Benefits and Harms of Mammography Screening

Magnus Løberg; Mette Lise Lousdal; Michael Bretthauer; Mette Kalager


Breast Cancer Res. 2015;17(63) 

In This Article

Information to Women

Screening differs from clinical practice. Individuals who undergo a screening procedure are invited to participate with the implied expectation that they will benefit. This contrasts with clinical practice, where the patients approach the medical practitioner with a symptom or complaint for help.[3] Thus, it is of utmost importance that information about benefits and harms of mammography screening is balanced. However, the harms of screening have not been communicated to the public as well as the benefits.[63,64] With increasing evidence of overdiagnosis, this is of concern and violates the individual's possibility to make an informed choice.

However, proper information on risks and benefits is not easy. Firstly, how do clinicians communicate benefits and harms? The use of relative risks may suggest greater effects than exist, whereas the use of absolute risks (or equivalents, such as the number needed to screen) prevents this misunderstanding. The use of relative risks should be avoided or employed only in combination with more comprehensible forms of communicating risk, such as absolute risks or numbers needed to screen.[65] Secondly, many cannot interpret numbers as well as words and have difficulty understanding numerical expressions of risk.[66] In medical schools, courses in statistics usually do not go far enough in teaching statistical or probabilistic thinking, and few teach strategies for effective communication. Hence, most physicians are poorly equipped to discuss risk factors in a way that is readily comprehensible to their patients. This deficiency puts the ideal of informed consent in jeopardy.[65,67]

Framing is the presentation of logically equivalent information in different forms. Positive framing emphasizes the absence of disease; negative framing emphasizes the presence of disease[65] (Figure 6). Based on the 20-year risk for a woman in the UK to die of breast cancer, the risk of dying from breast cancer with mammography screening would be 15 per 1,000 women and 17 to 18 per 1,000 women without mammography screening.[49] Positive framing would be that the number of women that will not die from breast cancer rises from between 982 and 983 to 985 per 1,000 women with the addition of screening for breast cancer.[34,35] An example of positive framing is illustrated in Figure 6.

Figure 6.

Positive framing. Out of 1,000 women aged 50 to 69 years invited every second year, 781 are alive with screening and the same number without screening over the course of 20 years. Correspondingly, 985 women and 982 to 983 women without screening will not die of breast cancer aged 55 to 74 years. Negative framing: out of 1,000 women aged 50 to 69 years invited every second year, 204 women will die with screening and the same number without screening. Correspondingly, 15 women with screening and 17 to 18 women without screening will die of breast cancer between 55 and 74 years old. Number of women dying among women aged 55 to 74 years is based on the observed mortality rates in England and Wales in 2007 [68]. The number of women dying over a 20-year period is estimated by summing the mortality rates for the ages 55 to 74 [68].

Women are not only overestimating their risk of breast cancer, but also substantially overestimating the benefit of mammography screening.[67,69–71] Over 50% of all women asked thought mammography screening reduced the risk of dying from breast cancer by at least 50%.[67,69] Further, women wanted to have balanced information and share the decision with their physician,[71] but many reported they were never provided information on false positives and side effects.[71] A report from Norway, where women are invited with a prescheduled time and date of a screening mammography appointment, showed that if the invitation letter included an information leaflet aimed at enabling women to make a free and informed choice, the prescheduled appointment undermined the option of not participating.[72] The authors concluded that the current recruitment procedures gave priority to screening uptake at the expense of informed choice.[72] Thus, the principle of informed choice might be in jeopardy.[72]