A vaccine could help to significantly extend the lives of people diagnosed with the brain cancer that killed ex-Labour cabinet minister Tessa Jowell, 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".
Vaccine did 'what everyone said was impossible'
Kat Charles was told in 2014 that she had three months to live after NHS doctors ran out of options to treat her brain cancer.
"They said there was nothing more they could do for me," says Kat, now 36 from Milton Keynes.
"I was distraught."
After undergoing the standard forms of treatment, and even taking part in a clinical trial for another medicine, she and her husband Jason raised funds to pay for DCVax privately.
After receiving the treatment, Kat's most recent MRI scan showed no trace of the tumour.
"DCVax has done what everyone said was impossible," her husband Jason says. "If not for this treatment, I would be without my wife and without a mother for our child."
Kat continues to have regular injections of the vaccine.
"I go to London on the train, I have a shot in each arm and then I'm free to go home. It doesn't give me any side-effects. It's fantastic."
What is glioblastoma?
- Glioblastoma is the most common type of brain tumour that starts in the brain
- It is the most aggressive form of adult brain tumour and is often resistant to treatment
- It is believed that the variety of cells in a glioblastoma is one of the reasons it is so hard to treat because current drugs are not able to effectively target all the cell types in the tumour
- As with most brain tumours, the cause of glioblastoma is not known
Source: The Brain Tumour Charity
Keyoumars Ashkan, professor of neurosurgery at King's College Hospital in London, who was the trial's European chief investigator, said the results gave "new hope to the patients and clinicians battling with this terrible disease".
"Although definitive judgement 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.
"Cautious optimism is welcome in an area where for so long the disease and suffering have had the upper hand."
Dr 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."