Former President Jimmy Carter announced on Thursday that the cancer spreading through his body is melanoma and has been found on four spots on his brain. He said he will begin radiation treatment today. He has also started receiving a new type of drug therapy that could make his prognosis far more promising than it would have been just a few years ago.

In a news conference at the Carter Center in Atlanta this morning, the former president spoke calmly and candidly about his diagnosis, which he learned of after a cancerous mass was removed from his liver during a procedure on August 3.

He told reporters that while initially, he thought he only had weeks to live, he is now "at ease" with the situation.

Get Breaking News Delivered to Your Inbox

"I'm perfectly at ease with whatever comes," he said. "I'm ready for anything. I'm looking forward to a new adventure."

Mr. Carter, who has a family history of pancreatic cancer, said he followed the advice of his doctors to pursue treatment and has four radiation treatments scheduled at three-week intervals.

CBS News medical contributor Dr. David Agus said that the drug therapy Mr. Carter is receiving, called pembrolizumab or Keytruda, has been successful in many patients since it was approved by the FDA last year and provides hope where even a few years ago, there was none.

"What it does is it takes a brake off the immune system. Cancer cells, particularly melanoma, have a 'don't eat me' signal on their surface for the immune system, and this blocks it," Agus said. "It's an antibody, a protein that's administered every three weeks, and in about 40 to 50 percent of people there's a pretty dramatic response. That response can sometimes last for a year or two."

Though there are some side effects, Agus said the drug is "very well tolerated" overall.

"There is significant hope with this disease," he said. "Several years ago there was almost no hope with metastatic melanoma."

And while Mr. Carter's age - he is 90 years old - has been a topic of speculation since he first announced that he has cancer last week, experts say it may not be as big a challenge as one might think.

"The therapies that have been developed in the past several years, including the drug that he's being offered, can be offered with reasonable consonance in terms of effectiveness and relatively little concern that an older person would suffer significant side effects," Dr. Keith Flaherty, an oncologist specializing in melanoma and Director of Clinical Research at Massachusetts General Hospital Cancer Center, told CBS News. "In our practice, we generally treat patients, including patients in their 90s, with a reasonable degree of consonance that they can respond to the same degree as patients in their 60s or 40s."

Though most cases of melanoma begin as skin cancer, Mr. Carter said his doctors have not yet determined where his cancer originated.

"Most if not all melanomas really are skin cancers," Agus explained. "Most of the time you can find the original lesion in the skin where it came from, but sometimes that lesion actually goes away and you can't find it."

While the majority of melanoma cases can be detected early and are treatable, "it is when it spreads internally to the body that it can be life-threatening. Then it is really no different from other cancers that have spread for its site of origin," Flaherty said.

Mr. Carter said Thursday that he hasn't felt any weakness, and that any pain associated with his cancer or treatment has been very slight thus far.

When asked about Mr. Carter's prognosis, Agus said there is reason to be optimistic.

"Several years ago if he had been diagnosed with this, the prognosis would have been dismal and we probably wouldn't have treated him at all," he said. "But in today's world with this immunotherapy he's getting, along with this very targeted radiation therapy of the small lesions in the brain, his prognosis for the next year or two is actually somewhat optimistic in that about half the people can live for a year or two with very little symptoms with this particular treatment."