—To determine factors that influence the sensitivity of modern first screening mammography.
—Nine counties in northern California.
—A total of 28 271 women aged 30 years and older referred for first screening mammography to the Mobile Mammography Screening Program of the University of California, San Francisco, from April 1985 to March 1992, of whom 238 were subsequently diagnosed as having breast cancer.
—Breast cancer risk profile, 2 standard mammographic views per breast, breast density, and follow-up of abnormal and normal mammography by contacting women's physicians and by linkage to the regional Surveillance, Epidemiology, and End Results tumor registry to determine the occurrence of any invasive cancer or ductal carcinoma in situ.
—For women aged 50 years and older, the sensitivity of first screening mammography was relatively high and decreased slightly with increasing length of follow-up after mammography: 98.5% for 7 months of follow-up, 93.2% for 13 months, and 85.7% for 25 months. Sensitivity was higher among women aged 50 years and older when breast density was primarily fatty rather than primarily dense (98.4% vs 83.7%; P<.01). For women younger than 50 years, the sensitivity of first screening mammography also decreased with increasing length of follow-up but was significantly lower than for older women: 87.5% for 7 months of follow-up, 83.6% for 13 months, and 71.4% for 25 months. For women younger than 50 years, breast density did not affect the sensitivity of mammography (81.8% for those with primarily fatty breasts vs 85.4% for those with primarily dense breasts) and was lower among those with a family history of breast cancer (68.8%).
—The sensitivity of modern mammography is highest among women aged 50 years and older who have primarily fatty breast density. Sensitivity is lowest among women younger than 50 years and particularly low when the time between screenings is about 2 years or when women have a family history of breast cancer, possibly because of rapid tumor growth.