Below is information about breast screening and overdiagnosis.

Overdiagnosis in relation to breast cancer refers to a small number of cancers or groups of abnormal cells (usually DCIS) that are detected through breast screening which even if left untreated would not progress to become life threatening.


The term overdiagnosis does not mean an error or a misdiagnosis but refers to cancers or groups of precancerous cells that are present in the breast Overdiagnosisbut would not grow to become life threatening.

Estimating the level of overdiagnosis is difficult, mainly because it is not possible to tell which cancers will not progress. Most well designed research studies suggest the level of overdiagnosis is low. The World Health Organization estimates that for every one or two overdiagnosed cases, at least one death due to breast cancer was avoided[1].

Breast screening saves lives and reduces harm because it detects breast cancers before they can be seen or felt. This allows for less invasive treatment, better recovery and peace of mind. The effectiveness of breast screening in saving lives and reducing the impact of breast cancer, was recently confirmed by the World Health Organization’s International Agency for Research on Cancer (IARC).

The IARC also found that the Cochrane Review (which raises questions about the benefits of screening mammography) did not look at the best sources of evidence to understand the effectiveness of population breast screening. 

Since the introduction of the BreastScreen program in 1991, there has been a sharp decline in breast cancer-related deaths due to the combination of early detection and advances in management and treatment. Even with advances in treatment, early detection still plays a major role in preventing breast cancer deaths.

All women aged 50-74, if they have no symptoms of breast cancer, should have their free mammogram through BreastScreen NSW every two years.

If a breast screen shows the need for further assessment, a specialist team of doctors, nurses, radiologists, surgeons, breast physicians and counsellors will support women to understand their assessment results in order to make informed choices about their treatment. For more information download the brochure, BreastScreen and You.  

The Canadian national breast cancer screening trial

A recent study which reports results of a 25 year follow-up of the Canadian breast screening trial[2] has often been held up as evidence that mammography screening does not save lives and can result in high levels of overdiagnosis. The findings of the Canadian trial are at odds with all other mammography screening trials.

Preventing deaths from breast cancer through screening depends on finding cancers early. The Canadian trial has been criticised because it failed to detect breast cancers at an early stage[3]. This was possibly due to poor quality mammography. Concerns have also been raised about whether women who were allocated to receive screening may already have had signs of advanced cancer[4].  

Estimates of the level of overdiagnosis from the Canadian trial may not be accurate because many of the women who participated may have continued screening through organised screening programs introduced shortly after the end of the trial[5]. Also, if the level of overdiagnosis reported by these researchers was calculated over a woman’s lifetime, the risk is very low (around 3%).

A review of all the available evidence, as undertaken by the International Agency for Research in Cancer, indicates that the benefit of mammography screening in reducing breast cancer deaths outweighs the potential harm due to a small chance of overdiagnosis[6].

[1] WHO position paper on mammography screening 

[2] Miller AB, Wall C, Baines CJ, Sun P, To T, Narod SA. Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial. BMJ. 2014;348:g366.

[3] Kopans DB. An open letter to panels that are deciding guidelines for breast cancer screening. Breast Cancer Res Treat. 2015;151(1):19-25.

[4] Freedman DA, Petitti DB, Robins JM. On the efficacy of screening for breast cancer. Int J Epidemiol. 2004;33(1):43-55.

[5] Njor SH, Garne JP, Lynge E. Over-diagnosis estimate from The Independent UK Panel on Breast Cancer Screening is based on unsuitable data. J Med Screen. 2013;20(2):104-5.

[6] Lauby-Secretan B, Loomis D, Straif K. Breast-Cancer Screening--Viewpoint of the IARC Working Group. N Engl J Med. 2015;373(15):1479.

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