One-Third of Breast Cancer Cases Treated Unnecessarily

A new study suggests that one in three women with breast cancer detected by a mammogram may be treated unnecessarily.

Researchers at the Nordic Cochrane Centre, the University of Oslo, and the Norwegian Institute of Public Health conducted a cohort study to assess the association between biennial mammography screening and the size of detected tumors in women aged 35 to 84 years, measuring trends in the incidence of advanced (>20 mm) and non-advanced (≤20 mm) breast cancer tumors in both screened and non-screened women.

Two approaches were used to estimate the amount of over-diagnosis. The first compared the incidence of advance and non-advanced tumors among women aged 50-84 years in screening and non-screening areas; the second compared the incidence for non-advanced tumors among women aged 35-49, 50-69, and 70-84 years in screening and non-screening areas. The researchers assumed that if screenings work as intended, the number of small, curable breast tumors should increase, reducing the number of large cancers by about the same amount.

The results revealed that screening was not associated with lower incidence of advanced tumors. The first approach found 271 invasive breast cancer tumors and 179 ductal carcinoma in situ (DCIS) lesions over-diagnosed in 2010, an over-diagnosis rate of 24.4% (including DCIS) and 14.7% (excluding DCIS). The second approach, which accounted for regional differences in women younger than the screening age, found 711 invasive tumors and 180 cases of DCIS over-diagnosed in 2010, an over-diagnosis rate of 48.3% (including DCIS) and 38.6% (excluding DCIS). The study was published on January 10, 2017, in Annals of Internal Medicine.

“Breast cancer screening was not associated with a reduction in the incidence of advanced cancer. It is likely that one in every three invasive tumors and cases of DCIS diagnosed in women offered screening represent over-diagnosis,” concluded study coauthor Karsten Jorgensen, PhD, of the Nordic Cochrane Center, and colleagues. “Although mammograms in Denmark detected a lot more breast cancers, these were mostly small, early-stage tumors. The number of advanced cancers did not fall.”

DCIS is the most common type of non-invasive breast cancer, in which the abnormal cells are contained inside the milk ducts. If DCIS is not treated, it may eventually develop into invasive breast cancer, which can spread outside the ducts into the breast tissue and then possibly to other parts of the body. Since DCIS cannot usually be felt as a breast lump or other breast change, most cases are diagnosed following routine screening with mammograms or ultrasound, appearing as micro-calcifications.