A new genetic test to gauge the aggressiveness of prostate cancer may help tens of thousands of men each year decide whether they need to treat their cancer right away or can safely monitor it. The new test, which goes on sale Wednesday, joins another one that recently came on the market. Both analyze multiple genes in a biopsy sample and give a score for aggressiveness, similar to tests used now for certain breast and colon cancers. Doctors say tests like these have the potential to curb a major problem in cancer care — overtreatment. Prostate tumors usually grow so...