cIn what the senior study author is calling likely “the first time this has happened in the history of cancer,” the study sponsored by the drug company GlaxoSmithKline had patients take monoclonal antibody dostarlimab every three weeks for half of a year. The cancer was nowhere to be seen in follow-ups on MRI scans, PET scans, biopsies, endoscopic tests, and physical exams.

“There were a lot of happy tears,” Dr. Andrea Cercek, an oncologist at Memorial Sloan Kettering Cancer Center and a co-author of the paper, told the New York Times

The medication the patients took—which costs about $11,000 per dose—unmasks cancer cells so the immune system can destroy them. One in five patients usually have an adverse reaction to the type of drugs that they took, the Times reports. Dr. Alan P. Venook, who was not part of the study, said that an absence of major side effects in this particular study meant that “either they did not treat enough patients or, somehow, these cancers are just plain different.”

While researchers share that the result must be repeated in larger studies to see how it all plays out, because the patients in this study had a rare genetic signature in their tumors, it still serves as a “cause for great optimism,” per oncologist Hanna Sanoff.

“Very little is known about the duration of time needed to find out whether a clinical complete response to dostarlimab equates to cure,” she wrote.