A new study has reported that premenopausal women who undergo regular screening mammograms may benefit if they schedule the procedure during the first week of their menstrual cycles. The study was designed to investigate the sensitivity, specificity, and cancer detection rate of screening mammograms according to week of menstrual cycle. The sensitivity of a test refers its ability to detect cases of a disease. Numerically, sensitivity is the number of true positive results divided by the sum of true positive and false negative results. A very sensitive test may result in a number of false-positive results, but almost no true cases of the disease will be missed.

Breast density varies slightly with menstrual cycle and this may influence the test's accuracy. In particular, in the first week (which starts with the first day of the menstrual period), breast tissue may be less dense and not engorged, improving mammogram readability. The study included a review of prospectively collected data concerning 387,218 screening mammograms linked to 1,283 breast cancers in premenopausal women in the Breast Cancer Surveillance Consortium (a nationwide network of research sites which has collected breast cancer screening data since 1994).

Screening mammography performance was not found to differ according to week of menstrual cycle in the overall population. However, different patterns emerged when the analyses were conducted according to prior mammography history. Among the 66.6% of women who had undergone regular screening (a mammogram had been performed within the previous two years), sensitivity was found to be higher (79.5%) in week 1 than in subsequent weeks (week 2, 70.3%; week 3, 67.4%; week 4, 73.0%). However, among the 17.8% of women who had mammograms for the first time in this study, sensitivity tended to be lower during the first week (week 1, 72.1%; week 2, 80.4%; week 3, 84.6%; week 4, 93.8%). Sensitivity was not found to vary significantly by week in menstrual cycle among women who had undergone mammography more than three years earlier. There were no meaningful differences in specificity or cancer detection rate. Note that the findings do not apply to diagnostic mammography performed to work up a symptom such as a breast lump. The authors conclude that premenopausal women who undergo regular screening may benefit from higher sensitivity of mammography if they schedule screening mammography during the first week of their menstrual cycles.

In a separate interview, study author Diana Miglioretti addressed the puzzling finding that screening mammography performance varied according to whether women were having regular mammograms. In general, when a first screening mammogram finds a tumor, it’s relatively large. Low breast density is more important for detecting small tumors, therefore the menstrual cycle influence might not have been observed for first screens. “Larger tumors may be easier to see later in the menstrual cycle, but this needs to be studied,” according to Miglioretti.