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Jimmy Carter’s melanoma: The former president will be treated with one of the newest drugs available

Former president Jimmy Carter has metastatic melanoma, he announced at a news conference at the Carter Center in Atlanta Thursday. A small mass was removed from the 90-year-old's liver on Aug. 12, and scans at Emory University later revealed four "spots of melanoma," each about 2 millimeters, on his brain.

"When I found out it was in four places in my brain," he said, "I just thought I had a few weeks."

He added that he was surprised that he "didn't go into an attitude of despair."

[Former president Jimmy Carter says cancer has spread to his brain]

Carter did not discuss his prognosis, but said he will begin radiation treatment later Thursday. He will undergo four treatments, each three weeks apart. Calling the recent events a "new adventure," he will also undergo infusions of one of the newest drugs available for the treatment of melanoma.

Keytruda, which will be used to enhance Carter's immune system, has only been available for about a year, according to Tim Turnham, executive director of the Melanoma Research Foundation.

"Since March of 2011 more than a half dozen new drugs," have been developed to fight melanoma, Turnham said. "Five years ago [Carter's] doctors would have had nothing available, especially for someone his age."

The side effects of both radiation and immunotherapy, he said, should be well tolerated by Carter, who is otherwise healthy.

[The ‘breakthrough’ melanoma drug doctors have given Jimmy Carter]

Because it is a drug to boost the immune system, Keytruda's side effects could cause inflammatory responses in other parts of the body, such as the gut, potentially causing diarrhea and dehydration, but "there's no reason to think a healthy 90-year-old couldn't tolerate it," Turnham added.

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, according to the American Cancer Society.

Most skin cancers are caused by excessive exposure to ultraviolet radiation, according to the American Melanoma Foundation. White males over the age of 50 are at a higher risk than the general population.

Carter said his doctors have not yet identified where his melanoma originated. Only rarely does it begin someplace other than the skin, but experts says about 4 percent of cases originate in the eyes and  2 or 3 percent in the mucosal membranes, such as the sinus and the mouth. But it is not unusual for a skin cancer to go undetected for years.

"Melanoma can sit idle for a long, long time," Turnham said. "Some people are diagnosed their 50s and 60s that go back to blistering sunburns when they were children."

In cases where the lesions or tumors are unable to be removed by surgery or radiation, the average length of survival is 11 months.

The radiation that Carter likely will undergo, said Turnham, is stereotactic. Instead of a single beam of radiation pointed at a brain lesion, which essentially destroys everything in its path, several lower-powered beams will be directed at angles, converging on the tumor site. The most typical side effect would be a headache.

After removal of the mass and one-tenth of his liver in early August, Carter said he has not had "any unpleasantness yet" as a result of his medical treatment.

The 39th president of the United States, who has maintained an active life in the decades since his presidency, said he and his wife, Roslyn, will be cutting back on their activities at the Carter Center, though he will "continue to sign letters asking for contribution.

[Opinion: Jimmy Carter has long pursued peace, justice and care for those on the margins]

In October, if he is medically cleared, he said he "really wants to go to Nepal to build houses" for Habitat for Humanity, which would mark his 33rd year working for the charity.

In May, Carter said, he had a bad cold, so doctors did a thorough physical exam. A PET scan discovered a likely cancerous mass on his liver. There are no signs, he said, of pancreatic cancer, which killed his father, a brother and two sisters,

"I've been as blessed as any human being in the world," he said. "I'm ready for anything."

Turnham said Carter's treatments are by no means extreme measures or out of the norm for the treatment of melanoma, even in a man as elderly as the former president. But because the drug is so new, he said it was impossible to predict how long Carter could live with the disease.

"We just don't know at this point," he said. "His odds are so much better now than three or four years ago,"