Here are two articles which discuss the changing science on mamography:
- http://www.hsph.harvard.edu/news/features/vatten-mammography-screening/ (link)
- http://well.blogs.nytimes.com/2012/11/28/ignoring-the-science-on-mammograms/ (link)
The first indicates that screening does not correlate with reductions in breast cancer mortality rates. Also, screening tends to find cancers that "would not have harmed them". The second reinforces that "that screening mammography doesn’t save lives" and "the number of cancers diagnosed at the advanced stage was essentially unchanged".
I thought about this for a while. How could screening mammograms, which find early stage cancer, not reduce advanced stage cancers or cancer deaths? I came up with five reasons:
- The early cancers detected would not have become malignant, so finding and treating them doesn't affect mortality rates.
- The early cancers that may have become malignant would have been caught at a later time. Treatments are now so advanced that the delay doesn't appreciably affect the mortality rate.
- Other screening methods are finding cancers so mammogram screening is less effective at finding cancer. Maybe women who do self exams find more cancers compared to women who wait for their yearly mammograms. This would strangely lead to a correlation between mammograms and increased mortality rates.
- The treatments may negatively affect mortality. Getting surgery or radiation or chemotherapy on early cancers or benign cancers may damage health enough to outweigh any benefits. I couldn't find a clear answer whether the mortality rates were just due to cancer, or due to anything (I'm guessing the latter).
- The advanced cancers appear very quickly and are very deadly. This makes some sense because cancer is uncontrolled growth of previously normal cells. Fast growing cancers would seem to be the most deadly. In this case, the mammography screening rate might have to be monthly to have a hope of finding these advanced cancers in time to make a large difference in mortality.
I decided to actually read the original New England Journal of Medicine article, so see if I could understand what was going on.
It enlightened the situation, and also showed the difference between scientists and science journalists. Here are some quotes for the NEJM study Discussion section:
- "Over the same period, the rate of death from breast cancer decreased considerably. Among women 40 years of age or older, deaths from breast cancer decreased from 71 to 51 deaths per 100,000 women — a 28% decrease. This reduction in mortality is probably due to some combination of the effects of screening mammography and better treatment. Seven separate modeling exercises by the Cancer Intervention and Surveillance Modeling Network investigators provided a wide range of estimates for the relative contribution of each effect: screening mammography might be responsible for as little as 28% or as much as 65% of the observed reduction in mortality (the remainder being the effect of better treatment). Our data show that the true contribution of mammography to decreasing mortality must be at the low end of this range."
- "As treatment of clinically detected disease (detected by means other than screening) improves, the benefit of screening diminishes"
- "Women now widely recognize the significance of a new breast lump and the need for diagnostic mammography. Ironically, increased awareness confers less opportunity for screening mammography to reduce the incidence of advanced cancer. ... We agree that women should understand that screening raises their risk of becoming a patient with breast cancer and that there is uncertainty about the benefit of screening. The assessment of how to cope with that uncertainty, however, remains a value judgment that we believe should be left to women and their doctors." - Authors response in Letters section
It should also be noted that the study did not follow women to see what they were doing, but looked at general statistics. It did not look at a woman's family risk of breast cancer, or whether they did self exams. Also, some fairly large adjustments had to be made to account for hormone-replacement therapy's effect on cancer rates.
What I take away from this is screening, in all forms, is useful, but results should be reflected on before health-affecting treatments are begun to make sure they are necessary. But this is a complicated subject, and my heart goes out to anyone who has to deal this these issues.