Prevention is the best medicine, doctors say, and nowhere is that truer than in cancer. Picking up early signs of disease is the best way to prevent cancer from taking root, so doctors have urged people get screened for all types of cancer on a regular basis. The assumption is that screening will save them from developing advanced cancer, which is less treatable and deadlier.

That was the reason behind a worldwide push to have every woman get regular mammogram screenings. The idea was to lower the rates of advanced cancers and ultimately deaths from the disease. But in a new study published in BMJ, researchers show that mammography did little to reduce either deaths or advanced breast cancer over a period of 23 years in the Netherlands. Instead, they found that the X-ray based test designed to pick up tumors led to overdiagnoses 60% of the time.