A mammogram uses x-rays to detect masses in the breast tissue, allowing doctors to detect breast cancer in its early stages when it’s most likely to be cured. Women over the age of forty are encouraged to get a mammogram every 1-2 years. Although a mammogram is beneficial when lethal cancers are detected, it may be harmful when perceiving those that are not.

It is estimated that one in three breast cancer mammography screenings are overdiagnosed. Overdiagnosis occurs when the patient dies before cancer complications occur, whether it's because the cancer grows too slowly or simply remains dormant.

Because it is impossible to discriminate between types of cancers that will not cause death or symptoms and those that will, all types of cancer are treated. In fact, 37% of women age 40-54 who died from causes other than breast cancer had invasive or non-invasive cancer legions at autopsy, half of which were visible on radiography.

Researcher Karl Julh Jørgensen, co-author and researcher of the study, wrote “The increase in incidence of breast cancer was closely related to the introduction of screening and little of this was compensated for by a drop in incidence of breast cancer in previously screened women.”

Jørgensen, along with director Peter C Gøtzsche, both of The Nordic Cochrane Center in Copenhagen, Denmark, gathered information for the review by compiling several studies from Norway, Sweden, Australia, Canada, and the United Kingdom, using data seven years prior to screening and seven years after.

Researchers obtained information from women too young to be screened to use as a control. This information was compared with pre-screening trends of the screened group to establish a background incidence.

The rate of diagnosis for women younger than 50 and greater than 74 remained constant, but when the post-screening trend in women age 50-69 was examined, breast cancer diagnosis was almost 42% higher than expected.  This was followed by a 15% decline in diagnosis for women over the age of 70. Over-diagnosis for breast cancer alone was 35%, which jumped to 52% when carcinoma in situ (at its original site) was included.

Although factors such as the use of hormone replacement therapy (associated with an increased risk of breast cancer) and incidence of ductal carcinoma in situ (DCIS) may play a factor in the statistics, researchers still attribute much of this change in diagnosis to organized and more frequent screening.

Other deviations due to the nature of different types of cancer must still be accounted for, but the study’s findings do stress taking precautions in breast cancer screening.

Digital mammography readings, for example, in lieu of traditional mammograms (that tend to detect a higher rate of false positives due to poorer image quality) may be the best procedure, but knowing the facts about breast cancer’s risks can help prevent complications as a result of overdiagnosis.