The Million Women Study in the United Kingdom revealed three patient characteristics that decrease the sensitivity and specificity of screening mammograms in women aged 50 to 64 years: use of postmenopausal HT, prior breast surgery, and body mass index below 25. Another factor that affects sensitivity and specificity is the interval since the last examination. One study used data from seven registries in the United States to examine mammographic data and cancer outcomes in 1,213,754 screening mammograms in 680,641 women. With longer intervals between mammograms, sensitivity increased, specificity decreased, recall rate increased, and cancer detection rate increased.
The optimal interval between screening mammograms is unknown, and practice varies widely. A prospective trial that was undertaken in the United Kingdom randomly assigned women aged 50 to 62 years to annual or the standard 3-year interval for screening mammograms. More cancers of slightly smaller size were detected in the annual screening group with a lead time of 7 months; however, the grade and node status were similar in both groups. A large observational study found a slightly increased risk of late-stage disease at diagnosis for women in their 40s who were adhering to an every-2-year versus every-1-year schedule (28% vs. 21%; odds ratio = 1.35; 95% CI, 1.01-1.81). A 2-year interval was not associated with late-stage disease for women in their 50s or 60s.
As a general rule, cancers that arise between screening examinations (interval cancers) have characteristics of rapid growth [9,45] and are frequently of advanced stage. The likelihood of diagnosing cancer is highest with the prevalent (first) screening examination, ranging from 9 to 26 cancers per 1,000 screens, depending on age. The likelihood decreases for follow-up examinations, ranging from one to three cancers per 1,000 screens.
Digital mammography is rapidly increasing in use. Digital mammography is more expensive than screen-film mammography (SFM), but more amenable to data storage and sharing. Performance of both technologies has been compared directly in three trials with similar results noted in the studies.
A large cohort of women undergoing both types of mammography was evaluated at 33 U.S. centers in the Digital Mammographic Imaging Screening Trial, showing no differences in mammographic sensitivity and specificity. Digital mammography had a higher sensitivity in premenopausal and perimenopausal women, in women younger than 50 years, and in women with dense breasts, according to a planned subset analysis.
An Italian trial of parallel cohorts of 14,385 women matched for age and interpreting radiologist were screened by either full-field digital or SFM. Recall rate and cancer detection rate, especially for clustered microcalcifications, were higher for digital mammography, whereas the recall rate for poor technical quality was higher for SFM. There was no difference in positive predictive value (PPV).