A vaccine could add years to the lives of people with the aggressive form of brain cancer, trials suggest.
The treatment for people with glioblastoma works by using the immune cells of patients to target their tumour.
Early findings from an 11-year trial involving more than 300 sufferers worldwide show those given the vaccine "are living longer than expected", according to a paper published in the Journal of Translational Medicine.
The Brain Tumour Charity said the preliminary results were "remarkably promising".
The standard treatment for glioblastoma is surgery followed by radiotherapy and chemotherapy. Patients who receive this care live for between 15 and 17 months on average.
Of the 331 patients with glioblastoma who took part in the trial, 232 were injected regularly with immunotherapy vaccine DCVax in addition to standard care, while the remaining group were given a placebo.
Every patient whose tumour recurred during the trial was automatically offered the vaccine, meaning around 86.4% received the treatment at some point.
The study found patients involved in the trial survived for more than 23 months on average after surgery.
Almost a third (30%) of participants are classed as "extended survivors" and lived for an average of 40.5 months after surgery, the authors said.
The longest survivors have lived for more than seven years.
The researchers said: "It appears that patients who survive past certain threshold time points may continue onwards to unusually long survival times."
Only seven participants, who were from the UK, US, Canada and Germany, reported any adverse side effects from the vaccine, the researchers said.
Professor Keyoumars Ashkan, professor of neurosurgery at King's College Hospital in London, and one of the 69 co-authors of the paper, said the interim results of the trial "give new hope to the patients and clinicians battling with this terrible disease".
"Although definitive judgment needs to be reserved until the final data is available, the paper published today hints at a major breakthrough in the treatment of patients with glioblastoma," he said.
"Cautious optimism is welcome in an area where for so long the disease and suffering have had the upper hand."
David Jenkinson, chief scientific officer for the Brain Tumour Charity, said: "These results appear remarkably promising for a community of patients who have been given little hope for decades.
"We need further analysis of the data from this trial and more research in this area to ascertain the role that immunotherapy can play in the battle against brain cancer."
DCVax has been developed by American company Northwest Biotherapeutics.