An experimental "vaccine" that offers a new approach to fighting prostate cancer added four months to the lives of men with advanced stages of the disease, doctors report.

The treatment, called Provenge, is called a therapeutic "vaccine" even though it treats disease rather than prevent it. It works by training the immune system to fight tumours.

Doctors are quick to point out this is not a cure. But it could be an important new way of approaching cancer treatment, say researchers, and could help patients avoid some of the surgery, radiation, hormones and chemotherapy treatments used now.

While the new immunotherapy treatment improves survival by only four months, that is still longer than the three months afforded by Taxotere, a chemotherapy drug that is currently the only treatment approved for late-stage, metastatic prostate cancer.

Doctors hope for even greater benefit if they give the drug earlier in the course of the disease.

The new study, which was funded by Dendreon, the manufacturer of Provenge, involved 521 men whose cancer had spread and who were no longer responding to standard hormone treatments.

Two-thirds were given three Provenge infusions two weeks apart, with each dose customized for each patient. The other one-third of men in the study had a dummy infusion.

To create the vaccines, doctors collected special blood cells from each patient, called dendritic cells, which help the immune system recognize cancer as a threat.

Those were mixed with a protein found on most prostate cancer cells, and another substance to rev up the immune system, to create the "vaccine."

Median survival was 26 months in men given Provenge and 22 months in the others. Three-year survival rates were 32 per cent for the Provenge group and 23 per cent for the others -- a 38 per cent improvement.

Results were reported Tuesday at an American Urological Association conference in Chicago.

Overall, the rate of serious side effects was the same in each group.

High blood pressure was twice as common with Provenge. Four men given Provenge suffered lung clots, though none were fatal. Strokes and other brain-related problems were no more common with Provenge -- a worry raised by earlier studies.

The results from the earlier studies led advisers to the Food and Drug Administration to recommend Provenge's approval two years ago. But the FDA delayed action and asked for more data, prompting outcry from prostate cancer survivors.

Dendreon officials say the company plans to seek approval of the drug once again, later this year.

Dr. Otis Brawley, the American Cancer Society's chief medical officer, said the FDA had been right to ask for more study of the drug. But he added that even if the drug had only extended survival by 10 days, he would vote for approval.