that procedure, a scan of Carter’s head and neck that same day revealed four “very small spots” on his brain measuring about 2 millimeters, he said. “So far the only place I’ve known about the cancer is on my liver and my brain,” Carter said.

Carter said he felt good, with only slight pain after the liver surgery. He first discovered he had a “spot” on his liver toward the end of May, after he fell ill with a cold while in Guyana to observe elections and had to cut his trip short. Doctors discovered the mass during an examination.

Carter said he didn’t tell his wife, Rosalynn Carter, about the suspicious mass until June 15, and he didn’t tell key members of the rest of his family until a biopsy had confirmed it was cancer. Carter, who will turn 91 in October, has largely been in good health through his old age, traveling extensively for nonprofit work around the world. The Democrat from Plains, Georgia, whose full name is James Earl Carter Jr., is the second-oldest living president after George H.W. Bush, 91. Raised on a peanut farm, he was a relative unknown as Georgia governor when he launched his campaign that unseated President Ford in the 1976 election.

At the time, the nation was reeling from the Watergate scandal, President Nixon’s resignation and Vietnam.Carter’s signature achievements as president were primarily on the international front. They included the Camp David peace accords between Egypt and Israel, which he personally brokered and which have endured through more than three decades of strife in the Middle East. At home, though, his presidency was buffeted by crises - rampant inflation, gas lines and high unemployment - as well as by his administration’s inability to win the release of 52 Americans held hostage in Iran for 444 days, the last 15 months of his term in office.

In 1982, Carter and his wife, Rosalynn, founded the Carter Center, which pressed for peaceful solutions to world conflicts, promoted human rights and helped work to eradicate disease in poor nations. The center, based in Atlanta, launched a new phase of Carter’s public life that would earn him a Nobel Peace Prize in 2002.