Hormones and breast cancer

Dr. Emily DeVoto at the Antidote wrote a careful analysis — "Startling" if true — of the news reports attributing the 15% drop in breast cancer rates to reduced use of hormones among women.

...The report is fascinating. It really is a bunch of experts - evidence-based experts at that - sitting around saying, in effect, "Huh. Yup. I guess that's what it must mean," but tentatively, because, as one of them points out, epidemiology (or non-randomized observational studies) cannot prove causation. Particularly this type of study, of ecologic design, which compares population-wide rates of hormone use with cancer incidence. The drop in hormone use is what some people - environmental scientists, for example - call a "natural experiment." The Women's Health Initiative was the only randomized study (read: experiment) to look at health effects of hormone therapy (Prempro, specifically), and breast cancer was one whose incidence was slightly elevated, along with heart attacks, stroke, and blood clots. The Wyeth spokesperson interviewed for this article was skeptical of the explanation, and said that more studies need to be done; sure (see below), but there is not going to be another study on the scale of the WHI to replicate a causal relationship. Sorry.

A good point made in an AP article is that stopping hormones may in part have slowed the growth of existing tumors, which could still come to light at a later date. Another factor to look at is screening, but you'd really have to see dramatic changes in screening utilization to explain these results. Clearly, there's a lot more interesting work, and closer looks at the numbers, to be done on this finding...