(ChemotherapyAdvisor) – Mammography screening was found to lead to an overdiagnosis of breast cancer in 15% to 25% of cases, results of a decade-long study in Norway published in the April 3 issue of Annals of Internal Medicine has found.

The investigators defined overdiagnosis “as the percentage of cases of cancer that would not have become clinically apparent in a woman’s lifetime without screening.” They compared invasive breast cancer incidence with and without screening using data from a nationwide mammography screening program in Norway gradually implemented from 1996 to 2005 in which women aged 50 to 69 years were invited to be screened.

A total of 39,888 patients with invasive breast cancer were included, 7,793 of whom were diagnosed after the screening program was initiated. Results showed that 15% to 25% of cases of cancer were overdiagnosed, translating to six to ten women overdiagnosed for every 2,500 women invited. Ductal carcinoma in situ was not studied.

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The authors recommend “watch-and-wait” strategies with non-invasive, early-stage tumors. However, they recognize this approach may be difficult for women to support. “Ultimately, better tools are needed to reliably identify which breast cancer will be fatal without treatment and which can be safely observed over time without intervention, but we cannot wait for these tools to be developed,” the investigators wrote. They added that clinicians “have an ethical responsibility to alert women to this phenomenon,” given that most women are not aware overdiagnosis can exist.

The authors of an accompanying editorial point out under-recognized harms of overdiagnosis, including the anxiety of false-positive results; in addition, once diagnosed with cancer that otherwise would not have progressed, women are forever altered by that diagnosis.