According to this article in the journal Radiology, menstrual timing affects how sensitive screening mammography is. The authors analysed the results of screening mammographies collected as part of the Breast Cancer Surveillance Consortium. They used menstrual timing (week of the menstrual cycle).
Overall, they found that there were no patterns, but when they subdivided by how recently women had had a mammogram, they found some interesting patterns. For women who had had a screening mammogram during the previous 2 years, they found that the mammography was more sensitive (correctly identified those with breast cancer as having breast cancer) during the first week of the cycle. For women having their first mammogram, the pattern was reversed, with screening during the first week tending to be less sensitive. And for women who had a prior mammogram more than 3 years previously, there was no pattern. The abstract says that menstrual timing did not affect the overall rate of detecting cancer. Other online coverage of this result quotes the author as saying that the difference between the first time detections and the detections in regularly screened women might be due to a difference in the size of tumours found – large tumours are picked up on first screenings, while subsequent screenings tend to find smaller tumours.
The study involved fairly large numbers (about 1200 cases of breast cancer in 380 000 mammograms), the results were found in subgroup analysis and the significance levels were around the 0.05 level. So it’s worth viewing these results with caution, and looking for confirmation in future studies.
In any case, those women with cyclic breast tenderness probably already know that mammograms are less painful if you time them appropriately, and it’s good that that coincides with the best sensitivity for women getting regular screening.