Oct. 29 (HealthDay News) -- Women who have had advanced precancerous lesions of the cervix are still at risk for invasive cancers up to 25 years later, Swedish researchers report.

Currently, the American Cancer Society recommends that women who have had precancerous lesions called severe dysplasia/carcinoma in situ (CIS) continue getting Pap tests for 10 years after treatment. But, based on this study, these guidelines may need to be changed, said Debbie Saslow, the society's director of breast and gynecologic cancer, who was not involved with the research.

Saslow added, however, that even though these women continue to be at risk for developing cervical or vaginal cancer, the risk is low. "Women who have been treated for advanced precancer do need to remain vigilant," she said.

"This paper is going in my file for when we update our guidelines in the next two years," Saslow added. "We will see if we want to stick with 10 years or go to a much wider interval."

The study was led by Dr. Bjorn Strander, a senior consultant with the Department of Obstetrics and Gynecology at Sahlgren's Academy at the University of Gothenburg. The researchers collected data on 132,493 women who had a diagnosis of severe dysplasia/CIS between 1958 and 2002. The statistics came from the National Swedish Cancer Register.

The researchers found 881 women had developed cervical cancer, and 111 had developed vaginal cancer more than one year after the initial diagnosis. This was almost seven times higher than expected, the researchers said.

Women with a diagnosis of severe dysplasia/CIS were more than twice as likely to develop cancer compared with the general female population. The women were also twice as likely to develop invasive cervical cancer after diagnosis of CIS if that diagnosis was made between 1991 and 2000, compared with the same diagnosis made from 1958 to 1970. This increased risk might be due to changes in treatment over that period, particularly because fewer hysterectomies are being done as part of treatment for CIS, the study authors said.

Strander's team also found a particularly high risk for women over age 50, and this risk continued to increase with age. "The risk after treatment hardly decreases at all after treatment and is still sustained after more than 25 years," he said.

"While well-screened women after 50 to 60 years of age are very well protected from cervical cancer and have little, if any, further use of screening, this does not apply to women who have been treated for grade 3 CIS," Strander said. "They need, and should have, long-term follow-up, perhaps lifelong," he said.

The results are published in the Oct. 26 edition of the British Medical Journal.

SOURCES: Bjorn Strander, M.D., senior consultant, Department of Obstetrics and Gynecology, Sahlgren's Academy, University of Gothenburg, Sweden; Debbie Saslow, Ph.D., director, breast and gynecologic cancer, American Cancer Society, Atlanta; Oct. 26, 2007, British Medical Journal