May 2015 Health Bulletin

Screening mammograms

When each Australian woman turns 50 she will receive her first invitation to attend BreastScreen to have a mammogram. This is a 'screening' program which means women have a mammogram even though they have no symptoms. So the aim is to detect breast cancer before any symptoms occur (for example a breast lump), so that early treatment can reduce the likelihood of death from the disease.

Of course, for some women at high risk of breast cancer (for reasons such as a genetic mutation or strong family history), screening or even preventive management may have already started, but for the majority of women at average risk of breast cancer, the decision about whether to participate in breast cancer screening will be made at age 50.

No screening method is perfect. There will always be some "downsides" to screening so it is important to know about the benefits and harms before you decide whether to participate in the program.

Mammography is a sensitive test, so few cancers are missed. However there will be some women who are recalled after a mammogram who, on further investigation, are shown not to have breast cancer, so-called "false positives". The experience of being called back for a repeat mammogram or going on to have a biopsy for a woman who does not have breast cancer can cause considerable anxiety.

Another disadvantage of screening is the issue of "over-diagnosis". We know that screening tends to identify a different spectrum of breast cancer than the type that presents clinically. Breast cancers detected by screening tend to be more slow-growing (or indolent) than tumours that present clinically. The most extreme version of an indolent tumour is one that is growing so slowly that it would never be detected in the woman’s lifetime if she never had a mammogram- so that she would never have known she had breast cancer if she had not been screened. This is not a trivial matter as for a woman in this category, the treatment she will receive when diagnosed with breast cancer (surgery with radiotherapy and possibly endocrine therapy*) is all harm, with no prospect of benefit.

A group of researchers at the University of Sydney have done a study using focus groups which showed that women did not know about over-diagnosis and that doctors had not raised the issue with them (Hersch et al, 2013). However when the concept was presented to them they understood the implications of it and indicated that if over-diagnosis was sufficiently common, it would influence their decision about whether to undergo screening or not.

Different research groups have reported different estimates of the frequency of over-diagnosis. There are a number of factors that contribute to this variation, but it complicates explaining over-diagnosis to women seeking advice about screening.

One way of weighing up the benefits and harms of screening mammography is to use a decision aid. Decision aids have been developed for a range of medical tests. In a decision aid information is presented simply and clearly for both benefits and harms.

The research group at the University of Sydney have now completed a randomised controlled trial of the use of 2 different decision aids in women aged 48-50 years randomly selected from the electoral roll of New South Wales (so about to be part of the system of invitation to screening mammography) (Hersch et al, 2015). The women were divided into 2 groups. The first group, the control group, was provided with a decision aid that included information about the benefit of screening in terms of the reduction in deaths from breast cancer and the possible “harm” of a false positive diagnosis, but the issue of over-diagnosis was not mentioned. The second group, the “intervention group”, was provided with a decision aid that included the same information presented to the control group, as well as a section explaining over-diagnosis. The aim of the trial was to establish whether the women understood the issues well enough to make an “informed choice”. A woman was considered to have made an informed choice if she had adequate knowledge and her attitude to screening and intention to undergo screening or not undergo screening, were consistent.

The study found that after using either of the decision aids only a minority of the women were making an informed choice. However, the women in the intervention group were more likely to make an informed choice (24%) than the women in the control group (15%). Furthermore, although the majority of both groups did say that they intended to be screened, fewer women in the intervention group intended to be screened (74%) compared with women in the control group (87%).

The decision aid from the University of Sydney said that for 1000 women screened for 20 years, there would be 73 women diagnosed with breast cancer compared with 54 amongst those not screened- so a total of 19 women in the over-diagnosed category. As a fraction of the total number of women diagnosed, an extra 19 is a considerable proportion - it is not a marginal issue. This was compared with 12 women who might die of breast cancer in the unscreened group compared with 8 in the screened group (so 4 extra breast cancer deaths prevented by screening). This gives a ratio of breast cancer deaths prevented to cases over-diagnosed of 1:5. This is similar ratio to that found by the Independent UK panel of breast cancer screening in 2012 (Independent UK panel on Breast Cancer Screening, 2012).

Thus, in order to make an informed choice about whether to participate in mammographic screening most women will need some assistance to understand the issue of over-diagnosis. Health care providers need to be aware of this issue in order to assist women to make an informed choice.

*drug treatment such as tamoxifen

Hersch J, Barratt A, Jansen J, Irwig L, McGeechan K, Jacklyn G, ... McCaffery K. (2015). Use of a decision aid including information on overdetection to support informed choice about breast cancer screening: a randomised controlled trial. Lancet. doi: 10.1016/S0140-6736(15)60123-4.

Hersch J, Jansen J, Barratt A, Irwig L, Houssami N, Howard K, ...McCaffery K. (2013). Women's views on overdiagnosis in breast cancer screening: a qualitative study. British Medical Journal. doi: 10.1136/bmj.f158

Independent UK panel on Breast Cancer Screening. (2012). The benefits and harms of breast cancer screening: an independent review. Lancet. doi: 10.1016/S0140-6736(12)61611-0.

Information provided might not be relevant to a particular person's circumstances and should always be discussed with that person's own healthcare provider.