attempted to develop a standard quantitative approach to mammographic density measurement, such as by using x-ray digitizers and quantifiable detection systems using density algorithms. These have been carefully studied, but are not yet widely available for clinical purposes.4,79,102

Breast density is a risk factor for missed cancers, and both false-positive and false-negative mammographic interpretations are more likely with dense breasts.34 In a study of more than 11,000 women with no clinical symptoms of breast cancer, the sensitivity of mammography was only 48 percent for the subset of women with extremely dense breasts compared to 78 percent sensitivity for the entire sample of women in the study.53 Technologies that are not based on x-rays, such as magnetic resonance imaging and sonography (ultrasound), are less affected by breast density.

Many factors influence breast density, such as obesity, ethnicity, age, stage of menstrual cycle, and number of live births (parity). Overall, menopausal status, weight, and parity account for 20 to 30 percent of the age-adjusted variation in the percentage of dense breast tissue.5 Younger women tend to have more dense breasts and thus often have mammograms that are difficult to interpret. Hormone replacement therapy increases breast density, although few women show dramatic changes, and the changes depend on the particular hormone regime (reviewed by Slanetz, 2002).93 For example, estrogen and progestin combination therapy increases breast density to a greater degree than estrogen alone.15

Breast density varies within individual women as well as among different women. Breasts that are mammographically dense also tend to have areas that are not dense. Women are slightly more likely to have extremely dense breasts during the last 2 weeks of the menstrual cycle (luteal phase),103,111 although this is not generally clinically significant. Nevertheless, performing mammography during the first 2 weeks of the menstrual cycle may increase mammographic accuracy,111 probably because women do not feel as much discomfort during breast compression. This increases the probability of obtaining an examination without noticeable patient motion, which can degrade image quality and limit the ability to find cancers.

Obesity is commonly associated with fatty breasts and accounts for more than 40 percent of the variance in breast density.6 Native American populations typically have lower density breast tissue, and Asian populations have greater density breast tissue than African American and white populations overall. One solution to the difficulties posed by dense breasts might be to perform ultrasound on all women with particularly dense breasts. This is standard practice in Korea and is done in many facilities in the United States, but, to date, no data have been published to indicate this would improve outcomes and which women would benefit.

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