A prostate cancer “vaccine” made by Dendreon Corp. can help patients with severe, advanced disease live a little bit longer, U.S. researchers reported on Wednesday.

The so-called therapeutic vaccine, designed to help a patient’s own immune system fight the cancer, added a few months to the lives of men with otherwise untreatable prostate cancer, the researchers said.

It is one of several experimental tailored approaches to cancer treatment.

“A therapy that prolongs life yet avoids the side-effects of other therapeutic approaches is clearly attractive to patients and physicians alike,” said Dr. Eric Small, who led the study at the University of California San Francisco School of Medicine.

The vaccine, called Provenge, helped patients live an average of nearly four months, or 18 percent longer, the researchers told a joint meeting of the American Society of Clinical Oncology, the Prostate Cancer Foundation, the American Society for Therapeutic Radiology and Oncology, and the Society of Urologic Oncology.

Dendreon’s vaccine, made with South San Francisco-based Titan Corp., is designed to stimulate the immune system to attack the 95 percent of prostate cancer cells that generate prostatic acid phosphatase.

A patient’s own immune cells are collected and trained to attack the protein, then reinfused into the patient.

Small and colleagues tested this highly tailored therapy on 127 men with prostate cancer that had spread and that no longer responded to hormone therapy.

As part of a phase III trial for Food and Drug Administration approval they gave the vaccine treatment to 82 men while 45 got a placebo treatment. On average the men who got the vaccine lived 26 months, while those who got a placebo lived 22 months.

Three years later, 34 percent of vaccine patients were still alive, compared 11 percent of unvaccinated patients.

Dendreon shares fell last month when the company said the trial failed to slow the progression of the prostate cancer in the patients but the researchers said extending life may be more important.