Former US president Jimmy Carter says his brain cancer has disappeared. His doctors found four small “spots of melanoma”, he said in August, after an operation to remove a tumour from his liver. It sounded like a fatal diagnosis at the time, but the former president said then he was “perfectly at ease with whatever comes”. And now it appears to have gone.

It is not a miracle, however much it may sound that way. The former president’s doctors believe the melanoma – normally a skin cancer but sometimes forming inside the body – was the primary cancer, and that it had spread to the liver. Carter was given pembrolizumab, one of the most exciting new drugs in cancer treatment today.

Pembrolizumab, sold under the brand name Keytruda in the United States, is one of the first immunotherapy drugs. Instead of killing cancer cells, these drugs boost the immune system to do the job. The theory behind immunotherapy has been around for decades, but it is only in very recent years that scientists have been able to put it into practice with, so far, just a couple of remarkably successful drugs.

Just over a year ago, Keytruda was approved for use in melanoma by the Food and Drug Administration (FDA), which is the licensing authority in the US. Trials showed it was a bit more effective and had fewer side effects than ipilimumab, which was the first breakthrough immunotherapy drug. Survival rates among patients with advanced melanoma who were given pembrolizumab were good at 12 months – 74% given doses every two weeks were still alive a year later.

It is possible that pembrolizumab has cleared the cancer in Carter’s brain, but the drug is so new that nobody can yet know how likely a recurrence is or when that might occur.

The other concern is that Carter’s cancer has already spread from the brain to the liver. Although surgeons no doubt removed all of the liver tumour and Carter will have had radiotherapy and probably chemotherapy as well, there is the possibility of further spread.

But the cancers have been slow growing and it is entirely possible, at Carter’s age of 91, that he will die with it, rather than from it.