Mammograms are probably the most important tool doctors have to help them diagnose, evaluate, and follow women who've had breast cancer. Safe and highly accurate, a mammogram is an X-ray photograph of the breast. The technique has been in use for about thirty years. Mammograms don't prevent breast cancer, but they can save lives by finding breast cancer as early as possible. For example, mammograms have been shown to lower the risk of dying from breast cancer by 35% in women over the age of 50; studies suggest for women between 40 and 50 they may lower the risk of dying from breast cancer by 25–35%. Leading experts, the National Cancer Institute, the American Cancer Society, and the American College of Radiology now recommend annual mammograms for women over 40. Finding breast cancers early with mammography has also meant that many more women being treated for breast cancer are able to keep their breasts. When caught early, localized cancers can be removed without resorting to breast removal (mastectomy). Mammograms aren't perfect. Normal breast tissue can hide a breast cancer, so that it doesn't show up on the mammogram. This is called a false negative. And mammography can identify an abnormality that looks like a cancer, but turns out to be normal. This "false alarm" is called a false positive. To make up for these limitations, more than mammography is needed. Women also need to practice breast self-examination, get regular breast examination by an experienced health care professional, and, in some cases, also get another form of breast imaging, like ultrasound or MRI scanning.