Mammography is a type of radiography used on the breasts. It is typically used for two purposes: to aid in the diagnosis of a woman who is experiencing symptoms or has been called back for follow-up views (called "diagnostic mammography"), and for medical screening of apparently healthy women (called "screening mammography"). Mammography is not very useful in finding breast tumors in dense breast tissue characteristic of women under 40 years. In women over 50 without dense breasts, breast cancers detected by screening mammography are usually smaller and less aggressive than those detected by patients or doctors as a breast lump. This is because the most aggressive breast cancers are found in dense breast tissue, which mammograms perform poorly on. The presumption was that by detecting cancer in an earlier stage, women will be more likely to be cured by treatment. This assertion, however, has been challenged by recent reviews which have found the significance of these net benefits to be lacking for women at average risk of dying from breast cancer. Screening mammography is usually recommended to women who are most likely to develop breast cancer. In general, this means women who have risk factors such as having a personal or family history of breast cancer or being older women, but not being frail elderly women, who are unlikely to benefit from treatment. Women who agree to be screened have their breasts X-rayed on a specialized X-ray machine. This exposes the woman's breasts to a small amount of ionizing radiation, which has a very small, but non-zero, chance of causing cancer. The X-ray image, called a "radiograph", is sent to a physician who specializes in interpreting these images, called a "radiologist". The image may be on plain photographic film or digital mammography on a computer screen; despite the much higher cost of the digital systems, the two methods are generally considered equally effective. The equipment may use a computer-aided diagnosis (CAD) system. There is considerable variation in interpreting the images; the same image may be declared normal by one radiologist and suspicious by another. It can be helpful to compare the images to any previously taken images, as changes over time may be significant. If suspicious signs are identified in the image, then the woman is usually recalled for a second mammogram, sometimes after waiting six months to see whether the spot is growing, or a biopsy of the breast. Most of these will prove to be false positives, resulting in sometimes debilitating anxiety over nothing. Most women recalled will undergo additional imaging only, without any further intervention.