The chance of surviving cancer is usually much better if it is found early; ideally before a person has any symptoms. One way to achieve earlier detection of cancer is by screening.
Breast cancer screening is one of the three NHS cancer screening programmes and is currently offered to all women aged 50 to 70 in the UK. This is done using mammography, which can detect cancers that are too small to see or feel.
Breast cancer screening is a form of secondary cancer prevention because it does not prevent breast cancer from happening in the first place; it only helps to find it earlier. Primary breast cancer prevention, on the other hand, is everything a person does to try and prevent breast cancer from ever developing; for example by not smoking, keeping alcohol within sensible limits and keeping a healthy weight. Although doing those things does not guarantee that a person will never get breast cancer, there is good evidence that it will reduce the chance.
However, any type of cancer screening not only has benefits, but also the potential to cause harm. An independent expert panel was asked not long ago to weigh up the benefits and harms of breast screening. They concluded that on balance, breast screening has more benefits than harms. Therefore, it is still recommended.
One particular risk of harm that the breast screening panel identified was overdetection (sometimes known as overdiagnosis). Overdetection happens when a cancer is picked up by a mammogram which would have never caused a problem during a woman’s lifetime – either because it was slow-growing, or because she would have died of something else before the cancer became a problem. However, because it is currently impossible to tell whether a cancer is ‘dangerous’ and fast-growing, or won’t cause further problems because it’s growing only very slowly, all cancers are treated as ‘dangerous’. This means that some women will have treatment that is very invasive and distressing (for example breast surgery or chemotherapy) when actually the cancer would not have caused them any harm. Currently, it is estimated that for every life saved by screening, three cancers are detected that would have not caused any problems.
Whether or not to attend breast cancer screening is a personal choice .
However, for women to make an informed choice about breast screening, they need to know about all the benefits and harms, including the risk of overdetection.
Our researchers carried out a survey with 2,272 women from the general population to find out whether women knew about overdetection, and whether getting some information on it would influence their decision to go for breast screening. Therefore, they asked women about screening intentions, before and after giving them information on overdetection. Women were told about the problem of treating cancers that never would have caused harm, and that for every woman who has her life saved by breast screening, three will have treatment for a cancer that would never have become life-threatening. The researchers thought that younger women (<47) who were not yet eligible for screening would have lower intentions to go for screening after hearing some information about overdetection than women who were already eligible for breast screening, because the latter may already have made up their minds.
The results showed that about half of the women (53%) were already aware of overdetection, with greater awareness among women who were already eligible for screening. However, even after getting some information on overdetection, only about two thirds of the sample (64%) felt that they understood what the concept meant, and a similar number (57%) understood that women who go for breast screening are more likely to be diagnosed with breast cancer than women who don’t. Interestingly, only a small number of women (7%) showed a decrease in screening intentions after receiving information on overdetection. As predicted by the researchers, a greater number of these women were not yet eligible for breast screening.
These findings suggest that the concept of overdetection may be difficult to understand for some women, and that brief information may not be enough to help them make an informed choice. However, one limitation of this study was that women had very little time to take in the information that was given to them. Perhaps understanding would have improved if women had had more time to process what it meant. Alternatively, it is possible that women took a ‘better safe than sorry’ approach to overdetection, which has been suggested by findings from some focus groups that our researchers did with 40 women.
Future work will help to understand how best to communicate the benefits and harms about cancer screening to different groups of people, so that they can make a truly informed choice about whether or not to participate.
Waller, J et al. A survey study of women’s responses to information about overdiagnosis in breast cancer screening in Britain (2014) British Journal of Cancer. doi: 10.1038/bjc.2014.482
Waller J, Douglas E,Whitaker KL, et al Women’s responses to information about overdiagnosis in the UK breast cancer screening programme:a qualitative study. BMJOpen 2013;3