"Although the medical community recognizes the benefits from screening are smaller and slower to appear in younger compared to older women, we still do not know why," says Dr. Cornelia Baines, the study's principal investigator and an associate professor in the department of public health sciences. "From our research we've learned menstruation may partially explain why women under 50 years of age do not benefit from screening mammograms as much as older women."
The research involved examining data from the Canadian National Breast Screening Study collected from 8,887 menstruating women who were 40-44 years old when they entered the study. The investigators compared mammogram results of women in the follicular phase of their menstrual cycle (days 1 to 14) with those in the luteal phase of their cycle (days 15- 28).
Baines and her U of T colleagues also looked at whether the women had taken hormones in the form of oral contraceptives or replacement therapy. The analysis showed a total of 1,898 women in study had never used hormones, 6,573 women were past users and 416 were current users.
A false-negative mammogram occurs when the x-ray is read as not suspicious for cancer when in fact cancer is present. When a mammogram fails to pick up cancer, the cancer can sometimes be detected without delay if a clinical breast exam is simultaneously performed, as was the case in the Canadian National Breast Screening Study. However, sometimes breast cancer is missed by both methods.
Among women who had ever used hormones, those who had their mammogram while in the luteal phase were twice as likely to have a false-negative mammogram compared to women who had mammograms in the follicular phase. For luteal mammograms, the unadjusted odds ratio for false-negatives versus true-negatives was 2.16 and the adjusted odds ratio was 1.47, both statistically significant. In contrast, women who had never used hormones appeared to have a decreased chance of having a false-negative mammogram in the luteal phase, but this decrease was not statistically significant.
"Mammography is known to be less accurate in younger than older women," says Baines, who notes that currently many women in their 40s receive mammography without an accompanying clinical breast examination. "Without that exam, their false-negative mammograms will not be recognized and the diagnosis and treatment of breast cancer will be delayed."
"If some breast cancers are not detected by mammography, obviously the overall benefit of screening programs is likely to be reduced," explains Baines. "Our study shows there is an opportunity to improve the accuracy of mammograms by choosing to have mammograms during the first half of the menstrual cycle."
Investigators Baines and Dr. Marjan Vidmar received support from the National Cancer Institute of Canada while working on the study with Professors Gail McKeown-Eyssen and Robert Tibshirani, all of the department of public health sciences.