Jimmy Carter spoke plainly when he said Thursday that doctors had found “four spots of melanoma” on his brain. The 90-year-old former president was relaxed and matter-of-fact as he talked about the uncertainty he faces. “I’m perfectly at ease with whatever comes,” he said.

Flanked by family members and friends at a news conference at the Carter Center in Atlanta, he detailed the treatments that he has already begun and will continue in coming weeks, including radiation and the IV infusions of a new type of anti-cancer drug that tries to harness the body’s immune system to fight the disease.

The brain lesions, each no larger that 2 millimeters, were discovered after an Aug. 3 operation at Emory University to remove a tumor from his liver. During the surgery, Carter said, doctors suspected that the cancer had originated in another part of his body and performed full-body scans.

When he found out he had brain cancer, he said, “I just thought I had a few weeks” but “didn’t go into an attitude of despair.” He called the recent events a “new ad­ven­ture.”

Carter did not discuss his prognosis, other than to say that he had radiation treatment scheduled for later Thursday. His office said he could undergo additional treatments if needed. The former president added that several members of his immediate family — his brother, two sisters, mother and father — all died of cancer.

How the new drugs work against cancer

Jeffrey Gershenwald, the medical director of the Melanoma and Skin Center at MD Anderson Cancer Center in Texas, said that, historically, a melanoma patient diagnosed with metastases in the liver and brain would face a very poor prognosis.

“When caught in [such an] advanced stage,” said the surgical oncologist, who is not involved in Carter’s care, “we measure median survival in months, not years.” But he said that Carter has far more options now than he would have had even a few years ago.

Carter said he is receiving one of the newest drugs in the anti-cancer arsenal, pembrolizumab, better known as Keytruda. The drug, the first in a promising new class of medications called immunotherapy, has been on the market for 11 months.

Carter’s melanoma was diagnosed after he fell ill with “a very bad cold” in May during a trip to Guyana to monitor elections there. He returned to Atlanta early, and doctors performed a complete physical examination.

By the end of June, doctors were certain that he would need to have an operation on his liver, Carter said, but he chose to delay surgery so he could finish a book tour for his memoir, “A Full Life: Reflections at Ninety.”

“The doctors told me that it was a very slow-growing cancer,” he said. “Apparently, it wouldn’t make any difference between the middle of July and August, so we scheduled it when I got through with the book tour.”

Former president Jimmy Carter reflects on the “ease” he felt when learning his cancer had spread to his brain and on the best things he has done in his life. (AP)

About 3.5 million cases of skin cancer are diagnosed every year in the United States, primarily basal cell and squamous cell carcinomas. Melanoma is rarer and deadlier, with about 70,000 cases diagnosed a year and resulting in nearly 10,000 deaths, according to the American Cancer Society. Doctors are not sure where Carter’s melanoma originated.

“Melanoma can sit idle for a long, long time,” said Tim Turnham, executive director of the Melanoma Research Foundation. “Some people are diagnosed in their 50s and 60s” for cancers that “go back to blistering sunburns when they were children.”

In the span of only several years, immune therapies have quickly become one of the most important new tools in cancer treatment, especially for melanoma.

“Five years ago, [Carter’s] doctors would have had nothing available, especially for someone his age,” Turnham said.

Louise Perkins, chief science officer for the Melanoma Research Alliance, the largest private funder of melanoma research, agreed that “the landscape of treatment for melanoma has changed dramatically.” But, she added, “There’s no universal cure.”

Keytruda, manufactured by Merck, has shown remarkable results in some patients, according to Antoni Ribas, a researcher at the University of California at Los Angeles and the lead investigator of a crucial study of Keytruda.

Ribas, who is not involved in treating the former president, said patients who benefited from the drug “responded immediately,” with some tumors shrinking or vanishing in a matter of weeks. “I have a series of patients who had weeks or months to live and who are now going on with their lives.” But he said that only about one third of patients had positive results.

Experts said that Carter is likely to tolerate his treatments well. He’s not receiving traditional chemotherapy, which can have severe side effects, such as nausea.

Carter is the second-oldest living president, separated by just over 100 days in age from George H.W. Bush. He served in the White House from 1977 to 1981.

After his presidency, he continued his humanitarian and human rights work through the Carter Center. He was awarded a Nobel Peace Prize in 2002 for that work.

Beyond his international travel schedule, Carter and his wife of 69 years, Rosalynn, have maintained a quiet but active life in Plains, Ga., his home town. He noted that he doesn’t expect the cancer treatment to halt his usual contributions to his place of worship, Maranatha Baptist Church.

“I plan to teach Sunday school this Sunday — and every Sunday as long as I’m physically and mentally able,” Carter said.

Carter noted at the news conference that get-well wishes have poured in from President Obama and all living former presidents, as well as from Secretary of State John F. Kerry and former secretary of state Hillary Rodham Clinton.

“It’s the first time they’ve called me in a long time,” Carter joked.

The former president said he will be treated at the Winship Cancer Institute, about two miles from the Carter Center. The institute and Carter’s organization are both affiliated with Emory University.

“I’ve been as blessed as any human being in the world,” he said.