LONDON (BBC News) -- A vaccine has been developed which may be able to fight the most aggressive form of brain tumor, scientists say.

U.S. researchers say their vaccine increased survival times for the 23 glioblastoma multiforme patients they tested it on by at least 18 months.

Only four patients went on to die from the cancer, the study to be presented at a meeting of experts in the U.S. said.

A larger trial of the jab, which works by targeting a protein thought to drive the tumor's spread, is now planned.

It uses an artificial form of the protein, which is found on the outside of 30-50% of tumors, to alert the immune system to its presence and attack it.

The brain is tricked into thinking the protein, known as EGFRvIII, is foreign, and fighter cells in the immune system are sent in.

Amy Heimberger, assistant professor of neurosurgery at the MD Anderson Cancer Center in Texas, said the vaccine was an easy-to-use "off-the-shelf" treatment that could potentially help half of all patients with glioblastoma multiforme (GBM).

She said results from the trial showed the vaccine significantly delays the progression of tumors until the cancer finds a new way to grow.

But when tumors did grow again they did not display the EGFRvIII protein which led researchers to conclude that the vaccine had worked.

Professor Heimberger said: "This is a proof of concept, and optimal use of the vaccine may be with chemotherapy to further retard progression. "Still, this is exciting because people have been trying to use immunotherapy against gliomas for a long time."

She also said that the median survival times were five months longer than those for GBM patients treated with the most up-to-date combination of chemotherapy and radiotherapy.

Patients with this type of tumor left untreated tend only to survive for four months.

Dr Kat Arney, science information officer at Cancer Research UK, said the research was interesting.

She added: "Results like this are exciting - they show that the idea of using the body's immune system to help beat cancer has the potential to make a real difference to patients in the future.

"But much of the work is still at an early stage. "This trial was very small, and larger studies need to be done before the researchers can be sure that the vaccine is safe and effective."

GBMs affected adults between 40 and 60-years-old and represent about a quarter of brain tumors.

The research will be presented at the annual meeting of the American Association of Neurological Surgeons and is being cited as one of the conference's most important findings.