By Matt Pearce, Los Angeles Times (TNS)

The nation’s 39th president, wearing jeans with a red tie and sport coat, strode in to deliver some bad news: He had cancer, it had spread to his brain, and he would be undergoing radiation treatment immediately.

But Jimmy Carter was smiling.

“I’ve had a wonderful life. I’ve had thousands of friends. I’ve had an exciting, adventurous, gratifying existence,” Carter, 90, said during a televised Thursday morning news conference at the Carter Center in Atlanta. “I’ll be prepared for anything that comes.”

Even while facing one of the greatest challenges of his life, Carter once again demonstrated the candor and vitality that has distinguished his post-presidential career as a Nobel Peace Prize-winning humanitarian and international icon.

Carter sometimes cracked jokes and grinned as he discussed the internal melanoma that had spawned a tumor in his liver, four small masses on his brain and was expected to spread.

When asked whether he would have done anything differently, Carter drew laughs when he said he wished he’d sent “one more helicopter” on the botched 1980 rescue attempt of American hostages in Iran — the defining failure of the Democrat’s one-term administration.

“We would have rescued them, and I would have been re-elected,” grinned Carter.

Carter did not directly give a prognosis but was clearly optimistic.

Even as he said he would “fairly dramatically” cut back his work with his beloved Carter Center — the nonprofit he created with his wife, Rosalynn, in 1982, and which he said he would have chosen over another four years in office — Carter promised to continue contributing and to teach Sunday school at church.

“It won’t be tough on my part,” Carter said of his treatment. “I’ll do what the doctors recommend for me to extend my life as much as possible.”

The facts Carter revealed were worrisome: Doctors still don’t know the source of the melanoma, which was first detected in his liver during an exam in May when Carter came down with a cold. (Carter had been in Guyana to observe an election and had to cut the trip short.)

Doctors removed a malignant tumor measuring 2.5 cubic centimeters and about one-tenth of Carter’s liver in early August. After they did, medical scans of his head and neck revealed four small spots on Carter’s brain measuring about 2 millimeters.

No cancer has been discovered in Carter’s pancreas. Pancreatic cancer has killed four of Carter’s family members.

On Thursday afternoon, Carter underwent radiation treatment, and he could undergo more radiation treatments if necessary, although none are now scheduled, his office said.

Carter is expected to also undergo a total of four intravenous treatments with the melanoma drug pembrolizumab at three-week intervals, according to his office.

He said at the news conference that he would have four rounds of radiation therapy, but his office later issued a clarification to say it would be four treatments of the drug.

Carter underwent his first a round of intravenous medication Wednesday and said he was in little pain, which he hoped would continue.

“I don’t anticipate any trouble and pain and suffering,” Carter said.

The former president also remained hopeful that he might be able to go to Nepal on his annual trip to build houses with Habitat for Humanity, though it coincides with his final scheduled treatment.

“I’m going to have to give the treatment regimen top priority,” Carter said, adding that he would follow the advice of his doctors at Emory University.

Carter’s announcement Thursday followed a short statement last week confirming he had cancer.

He also revealed Thursday that he initially kept doctors’ reports of a mass on his liver secret from his wife for two weeks in early June. He delayed surgery on his liver to continue a book tour because doctors said the cancer was slow-growing.

Though after Carter discovered in August that melanoma had reached his brain, his gut reaction was that he might have only two weeks left to live.

That obviously wasn’t true. More than two weeks later, Carter was looking vibrant Thursday as he spoke about his 12 grandchildren and 10 great-grandchildren and of feeling “surprisingly at ease” as he faced the unknown.

“I was pleasantly surprised that I didn’t go into an attitude of despair or anger or anything like that,” said Carter, who turns 91 in October.

He declared that marrying Rosalynn was the “pinnacle” of his life, and he urged other cancer patients to hope for the best but also accept what comes.

“We all thought and we all continue to think that Carter will live forever,” Paul Costello, a former assistant press secretary to Rosalynn Carter, said Thursday, noting that Carter has outlived many of his close advisers.

“What struck me today, the characteristics of what and how he spoke embodied his life,” Costello said. “There was grace, there was humility and there was the great love of the most important person in his life, Rosalynn.”

Costello also spoke admiringly of Carter’s endurance, recalling a “grueling” trip to Nepal with Carter seven or eight years ago in which the former president exuded his usual boundless energy.

Six or eight months from now, Costello hopes Carter will get the label “cancer survivor.”

“If there’s anyone who epitomizes a survivor, it’s Jimmy Carter,” Costello said.

The Democrat from Plains, Ga., 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, Carter 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 Richard 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 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.

(c)2015 Los Angeles Times. Distributed by Tribune Content Agency, LLC.

Photo: Former President Jimmy Carter discusses his cancer diagnosis at the Carter Center Thursday, AUg. 20, 2015 in Atlanta, Ga. The 90-year-old announced he had cancer after doctors removed small masses from his liver earlier this month. (Bob Andres/Atlanta Journal-Constitution/TNS)