Jimmy Carter credits the new cancer drug Keytruda for shrinking his brain tumors completely. It’s one more possible victory for the newest class of cancer drugs that empower the immune system to fight off tumors.

But cancer experts note that it's not at all clear that the drug itself is what shrank Carter's tumors. The former president was also treated with radiation to his brain and had a large tumor on his liver removed.

While most immune therapy drugs boost the immune system so that it can battle the cancer, Keytruda was the first drug to take a different approach that disrupts a trick tumors use to hide from immune cells.

Keytruda — known generically as pembrolizumab — targets the activity of genes called PD-1 and PD-L1. The interaction between the two genes lets some tumors escape detection and destruction by immune system cells.

“It's as good of an outcome as someone with advanced melanoma can hope for."

PD-1 stops immune cells from attacking normal healthy cells by mistake. Tumor cells make PD-L1 turn on PD-1 when immune cells approach. Keytruda, an engineered immune protein called a monoclonal antibody, disrupts this cloaking effect and lets the immune cells do their job and eat the tumor cell.

Carter’s Sunday school class erupted in applause when he announced his latest checkup showed his tumors appear to have shrunk completely. “And when I went this week, they didn't find any cancer at all, so I have good news,” he told his overflowing class.

It’s unusual, but not unheard of. Carter has stage 4 melanoma, meaning it has spread around his body. It’s almost never cured, but even stage 4 cancer can shrink so much that scans don’t find it — for a while. It almost always comes back.

“It's as good of an outcome as someone with advanced melanoma can hope for,” said Dr. Len Lichtenfeld, deputy chief medical officer for the American Cancer Society.

The Food and Drug Administration gave Keytruda accelerated approval in 2014 for patients, like Carter, whose melanoma has spread. It was on the basis of a big study that showed Keytruda could shrink the tumors of about a third of patients by as much as 90 percent.

Dr. Antoni Ribas of the University of California Los Angeles led that trial. Ribas says between 70 and 80 percent of the patients whose tumors shrank on Keytruda are still continuing to see their tumors shrink.

“That’s what we expect from the immune system,” he said.

Kathy Thomas of Torrance, California, was one of them. Thomas went from being in a wheelchair to walking three miles a day. She'd tried other immune therapies such as interleukin and a drug called Yervoy, but the side-effects made her sick.

And her cancer kept coming back. She started Keytruda in 2012 and felt the effects after two rounds of treatment.

"My hair is back and I am fat and happy," she said. Thomas celebrated her 60th birthday this year by traveling to South America and the Caribbean. She's not cured, but says tumors on her liver shrink after every new round of treatment with Keytruda.

"There needs to be a category of cancer patients like me. I am not cancer-free but I am surviving with it and basically doing well,"she said.

No one’s sure why not all patients are helped by the drug, but it may be because different tumors employ different tricks to evade the immune system and chemotherapy drugs.

"My hair is back and I am fat and happy."

“One hopes that by using immunotherapy the body can respond to whatever happens but cancer cells are clever and can develop workarounds for the various treatments,” Lichtenfeld said.

“It's always tough to label someone cancer-free because there may be cells that doctors haven't detected, but it's possible that removing the mass in his liver several months ago allowed his immune system -- along with radiation and the drug Keytruda -- to fight the four lesions in his brain,” Lichtenfeld added.

Ribas agreed.

"The surgery and radiation may have done it. We may not know if he is responding or not,” he said.