CHICAGO (AP) — An experimental treatment added four months to the lives of men with advanced prostate cancer in a study that tested an entirely new approach to fighting the disease, doctors reported Tuesday.

Dendreon Corp.'s Provenge vaccine trains the immune system to fight tumors. It's called a "vaccine" even though it treats disease rather than prevents it.

Doctors have been trying to develop such a therapy for decades, and this is the first to meet a preset goal for improving survival in late-stage testing.

"There have been a lot of false starts, but this is a real start," said Dr. Paul Schellhammer, a urologist at Eastern Virginia Medical School in Norfolk, Va., who led the study. Results were reported Tuesday at an American Urological Association conference in Chicago.

Seattle-based Dendreon paid for the study, and Schellhammer owns stock in the company. Dendreon shares fell sharply, and then trading was halted leading up to the release of the data. Shares fell $9.74, or 45.2 percent, to $11.81. The reason was not immediately clear.

Four months may not sound like a lot, but it is longer than the three months afforded by Taxotere, the only chemotherapy approved for men in this situation. Doctors hope for even greater benefit if they give the drug earlier in the course of the disease. Dendreon would give no cost estimate for Provenge, but other such biotech drugs cost several thousand dollars a month.

It remains to be seen if side effects will keep Provenge from winning federal Food and Drug Administration approval. Two years ago, the FDA went against its advisers and delayed a decision, asking for more proof of safety and effectiveness.

The new study involved 521 men whose cancer had spread and wasn't responding to standard hormone treatments. Two-thirds were given Provenge, a treatment that is customized for each patient.

Doctors collect special blood cells from each patient that help the immune system recognize cancer as a threat. They are mixed with a protein found on most prostate cancer cells and another substance to rev up the immune system. The resulting "vaccine" is given back to the patient as three infusions two weeks apart.

The other one-third of men in the study had a dummy infusion.

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

Strokes and other brain-related problems were no more common with Provenge — a worry raised by earlier studies. However, four men given Provenge suffered lung clots, though none were fatal. High blood pressure was twice as common with Provenge. Overall, the rate of serious side effects was the same in each group.

Improving survival "is the gold standard" for any treatment, and Provenge appears to do that, said Dr. Ira Sharlip, a urologist from the University of California in San Francisco and a spokesman for the urological association.