Roughly one in three breast cancers detected in publically organized mammography screening programs is over diagnosed - meaning that the cancer will not cause symptoms or death in the woman’s lifetime, according to a study released Thursday.

In the study, Dr. Karsten Juhl Jorgensen and Dr. Peter C. Gotzsche, from The Nordic Cochrane Center, Copenhagen, analyzed how breast cancer rates changed after publicly organized screening programs were introduced in the UK, Canada, Australia, Sweden, and Norway.

Information from at least 7 years before and after the programs were introduced was analyzed and was obtained from both screened and non-screened age groups.

An increase in breast cancer rates was noted in each country that was closely associated with the introduction of screening, the study team reports.

Moreover, very little of this increase was compensated for by a fall in breast cancer rates among previously screened women, the investigators say.

The overall rate of over diagnosis was 52% and ranged from 46% in Sweden to 59% in Canada. When only “invasive” breast cancers were considered, the rate of over diagnosis was 35%.

In a related editorial, Dr. H. Gilbert Welch, from the Dartmouth Institute for Health Policy and Clinical Research, White River Junction, Vermont, notes that “the amount of over diagnosis is a function of the mammographer’s “threshold” to recommend a tissue biopsy to get a better look.

He adds that “the time has come for a randomized controlled trial to test higher thresholds, such as only recommending biopsy for breast masses larger than a certain size.”

SOURCE: British Medical Journal, online July 10, 2009.