The cancer discovered in former President Carter's liver has spread to his brain, he announced Thursday during a press conference in Atlanta.

While doctors thought an operation performed earlier this month had removed all the cancer in Carter's liver, an MRI scan later found four new spots of melanoma on his brain. The cancer, Carter said, “is likely to show up in other places in my body.”

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"I'm perfectly at ease with whatever comes," Carter said, adding that his situation is "in the hands of God."

He will begin radiation treatment immediately.

Carter, who at age 90 is the second-oldest living president, behind George H.W. Bush, said he plans to cut back his work for the Carter Center "fairly dramatically," though he would continue to make calls and sign letters for donations.

He said he received numerous calls after his initial diagnosis, including from President Obama, Vice President Biden, Bill and Hillary ClintonHillary Diane Rodham ClintonTrump's economic approval takes hit in battleground states: poll This is how Democrats will ensure Trump's re-election The Hill's Morning Report - Trump takes 2020 roadshow to New Mexico MORE and George H.W. Bush, who called Wednesday.

Secretary of State John KerryJohn Forbes KerryLet's not play Charlie Brown to Iran's Lucy The Memo: Democrats struggle to find the strongest swing-state candidate 2020 caucuses pose biggest challenge yet for Iowa's top pollster MORE also called him, he said.

"First time he had called me in a long time," Carter quipped, drawing laughter.

Carter disclosed publicly last week that he had cancer, and would arrange to undergo treatment at Emory Hospital in Georgia. Doctors removed a tenth of his liver during surgery on Aug. 3, he said Thursday.

"I was surprisingly at ease. Much more than my wife was," Carter said.

Carter cut short a trip to Guyana in May due to "a very bad cold," he said. Later that month, he found there was a spot on his liver, though he did not tell wife, Rosalynn, about it until June 15.

A combination of factors, including a book tour and travel by his doctor, kept him from pursuing further medical care for several weeks.

Carter's family has a history of pancreatic cancer, which claimed the life of his father, two sisters and brother. His mother had the disease as well.

— This story was last updated at 10:56 a.m.