Independent UK Panel on Breast Cancer Screening replies to Michael Baum

Baum has raised interesting and important issues.1 The panel did not “duck” the effect of breast screening on all cause mortality. We indicated in the full report (p 32) that a 20% reduction in breast cancer mortality would reduce mortality from all cancers by only 3% and all cause mortality by 1.2%.2 The randomised trials performed lacked statistical power to examine all cause mortality.

The panel agrees that overdiagnosis is important, but it is impossible to say for individual women whether a cancer detected at screening represents overdiagnosis, so the extent of this problem is difficult to estimate. The panel does not accept Baum’s estimates—the Bleyer and Welch paper that he quotes is subject to many of the problems of observational studies (discussed on p49 of our full report). Hersch and colleagues’ paper confirms what would be expected—that the larger the scale of overdiagnosis the more it will affect women’s attitudes to screening.3 It is therefore important that any estimates of overdiagnosis are as accurate as possible.

Similarly, the panel agrees that the adverse effects of treatment are important, particularly for patients whose cancer has been overdiagnosed, because these women do not benefit from a compensatory decrease in breast cancer mortality. We therefore tried to quantify the risks of treatment in the full report (p 64), but our estimates were lower than Baum’s. The latest Oxford overview estimates that radiotherapy given after conservative surgery for breast cancer increases the risk of non-breast cancer related death by 0.2% at 15 years.4 This is clearly higher than desired, and changes in radiotherapy (such as the TARGIT trial supported by Baum) should reduce this further.5