The federally financed Women's Health Initiative has had difficulty recruiting participants in a long-term prevention study intended to follow 5,000 women over 70 years old for eight years.

The concepts of multistep and field carcinogenesis provide a model for prevention studies.

One of the largest breast cancer prevention studies ever, it included 22,000 women in 400 medical centers in the United States and Canada.

Two prevention studies will start in 2009.

But studies in the United States, including the 1998 prevention study, showed the endometrial tumors to be uncommon, easy to diagnose early and highly curable.

The smattering of controlled prevention studies in the scientific literature have decidedly mixed findings.

The prevention study was begun in April 1992.

As a result, critics argue, some of the subjects in prevention studies will receive treatment unnecessarily, or be treated for what turns out to be the wrong problem.

But in its announcement yesterday, the institute did not give full authorization to enroll new women to the prevention study.

Two prevention studies, where the vaccine is given to persons who have not yet developed diabetes are underway.