* Provenge improved 3-year survival by 38 percent

* Provenge patients lived 4.1 months longer on average

* No major adverse events seen with Provenge in study

* Shares rise 130 percent after hours to all-time high (Adds physician's and company comment, updates share price)

By Bill Berkrot

NEW YORK, April 28 (Reuters) - An experimental cancer vaccine developed by Dendreon Corp (DNDN.O: Quote, Profile, Research) improved three-year survival of patients with advanced prostate cancer by 38 percent compared with a placebo in what could be a breakthrough for tens of thousands of men who suffer from the disease.

The tiny biotech company has been on a roller-coaster ride that has seen its shares go from $2.55 in March to $25 earlier on Tuesday before a 45 percent plunge just before the news. [ID:nN28347736]

Patients in the 512-subject study who received Dendreon's Provenge lived an average 4.1 months longer than those who received a placebo with no major adverse side effects in the late-stage clinical trial, researchers said.

Patients lived 25.8 months after receiving the vaccine compared with 21.7 months for those taking placebos, according to data from the closely watched study unveiled on Tuesday.