Breast Ca overdiagnosed in 1 in 10 middle-aged women

About 1 in 10 women between the ages of 55 and 69 who is diagnosed with breast cancer is diagnosed unnecessarily because her disease would never have come to clinical attention if she were not screened. The findings come 15 years after the end of the population-based Swedish Malmö mammographic screening trial, a randomized, controlled trial of over 40,000 women.

The authors of the study compared a group of women screened for breast cancer using mammography to a group not screened (controls). Even after excluding cancer in situ, incidence of overdetected cancers was still 7% in the screened group.

The trial is significant because the trial time was longer and more screening rounds were completed than in any other trial of screening for breast cancer. And the investigators maintained the randomization design for almost 5 years after the trial's conclusion.

The authors of the article claim that interval cancer rates-"the proportion of women whose screening yields negative results but who then present with cancer before the next scheduled screening test"-are essential to evaluating any new screening test for breast cancer and to determining whether a test possesses truly enhanced sensitivity over older methods or whether it simply produces needless diagnoses.

Zackrisson S, Andersson I, Janzon L, et al. Rate of over-diagnosis of breast cancer 15 years after end of Malmö mammographic screening trial: follow-up study. BMJ. 2006;332:689-692.

Irwig L, Houssami N, Armstrong B, et al. Evaluating new screening tests for breast cancer: may require randomized controlled trials to assess overdetection. BMJ. 2006;332:678-679.