A group of "unique" patients with advanced lung cancer appear to have won their battle for survival after treatment with a new immunotherapy drug.
On average, only 1% to 4% of people diagnosed with the highly aggressive disease are still alive five years later.
But 16 of 129 trial patients treated with the drug nivolumab, which helps the immune system fight cancer, have "beaten the odds" by surviving at least 58 months, doctors reported.
In all but four of these cases, evidence suggests that the disease was stopped in its tracks. No sign of worsening cancer was seen in the patients when their condition was assessed in November last year.
One British expert described the results as a "landmark".
Scientists are now trying to find out what made this small group of patients with advanced non-small cell lung cancer (NSCLC) so special.
Lead researcher Dr Julie Brahmer, director of the Johns Hopkins Kimmel Cancer Center in the US, said: "A small subset of non-small cell lung cancer patients appear to respond to nivolumab and have beaten the odds that most patients with this cancer face.
"Now, we need to figure out how to make more patients responsive to immunotherapy by exploring combinations of immunotherapy drugs and other treatment agents.
"It's clear that the patients who beat the survival odds are in some ways truly unique biologically, and the goal now is to discover exactly how immunotherapy is keeping their disease in check."
Nivolumab is one of a new generation of "checkpoint inhibitor" antibody drugs that block the ability of cancer to shield itself from the immune system.
Results of the early-stage Phase 1b trial were presented at the American Association for Cancer Research annual meeting in Washington DC.
The 129 patients were treated at more than 11 hospitals across the US.
Twelve of the 16 - nine men and seven women - who remained alive after five years required no further therapy after it was shown that their cancers had stopped progressing. The remaining four patients went onto chemotherapy or joined other clinical trials.
Based on the findings, the scientists estimated that 16% of patients with advanced non-small cell lung cancer who receive nivolumab could expect to survive five years or longer.
The team is attempting to identify genes or proteins shared by the lucky 16 that may help explain why they responded to the treatment so well.
Participants in the trial received nivolumab once every two weeks for up to two years at a total cost of more than 200,000 dollars (£160,000).
Having the ability to predict which patients are likely to respond to the expensive therapy could provide major cost savings, said the researchers.
Early data from the trial appeared in the Journal of Clinical Oncology in 2015. At the time, reported side effects of the treatment included fatigue, thyroid problems, lung inflammation, rashes and gastro-intestinal complaints. Three patients died from lung inflammation.
British expert Dr Alastair Greystoke, from Newcastle Hospitals NHS Foundation Trust, said: "These data are a landmark step in this type of lung cancer and, for the first time, there is a renewed hope that we may see a significant shift in survival."
Each year in the UK, more than 46,000 people are diagnosed with lung cancer, 90% of whom have NSCLC.
Research suggests that In up to 76% of cases, the disease is only detected at an advanced stage. More than 35,000 Britons were killed by lung cancer in 2014.