May 12 (HealthDay News) -- Women previously treated for cervical intraepithelial neoplasia (CIN) are at higher risk for recurrence of severe CIN and invasive cervical cancer than women who haven't been diagnosed with CIN, a new U.S. study suggests.

CIN is a precancerous condition in which there's abnormal growth of cells on the surface of the cervix.

Researchers analyzed data on 37,142 women treated for grades 1, 2 or 3 CIN between January 1986 and December 2000, and compared them to 71,213 women with no previous CIN diagnosis. Both groups of women were followed until 2004.

The risk of CIN 2/3 recurrence and invasive cervical was highest among women who were older than 40, previously treated for CIN 3 or had been treated with cryotherapy, the study found.

The highest rates of CIN recurrence occurred in the first six years after CIN treatment, with most cases of recurrence diagnosed within the first two years. During the six years after initial treatment, recurrence rates for CIN 2 or 3 ranged from 2.3 percent in the lowest-risk group to 35 percent in the highest-risk group.

Overall incidence of cervical cancer among women who'd had CIN was 37 per 100,000 woman-years, compared with six per 100,000 woman-years among women with no previous CIN diagnosis.

The study was published in the May 12 issue of the Journal of the National Cancer Institute.

"This large, population-based cohort study with more than 300,000 women-years of observation in the CIN cohort provided important information that could contribute to evidence-based guidelines for follow-up of women treated for CIN," wrote Dr. Joy Melnikow, of the University of California, Davis, and colleagues. "Future randomized trials will need longer term follow-up to define the impact of treatment choice on subsequent CIN and invasive cancer."

SOURCE: Journal of the National Cancer Institute, news release, May 12, 2009