Overdiagnosis of breast cancer after 14 years of mammography screening

BACKGROUND: In 2004 we wrote in Tidsskriftet that mammography screening resulted in massive over-diagnosis and over-treatment of breast cancer. Our study was criticised because we had only five years of follow-up time and did not take account of the fact that increased use of hormone replacement therapy could lead to more breast cancer. We have now been screening women for 14 years, and during a period when the use of hormones has fallen by 70 %. MATERIAL AND METHOD: Age-specific incidence rates, detection rates and interval rates for breast cancer in the period 1991-2009 have been computed for 40-79 year-old women. Incidence trends have been calculated using Poisson regression. RESULTS: The incidence of breast cancer in the age group 40-49 was stable throughout the period, but rose by 50 % in the age group 50-69 years immediately after the start of screening. There was no significant reduction in the incidence of breast cancer in the age group 70-74. The number of new cases of breast cancer in the period increased from around 2000 to 2750. About 300 cases of ductal carcinoma in situ (DCIS) were also diagnosed. Today a total of some 1050 more women have been diagnosed than before screening started. Our calculations indicate that in the absence of screening, around 800 of these women would never have become breast cancer patients. INTERPRETATION: The figures from 14 years of mammography screening indicate that all increase in the incidence of breast cancer is due to over-diagnosis: findings of tumours that in the absence of screening would never have given rise to clinical illness.