This is a protocol for a Cochrane Review (Intervention). The objectives are as follows: To assess the effect of screening mammography for breast cancer on overdiagnosis in women aged 40 years and older at average risk of breast cancer.
Breast cancer is the most common cancer in women worldwide and a leading cause of cancer death (Ferlay 2015). During the late twentieth century, screening mammography was introduced in high‐income countries on the evidence that it reduced breast cancer mortality, without adequate consideration or knowledge of potential harms. During this time there was an increase in breast cancer incidence in women aged between 50 to 69 years. This was partly due to changes in risk factors such as alcohol intake, reproductive factors, obesity and hormone therapy use (Bray 2004; Jemal 2010); but also, as has now become apparent, because of widespread uptake of screening mammography and resulting overdiagnosis. In the context of cancer screening, overdiagnosis is the detection of cancer by screening that would never cause symptoms or harms in the absence of screening (Baker 2014; Marcus 2015; Welch 2010). Overdiagnosis involves the interaction between the biology of preclinical cancer and competing risks for mortality. Thus it can occur through the detection of non‐progressive preclinical breast cancer, or through the detection of progressive, preclinical cancer in women with limited life expectancy.
Overdiagnosis is now acknowledged as the major harm of screening mammography (Independent UK Panel on Breast Screening 2012). It should not be confused with a false positive result: when a screening test detects an abnormality but with further investigation, no cancer is found. By contrast, overdiagnosis is a cancer diagnosis which is correct according to contemporary professional standards for pathology reporting and classification. As it is currently not possible to identify individuals who will benefit or be harmed by early detection and treatment, almost all cancer patients are offered treatment. Thus, to the extent that overdiagnosis of cancer occurs, it leads to overtreatment (Brawley 2017; Independent UK Panel on Breast Screening 2012) ‐ unnecessary surgery, radiotherapy and other adjuvant therapy – that will not benefit individuals but may harm them through life‐long physical and psychological consequences that can impact quality of life and life expectancy (Esserman 2014). As such, there is a scientific and public health imperative to establish the frequency of overdiagnosis. This is the evidence gap this review seeks to address.