Breast Cancer Screening

Breast cancer screening tests women without symptoms, most often with mammography, to detect breast cancer at an earlier stage than it would otherwise present. Unlike cervical and colorectal screening, it generally detects cancer earlier rather than removing precursors, and the balance between its mortality benefit and harms such as overdiagnosis is one of the most extensively debated questions in screening.

Definition

Breast cancer screening is the examination of asymptomatic individuals, principally by mammography, to detect breast cancer at an earlier and potentially more treatable stage.

Scope

This topic covers mammography as the principal screening modality, the randomized-trial and review evidence on whether it reduces breast-cancer mortality, and the recognized harms of false positives and overdiagnosis that drive disagreement over age and interval. It is a reference account of the evidence and controversy; it gives no individual screening advice.

Core questions

• Does mammographic screening reduce breast-cancer mortality, and by how much?
• How large is overdiagnosis, and how should it be weighed against the mortality benefit?
• How do age of starting and screening interval change this balance of benefits and harms?

Key concepts

• Mammography
• Cause-specific mortality reduction
• Overdiagnosis
• False-positive results
• Recall and biopsy
• Breast density
• Age and interval trade-offs

Mechanisms

Mammography images the breast to reveal masses and microcalcifications that may represent cancer too small to be felt, allowing diagnosis at an earlier stage when treatment is more often effective. The intended benefit is a reduction in breast-cancer mortality, demonstrated in randomized trials and quantified in subsequent reviews (Nelson, 2009; Marmot, 2012). The same earlier detection produces harms: some screen-detected cancers, particularly ductal carcinoma in situ and slow-growing invasive tumours, would never have caused symptoms or death in a person's lifetime, so their detection constitutes overdiagnosis and may lead to overtreatment; screening also generates false-positive recalls and biopsies (Gøtzsche, 2013).

Clinical relevance

Mammographic screening programmes are widespread in primary care and public health, and major bodies issue differing recommendations on starting age and interval for average-risk women (Oeffinger, 2015). This entry summarizes the evidence and the points of disagreement for reference; it is not a recommendation and does not specify who should be screened.

Epidemiology

Randomized trials and pooled reviews indicate that inviting women to mammographic screening reduces breast-cancer mortality, with the relative benefit commonly estimated around one-fifth in reviews such as the Independent UK Panel, though estimates vary by trial quality and era (Marmot, 2012). Estimates of overdiagnosis differ widely between analyses, a divergence that largely explains why expert bodies reach different conclusions about the net value of screening (Gøtzsche, 2013).

History

Randomized trials of mammography from the 1960s onward, beginning with the Health Insurance Plan study and the Swedish two-county trials, established that screening could reduce breast-cancer mortality and led to the spread of population programmes. From the 2000s, systematic reviews increasingly emphasized the harms of false positives and overdiagnosis, producing sharply differing appraisals of the net benefit and prompting independent reviews to reconcile the evidence (Marmot, 2012; Gøtzsche, 2013).

Debates

How large is overdiagnosis from mammographic screening?

Estimates of the proportion of screen-detected cancers that are overdiagnosed range from a few percent to a third or more depending on method and assumptions; because overdiagnosis is the principal harm weighed against the mortality benefit, this uncertainty drives the disagreement between guideline bodies.

Key figures

• Michael Marmot
• Peter Gøtzsche
• Heidi Nelson
• Kevin Oeffinger

Seminal works

• marmot-2012
• nelson-2009

Frequently asked questions

Why is breast cancer screening more controversial than cervical or colorectal screening?

Mammography mainly finds cancer earlier rather than removing precursors, and a meaningful share of detected cancers may never have caused harm; this overdiagnosis, whose size is uncertain, must be weighed against the mortality benefit, which is why expert groups disagree on age and interval.

What is overdiagnosis in breast screening?

Overdiagnosis is the detection of a breast cancer that would never have produced symptoms or caused death during the person's life; because it cannot be distinguished from harmful cancer at diagnosis, it can lead to treatment that provides no benefit.