For the video version of this post, click here. The idea of screening mammography makes a lot of sense. Detect cancers early, treat them early, improve outcomes. In practice, though, screening mammography gets much more complicated. When should screening begin? When should it end? How frequent should it be? Each of these questions has its own fully-developed controversy.  Full disclosure: I am married to a breast cancer surgeon. Now a study, appearing in JAMA Oncology tries to crack the frequency question.

The study, by Diana Miglioretti and colleagues, used data from the Breast Cancer Surveillance Consortium, a national group that records data from regional radiology facilities. They identified all women in the dataset aged 40 - 85 who were diagnosed with a new breast cancer - around 15,000 women altogether.

Of those, around 12,000 had gotten annual mammograms, and 3000 or so had gotten biennial mammograms. The question was whether the tumors in the less-frequently-screened women would be bigger, or more advanced than those who were screened more frequently. In other words, does more frequent screening catch cancers when they are smaller?

The answer is a definitive kind of. Overall, there was no difference in tumor characteristics among those screened yearly or every other year. Among premenopausal women, though, annual screening did seem to find tumors with less advanced characteristics, an effect that was statistically significant, provided you don’t account for the multiple hypotheses being tested. But if the association is real, it’s interesting to note that there was no such effect when the cohort was stratified by age - so it seems that biological age, at least in terms of menopause, might be more relevant than chronological age here.

The study is shackled by several big limitations though.  #1 is that every woman in this study was diagnosed with cancer. We have no idea how many screenings were done to identify these 15,000 women with cancer and no way to tell if women undergoing every other year screening are treated differently. Perhaps mildly abnormal results get biopsied more often in the every other year group, since waiting to see how things look next year is not an option. The second big problem is that there is no link to any outcomes. Even if we buy that more frequent screening detects cancers earlier, we have no data to tell us whether that matters - ie, whether treatment is more effective at that point.  

In the end, the authors, like the guideline organizations, say that the frequency of screening should be decided between a patient and her doctor. But at some point, the decision to switch to biennial screening may be forced by insurance companies or medicare, and, at least according to this study, that might not be a bad thing.