Jimmy Carter's cancer has spread to his brain, the former president announced Thursday, and he is beginning a course of radiation and drug therapy to combat it.

Carter said he has been diagnosed with melanoma, a skin cancer that has progressed to his brain and liver. Doctors have discovered four  small spots of melanoma on his brain but have told Carter they are treatable. His surgeons removed the cancer from his liver in an operation earlier this month that took about 10 percent of the organ.

Carter, 90, announced the findings at a press conference Thursday at the Carter Center. First he thanked his wife, Rosalynn, then his Emory Healthcare doctors and then the large numbers of well-wishers who have expressed their support for him.

Wearing blue jeans and a coat and tie, Carter walked into the room with Rosalynn, but he appeared alone before the cameras. He said he will receive radiation and also drug therapy to treat the brain lesions. "It's likely to show up in other places in my body as the scans detect it," Carter said.

He spoke of the day of the surgery on his liver.

"At first, I thought it was confined to my liver and that the operation had completely removed it, so I was quite relieved," he said. But an MRI later that day revealed the spots on his brain.

"I just thought I had a few weeks left, but I was surprisingly at ease. I've had a full life, I have thousands of friends. .... So I was surprisingly at ease, much more so than my wife was. ...  I'm ready for anything and looking forward to a new adventure."

"I feel good," he said in response to a question. "I haven't felt any weakness or debility. The pain has been very slight."

He said President Obama has called to wish him well, as have Bill and Hillary Clinton and both Presidents Bush.

Melanoma is one of the most common cancers to spread to the brain, and a patient’s prognosis generally depends on factors such as the number and size of tumors, as well as their location in the brain.

Carter has an extensive family history of pancreatic cancer, but he said Thursday that his melanoma has not spread to his pancreas. He also acknowledged that this is a possiblility.

"Hope for the best, accept what comes," he said. "I think I have been as blessed as any human being in the world."

Asked about what he was most proud of, Carter replied: "The  best thing I ever did was marrying Rosalnn. That was the pinnacle of my life, and we've have 69 years together."

Asked whether he would do anything different, Carter drew laughs with a smiling but very frank statement, "I wish I'd sent one more helicopter to rescue the hostages.  I think I might have been re-elected. But if I had to choose four more years or the Carter Center, I think I'd choose the Carter Center."

Carter recently finished a nationwide tour for his latest book, called “A Full Life: Reflections at 90.” On Aug. 3, days after the tour ended, the Carter Center said the former president had had surgery to remove the mass from his liver.

Carter, a former peanut farmer who became Georgia’s governor, defeated Republican Gerald Ford in 1976 to become the nation’s 39th president. He established a national energy policy and brokered a landmark peace deal between Israel and Egypt. But the end of his one term in the White House was marred by an energy crisis and the Iranian hostage standoff.

In the 35 years since he returned to Georgia after his 1980 defeat to Ronald Reagan, he has logged millions of miles and visited dozens of countries on missions to promote voting rights, settle conflicts, advocate for human rights and combat deadly diseases such as malaria and guinea worm.

News of Carter’s cancer last week sparked an outpouring of prayer for the former president. Well-wishers at the Carter Center penned notes urging him to feel better, and the owner of Manuel’s Tavern hung a giant banner atop the popular Atlanta restaurant reading: “Get Well Soon, Jimmy.”

Residents of Plains, the town of about 700 where Carter lives, have rallied around their native son. On Sunday, he surprised many when he showed up at Sunday services at the nearby Maranatha Baptist Church, where he has long led Bible study. After the service, Carter seemed in high spirits.

“I plan to teach Sunday school this Sunday and every Sunday as long as I'm physically and mentally able," he said.