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From the January 2010 SA Menopause Matters website & latest newsletter::
It is difficult to calculate the precise value of screening mammography. There has been a significant reduction in deaths from breast cancer in developed countries over the last 20 years due to early detection, better initial care, more targeted adjuvant follow-up and a multi-disciplinary team approach. Quantifying the contribution of each of these components is the problem if one is trying to define the benefit of screening. A natural experiment occurred when Norway introduced a national screening programme state by state over a number of years so that the effect of screening, as opposed to, say, a more sophisticated team strategy could be teased out. Kalager et al (NEJM 2010;363:1203-10) showed that screening’s effect was modest – about 10% in women aged 50 – 70 years. In women over 70 years it was 2%.
Interesting data emerge if one applies these figures to the US population where the prevalence of breast cancer is high and the pressure on women to undergo screening is considerable. Welch in an editorial (NEJM 2010;363:1276-8) explains the effects of screening as follows:
The risk of a 50 year old woman dying from breast cancer in the next 10 years of her life is 0.4% (or 40 per 10 000) – this calculation includes screening.
Put in the reverse frame of reference: 9 960 per 10 000 will not die!
Screening contributes 10% to this survival, so without screening 9 956 women will not die. The number of lives saved by screening is thus 4 per 10 000 women per 10 years of screening.
Using “numbers needed to treat” 2 500 women would need to be screened for 10 years to save one life. This is the benefit of screening. In other words, 25 000 women are screened initially to save 1 life. The harms are what happen to the 2 499 women who are screened that do not die. Depending on screening techniques, roughly 1 000 of the 2 499 can anticipate a false-positive result. This number rises with the frequency of mammography and is calculated on screening every 2 years, ie 4 in 10 women are recalled.
Over-diagnosis and over-treatment is more sinister and would occur in 10 of the 2 499 survivors. This is unnecessary treatment of a condition that was never going to bother them, because of xray screening, 1 in 250 are going to be over-diagnosed and over-treated. False-positive diagnoses and over-treatments are the harms of screening.
Despite these statistics, screening in the US is given considerable status. The public “perceives screening mammography to be one of the most important services provided by modern medicine”. This is patently untrue but it is sustained by energetic marketing of a multimillion dollar industry that relies on “more screening” for its existence and profits.
There seems little publicity given to the downside of screening and plenty of push for increasing the percentage of women covered. At present, about 80% of middle-aged women are screened every 2 years (Vital Signs www.cdc.gov.mmwr 2010;59:813-6). It is very much part of the American psyche but as Welch concludes “The time has come for breast screening to stop being used as an indicator of the quality of our health care system.”
Dr Neil D Burman MBChB (UCT 1966), MRCP(UK 1974),
Specialist Physician (Menopause, Andropause)