Why can women choose to start screening every 2 years at age 55?

Although breast cancer is more common in older women after menopause, breast cancer grows more slowly in most women, and is easier to detect early because the breasts are less dense. Since most women are post-menopausal by age 55, and because the evidence did not reveal a statistical advantage to annual screening in post-menopausal women, the guidelines committee concluded that women should move to screening every 2 years starting at age 55. Still, the guideline says women may choose to continue screening every year after age 55 based on their preferences.