SEATTLE (AP) — The maker of an experimental treatment for prostate cancer says the vaccine has met a key goal in a late-stage study. Seattle-based Dendreon Corp. said Tuesday that its Provenge cancer vaccine improved overall survival when compared to a dummy treatment in a study of 512 men. The treatment is the first to use the body’s immune system. The study has been closely watched. Advisers to the Food and Drug Administration had previously recommended approval of the vaccine. But the FDA asked for more data. That sparked protests from men’s groups and cancer research advocates.