The increased incidence of breast cancer is closely related to the introduction of screening--only a small percentage of which was trumped by a drop in breast cancer occurences in previously screened women, according to a study published July 9 in the British Medical Journal (BMJ).In addition, the study found that one in three breast cancers detected in a population offered organized screening is overdiagnosed.

Karsten Juhl Jørgensen, MD, researcher, and Peter C Gøtzsche, MD, director at the Nordic Cochrane Centre, Rigshospitalet in Copenhagen, Denmark, conducted a systematic review of published trends in incidence of breast cancer before and after the introduction of mammography screening.

For this research, one investigator extracted data on incidence of breast cancer (including carcinoma in situ), population size, screening uptake, time periods and age groups, which the other investigator independently checked. They used linear regression to estimate trends in incidence before and after the introduction of screening and in older, previously screened women; and meta-analysis to estimate the extent of overdiagnosis.

They found that incidence data covering at least seven years before screening and seven years after screening had been fully implemented, and including both screened and non-screened age groups, were available from the United Kingdom; Manitoba, Canada; New South Wales, Australia; Sweden; and parts of Norway. The implementation phase with its prevalence peak was excluded and adjustment made for changing background incidence and compensatory drops in incidence among older, previously screened women.

Jørgensen and Gøtzsche estimated that overdiagnosis was 52 percent. Data from three countries showed a drop in incidence as the women exceeded the age limit for screening, but the reduction was small and the estimate of overdiagnosis was compensated for in this review, according to the authors.

In an accompanying commentary, H. Gilbert Welch, MD, professor of medicine at the Dartmouth Institute for Health Policy and Clinical Research in White River Junction, Vt., wrote that with the "advent of widespread efforts to diagnose cancer earlier, overdiagnosis has become an increasingly vexing problem."

He added that Jørgensen and Gøtzsche's findings are consistent with a growing body of observational evidence that "screening mammography is associated with sustained increases in the incidence of breast cancer in women of screening age, with little or no subsequent decrease in incidence in older women."

As a result, White said that the question is no longer whether it occurs, but how often it occurs.

White concluded that the information which will probably influence most women's choice will be data on the trade-off between the number of deaths from breast cancer avoided and the number of cancers overdiagnosed. He also said that more research is needed to confirm or dispute this assertion.