Life being complex, many simple principles turn out on examination to be not as simple as at first thought. For example, everyone knows, or thinks that he knows, that prevention is better than cure. But is it always? It is often very difficult to say with certainty.
Three articles in a recent British Medical Journal tackle the vexed question of mammography, whose purpose is to detect cancer of the breast early in its development on the assumption that early detection leads to more effective treatment. The advice to women, therefore, is to get themselves scanned regularly.
This seems straightforward and commonsensical, but in fact the question of whether the light of mammography is worth its candle is devilishly complex. For example, if the treatment of breast cancer has improved (and death rates in Britain have almost halved between 1990 and 2010, thanks mainly to improved treatment rather than to early finding), then the number of cases found by mammography in order to save a single life has to increase. This in turn means that old trials – and all trials to determine the long-term effect of mammography have to be old – may no longer be relevant to the present situation. Trials of mammography are, in effect, always trying to hit a moving target.
The main problem that has bedevilled mammography is that of the false positive: the diagnosis of cancer when in fact there is none. For example, it is estimated that approximately 70,000 women in America are falsely diagnosed with cancer annually by means of mammography, that is to say a half of all those who are diagnosed.