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Wednesday, August 3, 2011

Mammography and breast cancer mortality: correlation is not causation

Breast cancer mortality has been declining in the industrialized world over the last several decades. At the same time, mammographic screening has become the norm in most of Europe, Australia and North America. Does early detection explain the reduced mortality? A paper in the Aug 1 British Medical Journal concludes that it does not.

The study looked at three country pairs (Northern Ireland vs the Republic of Ireland, the Netherlands vs Belgium and Flanders, and Sweden vs Norway), in which breast cancer screening was introduced at different time periods. The paper points out that it would not be unprecedented for screening to lead to lower death rates because that's exactly what happened with cervical cancer mortality, whichhave been shown to affect risk of breast cancer) from nation-wide statistics on these, by age. Data on breast cancer management was also population-based; they looked at expenditures on anticancer drugs, the 'uptake of new anticancer drugs in general...after their introduction in the country, and the uptake of trastuzumab [a recently introduced cancer medication that is effective against a subset of breast tumors]...after its introduction in the country.'

National organized screening was first introduced in Sweden in1986, and by 1990 90% of Swedish women had been offered a screening. Screening rates there are among the highest in any country. In Norway, in contrast, organized screening wasn't introduced until 1996 as a pilot project, and was gradually expanded until in 2005 the program was nationwide. Breast cancer mortality fell by