Debate on screening for breast cancer is not over

Editor—In a news item, Mayor described a historically controlled study from Sweden that claimed that screening with mammography reduced deaths from breast cancer by nearly two thirds.¹ This was followed a week later by a comment in the Minerva section, which started off with the strong political message that the case for breast cancer screening programmes is now beyond debate and went on to speak about a meticulous report from Sweden, repeating the claim of a 63% reduction in breast cancer deaths in women screened.²

I am surprised that Minerva, who usually makes cautious reservations about the studies she quotes, on this occasion was not cautious, in particular as the quoted study is confounded and cannot reliably say anything about a possible effect of breast cancer screening with mammography.³ For example, there was no significant difference in the decline in breast cancer mortality in the age group that had been invited to screening, compared with a younger age group that had never been invited to screening, and the study did not take account of introduction of effective treatments such as tamoxifen. Furthermore, it seems implausible that such a dramatic effect should be seen in Sweden, where the trend in mortality from breast cancer has not changed throughout the past 30 years. In contrast, for the United Kingdom, where an upward trend has changed into a downward one, a report tentatively ascribed only 6% of the decline in breast cancer mortality to screening—and the authors even found this estimate impossible to test.⁴

Whether one considers the results obtained in the randomised screening trials or what one could hope for on the basis of tumour biology, the result is the same: if there is any effect of screening, which we currently do not know with certainty, it is likely to be much smaller than the 20-30% reduction in mortality from breast cancer that has often been claimed.⁵ It is therefore reasonable to continue the debate on screening for breast cancer.