In addition to providing badly needed hope for victims of one of the most common cancer killers, the findings mark a watershed for the long-sought goal of using vaccines to fight malignancies, experts said. 

"We are very excited," said Eric J. Small of the University of California at San Francisco, who led the study. "We think this is going to open up this whole field."

Other researchers agreed, saying that although the results need to be confirmed, they should reinvigorate a strategy for treating cancer that had been the focus of intense interest over the past decade but failed to fulfill initial expectations. 

"This is a significant development," said James L. Gulley of the National Cancer Institute. "It is both meaningful for patients who have prostate cancer but also for the field. It provides a proof of concept that vaccines can in fact work, and that has generally been lacking." 

Unlike traditional vaccines, cancer vaccines are designed to treat rather than prevent disease by stimulating the immune system to attack cancer cells as it would invading bacteria or viruses. Although hope had dimmed for the approach, some scientists have quietly continued to pursue it. 

"It seemed really good, and then the pendulum swung back because we didn't see promising results, but now this is clearly some of the strongest evidence we've seen of a clinical benefit," Gulley said. 

For this vaccine treatment, doctors removed certain immune system cells from patients with advanced prostate cancer, processed them in the laboratory with a protein on prostate cancer cells (called prostatic acid phosphatase, or PAP), and then injected the modified cells back into the patients in three infusions over the course of a month. The idea is to provoke other immune system entities known as T cells to seek out and destroy prostate cancer cells throughout the body. 

"The theory is that these [modified immune system] cells then communicate with the T cells, which go out and do their thing and kill the prostate cancer cells," Small said. 

In the new study, Small and his colleagues gave the vaccine to 82 men whose cancer had progressed after surgery and radiation treatment, and a placebo to 45 similar men. Those receiving the vaccine survived a median of 25.9 months, compared with 21.4 months for those receiving the dummy vaccine -- a 4 1/2-month difference that exceeds the benefit produced by chemotherapy. After 36 months, 34 percent of the men receiving the treatment were alive, compared with 11 percent of those who received the placebo. 

"That's a real, significant clinical impact," said Small, who is presenting the findings this week at a prostate cancer symposium in Orlando that is co-sponsored by the American Society of Clinical Oncology. Small has no financial interest in the treatment. 

The infusions can cause fever, chills and other flu-like symptoms in some patients, but no serious side effects. "It's very benign, especially compared to chemotherapy," Small said. 
It remains unclear how long the treatment can hold the cancer in check, he cautioned. 
But the results are encouraging because there are so few options for men with advanced prostate cancer, said Jamie Bearse of the National Prostate Cancer Coalition. 

"Whenever you have something that comes out that really kind of changes the treatment landscape in the prostate cancer field, it's very encouraging," Bearse said. "It gives a lot of hope." 

More than 232,000 cases of prostate cancer are diagnosed each year in the United States, making it the leading cancer among American men. Most men can be treated with surgery, radiation and hormone therapy. But more than 30,000 men die each year after their cancers progress, making it second only to lung cancer in causing deaths. There was no follow-up treatment available until May, when the Food and Drug Administration approved the drug Taxotere for prostate cancer. 

Other experts agreed that the new findings were encouraging but cautioned that they needed to be confirmed. 

"I think they are very exciting findings," said Philip Kantoff of the Dana-Farber Cancer Center in Boston. "The question is whether it is real -- that's the central question. It's very provocative and needs to be confirmed." 

Dendreon Corp. of Seattle, which is developing the vaccine, has begun a second study of the therapy, dubbed Provenge, involving 98 men with advanced prostate cancer. So far, the study appears to be confirming the findings, said Mitchell H. Gold, the company's president and chief executive. If the final results hold up, the company could file for approval by the FDA by the end of the year, he said. The company also plans to test the treatment on men with earlier stages of prostate cancer, Gold said. 

The findings suggest that scientists may have been too quick to discard some experimental cancer vaccines when they did not appear to shrink tumors, as was the case with Provenge, Gulley said. 

"It may be these biologic agents do little in causing shrinkage of disease, but if they generate a good enough immune response, they could continue to benefit the patient," he said. "This is very intriguing."