ATLANTA - Former President Jimmy Carter said Sunday that his latest brain scan found no evidence of melanoma, the serious form of cancer that doctors discovered this summer in his liver and brain.

Carter, who celebrated his 91st birthday in October, said four lesions on his brain were gone and the scan found no evidence of new cancer cells. Carter said he will continue receiving regular drug treatments to help his immune system fight the disease.

A look at more about his situation:

Does a cancer-free scan mean carter is cured? Short answer: No. Tim Turnham, executive director of the Melanoma Research Foundation, said melanoma has a "frightening ability" to return years into remission.

Doctors not involved with Carter's care said his medical team will perform regular scans of Carter's brain and liver where cancer cells had been, and they likely will check other parts of his body for any recurrence or new growth.

"For today, the news cannot be better," said Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society. "Circumstances may change over time, or he may be in a situation where it does not recur for many years or at all."

A spokesman for Emory's Winship Cancer Institute, where Carter is being treated, declined to comment Sunday due to patient privacy.

What kind of treatment will he receive now? Carter said in a statement Sunday that he will continue to receive doses of Keytruda every three weeks. The Merck & Co. drug was approved last fall for treating melanoma.

It removes a sort of cloaking mechanism that cancer cells use to evade attack by the immune system.

Dr. Douglas Johnson, a melanoma specialist at Vanderbilt-Ingram Cancer Center said Keytruda and similar immune therapy drugs have "really revolutionized" treatment of the disease.

"In general, people who've had a good response to these drugs continue the treatments," Johnson said, adding that clinical trials on the drugs didn't give a defined number of treatments.

How are these drugs changing melanoma care? Experts have called Keytruda and other immune therapy drugs "game-changing" for patients with the skin cancer that kills nearly
10,000 Americans each year.

Turnham said six drugs have been approved since 2011. They lift a brake on the immune system and encourage it to recognize and attack cancer cells. They also cause fewer side effects, unlike the nausea or hair loss often caused by chemotherapy, which targets all swiftly growing cells, like those in the stomach lining.

"Before these drugs were available, median life expectancy with melanoma (that had spread) was
11 months," he said. "Now about half of people will have good outcomes, living five or more years with no cancer or their cancer under control. That's amazing progress in a short period of time."

How has the treatment affected Carter? Carter told The Associated Press in November that he experiences few side effects from the drug treatments.

"I haven't been uncomfortable or ill after the treatments were over," he said. "So that part of it has been a relief to me and I think to the doctors."

In August, Carter had a portion of his liver removed. He also received a round of targeted radiation aimed directly at four small lesions on his brain.

He has remained active, despite telling reporters in August that he would scale back on work at The Carter Center, the human rights organization he founded after leaving the White House. Carter also has volunteered on a Habitat for Humanity build in Memphis, Tennessee, and is acting as a mediator for the Rev. Martin Luther King. Jr.'s children in a legal dispute.

Does his age matter? Johnson said even older patients can tolerate Keytruda and other immune therapy drugs well.

Someone like the 91-year-old Carter who has reported few side effects after several months is likely to see the same success. Less than 5 percent of people saw severe side effects, which can include inflammation of the colon or lungs, Johnson said.

"In the near-term, you'd say there is good reason to be quite optimistic," said Dr. Keith Flaherty, a melanoma specialist at Massachusetts General Hospital's Termeer Center for Targeted Therapies. "At the president's age, it's very likely he's going to enjoy excellent quality of life."

Winship Executive Director Dr. Walter Curran Jr. told reporters in August that a cure isn't the goal for Carter's medical team.

"The goal is control and to have a good quality of life," he said.