Former President Jimmy Carter is slated to begin radiation treatment for several melanoma spots on his brain Thursday afternoon, he said at a news conference in Atlanta.

Carter, 90, told reporters he had a mass removed from his liver on Aug. 3, which he learned was melanoma.

Asked how he felt when he learned of the diagnosis, Carter said: "I just thought I had a few weeks left. But I was surprisingly at ease. I've had a wonderful life, I've had thousands of friends, and I've had an exciting and adventurous and gratifying existence. So I was surprisingly at ease."

"I'll be prepared for anything that comes," he added.

His family has a history of pancreatic cancer. His father, both his sisters and his brother died of pancreatic cancer, and his mother had pancreatic cancer as well.

Carter, who has been active since leaving the Oval Office, has worked with groups such as Habitat for Humanity, winning the Nobel Peace Prize in 2002 and founding the nonprofit Carter Center in his home state of Georgia.

He will step back from his work at the Carter Center, however he will continue attending some meetings and sign letters as well as other lighter duties.

"I can't really anticipate how I'll be feeling obviously," he said of his expectation of radiation treatments.

He expects that other cancers will be discovered as treatment progresses.

"I am perfectly at ease with whatever comes," he said, adding that he has had a "good life", thousands of friends and has been involved in satisfying work. He also cited his deep spiritual faith as something that has helped sustain him after the diagnosis.

"Now I feel it's in the hands of God," he said.