Sunday 28 May 2017

How safe is breast cancer screening?

Dr Bridget O’Brien, general practitioner

Studies show that the benefits of organised breast cancer screening programmes save lives. But is that enough to close off debate?

'We cannot ignore the effects of over-diagnosis'

Breast cancer has become a very emotive subject over the last number of years and screening is recommended for all women aged 50-69 in Europe. The objective of a screening programme is to identify tumours at an early stage, and thus reduce mortality and improve patient outcomes.

Mammography is the screening method of choice here in Ireland, and involves the use of low energy X-rays. The breasts are compressed between two plates and the image obtained is then interpreted by radiologists. Computer-aided diagnosis is also incorporated into the diagnostic modalities used in screening.

A recent panel led by University College London's Professor Sir Michael Marmot analysed data from screening trials from a number of countries over recent decades. The report, published in 'The Lancet', concludes that screening reduces breast cancer mortality but that some over-diagnosis occurs.

It found that for every life saved in the UK, three women were over-diagnosed.

Similarly, a Norwegian study published in April 2012 suggests that between 15 and 25pc of invasive cancers are over-diagnosed.

This increase in incidence of breast cancer has been noted by population statisticians in all locations where mammography is used as a screening tool.

Another study of the Irish breast screening programme published in the 'American Journal of Roentgenology' in 2009 shows the incidence of breast cancer has further increased following the introduction of digital technology.

Digital mammography involves the utilisation of reduced radiation dose and increased compression to provide the image.

The study shows that there is a statistically significant increase in the incidence of breast malignancy, both in initial round screening and in subsequent screenings when this technology is used.

The health of those who have been exposed to this modality of screening and have subsequently developed breast cancer is also of concern.

A lady from my practice with a 7cm tumour had no particularly alarming features on her mammogram.

There was no evidence of a mass, and no abnormal calcification was noted. This report corresponds with the information that 1pc of tumours are not visible on mammogram.

Digital mammography results in a statistically significant increase in recall rates.

This increase can generate anxiety and distress in the women who are recalled.

Screening may be conducted in mobile units. In this event, the patient must be contacted, and a follow up appointment arranged. Levels of anxiety can be increased when a patient is contacted, and has to travel and attend clinics. Biopsies may be traumatic for the patient, and further increase the anxiety levels.

Radiation risk has long been recognised as a slight but measurable complication of breast cancer screening. The switch from analogue to digital imaging has produced remarkable results. There is a direct comparative analysis now available between the results from the analogue image, which uses more radiation and less compression, and digital, which uses more compression and less irradiation.

There is a demonstrable (30pc) increase in incidences of breast cancer in those screened using the digital technology, both on the first and on all subsequent screenings.

The Irish Breast Cancer screening programme has so far resulted in the diagnosis of breast cancer in more than 830 women.

Taking this 30pc figure into account, it is fair to speculate that around 240 fewer women might have been diagnosed with breast cancer had analogue imaging been utilised. Around 85 women may have a more aggressive cancer diagnosis as a consequence of this screening, taking the 10pc interval tumour rate into account.

This evidence might point to compression as an independent risk factor for the development of breast cancer.

The social, economic and health issues that arise for these women cannot be over-estimated. A screening program is designed to facilitate the early detection of cancers. It is important to recognise that the women called for screening are well, fit and asymptomatic. Thus, it is imperative that those offering a screening program would be in a position to guarantee the safety of women when they attend.

In my opinion it is unclear whether screening does more harm than good.

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