Breast cancer screening: well established but remains contentious
Since routine breast screening with mammography is well established in many countries one could well assume that mammographic screening must be based on sound evidence of benefits outweighing harms. As one US public health expert remarked in 2010: ‘No screening test has ever been more carefully studied. In the past 50 years, more than 600,000 women have participated in 10 randomized trials, each involving approximately 10 years of follow-up’. But he went on to say ‘Given this extraordinary research effort, it is ironic that screening mammography continues to be one of the most contentious issues within the medical community’. 
Why is mammographic screening so contentious? A fundamental reason is that it has been ‘sold’ to women as a sensible thing to do by those providing screening and by patient groups. The information provided to women who are invited for breast screening emphasizes the benefits while glossing over the harms, limitations, and consequences.  Yet mammography not only leads to early diagnosis but also, much as with prostate cancer, to diagnosis of cancers that would never have become apparent in a patient’s lifetime. And inevitably there will be false-positive results too.
The most reliable evidence comes from reviewing, systematically, the results of clinical trials in which women have been randomly allocated to screening or no screening. And the results make for interesting reading. They show that if 2,000 women are screened regularly for ten years, one will benefit from screening, as she will avoid dying from breast cancer. But at the same time, ten healthy women will, as a consequence of screening, become ‘cancer patients’ and will be treated unnecessarily. Mammography in these women has in fact detected lesions that are so slow-growing (or even not growing at all) that they would never have developed into a real cancer. These healthy women will go on to have either part of their breast removed, or even the whole breast, and will often receive radiotherapy and sometimes chemotherapy. 
Furthermore, 200 screened women out of 2,000 will experience a false alarm, and the psychological strain until the woman knows whether it was cancer, and even afterwards, can be severe. And mammography is often promoted to women alongside advice on breast self-examination or breast awareness, when both these methods have also been shown to result in more harm than benefit.12
A British public health expert noted that the potential for individual benefit from mammography is very small. He remarked: ‘this is not widely understood. In part this is due to obfuscation from organisers of mammography services assuming that a positive emphasis is needed to ensure reasonable compliance [with screening]’. Assessing the available evidence in 2010, he commented: ‘Mammography does save lives, more effectively among older women, but does cause some harm.’
The harms he is referring to are overdiagnosis and false positives. Critically, he observed that full examination of all the individual results from recent screening studies had yet to be examined dispassionately.  While such an impartial evaluation is awaited, women continue to be invited for mammographic screening. At the very least, they need to be given sufficiently balanced information to enable them to decide (together with their family and their doctor if they wish), whether to attend for screening – or not.
Next: Prostate cancer screening: clear harms with uncertain benefits
GET-IT Jargon Buster
GET-IT provides plain language definitions of health research terms