Why did ACS change its guideline to say routine screening should start at 45 instead of 40?

The evidence shows that the risk of cancer is lower for women ages 40 to 44 and the risk of harm from screenings (biopsies for false-positive findings, overdiagnosis) is somewhat higher. Because of this, a direct recommendation to begin screening at age 40 was no longer warranted. However, because the evidence shows some benefit from screening with mammography for women between 40 and 44, the guideline committee concluded that women in this age group should have the opportunity to begin screening based on their preferences and their consideration of the tradeoffs. That balance of benefits to risks becomes more favorable at age 45, so annual screening is recommended starting at this age.

Every life lost to cancer is important. But the fact is, even though mammography reduces deaths from breast cancer, it does not eliminate them, even in the age groups where it is agreed that women should be screened. The challenge of screening is maximizing the lifesaving benefits while minimizing its harms. These evidence-based guidelines represent the best current thinking on that balance.