An experimental treatment tripled the survival rate of men with advanced prostate cancer in a clinical trial, doctors said yesterday, a result they said represents perhaps the first significant success for therapies known as cancer vaccines. The treatment, developed by Dendreon, a Seattle biotechnology company, is called a vaccine not because it prevents disease but because it tries to harness the body's own immune system to fight cancer after the disease has developed. Many such cancer vaccines have failed in clinical trials and none have reached the market in the United States. But Dendreon's treatment, called Provenge, now seems to...