A vaccine could help to significantly extend the lives of people diagnosed with the brain cancer, early trial results suggest.
People with glioblastoma who took part in the study lived more than twice as long as those on standard treatments in many cases, researchers say.
The vaccine works by using the body's immune cells to target the cancer.
A cancer charity said preliminary results seemed "remarkably promising".
The standard treatment for glioblastoma, the most aggressive of brain tumours in adults, involves removing the tumour followed by radiotherapy and chemotherapy.
But it is difficult to treat and patients live on average for only 15 to 17 months after surgery.
For this phase three trial of 331 people from the UK, the US, Canada and Germany, 232 patients were given the immunotherapy vaccine DCVax on top of standard treatments while the rest received a placebo along with normal care.
The vaccine works by taking immune cells, known as dendritic cells, from the patients' bodies and then combining them with a sample of their tumours.
When the vaccine is injected back into the patient, the body's entire immune system recognises the cancer to attack.
Preliminary results from the 11-year study, published in the Journal of Translational Medicine, found those involved in the trial survived for more than 23 months on average after surgery, with 100 living for 40.5 months at the time of the researchers' analysis.
Because the study has not concluded yet, the data does not break down who received the vaccine and who had the placebo, but this will be released when the trial concludes.
The longest survivors have lived for more than seven years after surgery.
The study's authors said it appears that patients on the trial who reach a certain threshold beyond diagnosis "may continue onwards to unusually long survival times".
