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It has been said of screening programmes that  ‘all do harm; some do good as well and of these some do more good than harm at a reasonable cost’. This has been vividly illustrated by the issue of breast screening that has been in the news.

All women in the UK between the ages of 50 to 70 yrs are called up every three years for breast screening mammograms. Many of these women will have false positives – abnormal results but when the abnormality is biopsied [a needle put in and breast tissue removed] the sample is found to be entirely normal. Additionally every mammogram exposes a woman to a significant dose of radiation- there is no such thing as a safe dose of radiation.

Even when a biopsy shows cancer it is likely to be a carcinoma in situ; that is a cancer but not one that is invading surrounding tissues. When a woman is diagnosed with carcinoma in situ it is not know whether in future it will invade or whether it will just sit quietly never causing any problems –the so called ‘pussy cat’ cancer. An epidemiological study was commissioned which shows that women are three times more likely to be treated for a cancer that would never have harmed them than have their lives saved by screening.  So three women will go through the emotional stress of diagnosis and are likely to undergo surgery, possible radiotherapy and hormonal treatment unnecessarily for every four women diagnosed with cancer. We call this iatrogenic harm – these women will have been unnecessarily harmed by medicine. We have no way of predicting which cancers are ‘pussy cats’ and which are ‘tigers’ i.e. which will become aggressive and invasive. Most women will therefore opt for surgery, hormonal treatment and possible radiotherapy.

The benefits of breast screening are that one out of every four women found to have a very early breast cancer [carcinoma in situ] will have their life saved. The harms of screening are that millions of women have to go through an unpleasant procedure – mammograms, the stress of waiting for results, and are subjected to the possible radiation harm of a mammogram. Three out of four found to have early breast cancer will undergo surgery, radiotherapy and hormonal treatment and all the stress that this entails for no personal benefit as their cancer would never have harmed them.

Overall the benefits of breast screening probably outweigh the harms in breast cancer but only just. This illustrates the common fallacy that more screening is beneficial. Private screening companies prey on peoples anxieties so many undergo tests and scans without understanding that screening can harm and it has to be carefully and scientifically applied.

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