A good start is ensuring that all are in agreement that mammography does not prevent breast cancer. This point is made
quite clearly at the start of the article but is worth repeating as misleading slip-ups can occur when a message is being
repeated on many occasions to different people. Later in the article the authors fall prey to this error themselves when
they incorrectly suggest that women with a breast cancer gene can “...reduce their risk of developing breast cancer
with options including more frequent screening and starting (mammography) at a younger age”.
We do know that mammograms can detect a breast cancer before it is symptomatic, although this in itself does not mean
that the person will survive the breast cancer. This is where the statistics can begin to deceive. The authors state
the relative risk reduction (of death from breast cancer) for woman undergoing regular mammography as 25% to 30%. If
a thousand women are screened with mammograms for ten years two will die from breast cancer instead of three (the figure
for the unscreened population). A general practice of say 2000 patients might have 350 eligible women and would need
to run a 100% uptake rate mammogram programme with no drop-outs for thirty years to prevent one of these women from dying
from breast cancer. Because abnormal results are quite frequent, and 90% of those are false positives, by the time these
woman have completed all their free mammograms half of them will have had one or more positive results and undergone
further investigation to discover
that they do not have breast cancer.*
My point is not to attempt to address the good versus harm debate, but simply to ask if women are being given the opportunity
to make an informed decision for themselves? The most informative of Breastscreen Aotearoa’s various multilingual
information leaflets (HE1801) mentions the existence of false negatives and false positives but quotes no figures at
all in terms of either relative or absolute risk reduction. It, therefore, falls to clinical staff to answer patient’s
questions and we better be sure we have our facts right.
To allow women to make informed decisions about breast screening, general practitioners and practice nurses need to
be able to discuss with their patients the pros and cons of screening and to understand how New Zealand guidelines are
arrived at.
The aim of any cancer screening programme is to ensure that nobody with cancer goes undetected. As a result, some
people will be called back for a secondary examination due to suspicious or indeterminate results, but in the majority
of cases, cancer is not confirmed in these patients, i.e. a false positive. The harm (i.e. anxiety) associated with
false positive results needs to be weighed up with the benefits of screening.
The National Screening Unit recommendations aim to reduce the amount of breast cancer false positives by targeting
women in the age range of 45 to 69 years with biannual breast screening, because:
- Breast cancer rates are significantly elevated in this age group
- Biannual testing provides 70 to 99% of the benefits of annual testing1
Screening more frequently, or screening of a wider cohort is not performed because:
- Detection of breast cancer by mammogram is more difficult in younger women due to denser tissue and false positives
are more common
- Annual testing significantly increases the number of false positives2
It is generally accepted that the relative risk reduction for international breast screening programmes with a 70%
participation rate is 20-30%.3,4 What makes the relative risk reduction meaningful is the incidence of breast
cancer. Each year approximately 2300 New Zealand women develop breast cancer and 630 will die from it. This makes breast
cancer the leading cause of cancer death for women aged 45 to 69.5 Applying a 25% risk reduction to a New
Zealand setting means that if no screening were to occur at all, then each year approximately 840 women would die, i.e.
210 more than if screening did occur.
The absolute risk reduction is calculated by determining the risk of dying from breast cancer and applying the relative
risk reduction to this figure, if breast screening occurs. For example, if the risk of dying from breast cancer in a
60 year old woman in the next ten years was 9 in 1000, then screening would reduce this risk by 20-30%. This means that
a woman in this age group now has a 6 to 7 in 1000 chance of dying from breast cancer if she has biannual breast screening.
As the absolute risk of dying from breast cancer decreases with age, younger women derive less benefit from the relative
risk reduction achieved from breast screening.
However, perhaps a more important statistic is the number of women that need to be screened to prevent one death.
A meta-analysis, published in the United States, of six trials among women aged 50 to 59 years and two trials among
women aged 60 to 69, calculated that the number of women needed to be screened by mammography, every two years, to prevent
one death, was 1339.1 In New Zealand, the uptake of breast screening among eligible women (i.e. aged 45 to
69 years) is approximately 67%,6 equating to over 450 000 women screened every two years.
Although the New Zealand breast screening programme undoubtedly prevents deaths, the trade off is the anxiety of false
positives and the discomfort and potential pain of the procedures required for screening and investigation. Through
informed discussion with their GP and practice nurse, every woman should have the right to make her own decision on
whether she undergoes breast screening.
N.B. The correspondent is correct in stating that mammography does not prevent breast cancer from occurring, it enables
detection of tumours that can then be treated to prevent the cancer developing and therefore to reduce the risk of death.
Mammography does not detect all tumours and the two year interval between screening means that some fast-growing tumours,
which are associated with a higher risk of mortality, may not be detected.
References
- U.S. Preventive Services Task Force. Screening for breast cancer: U.S. Preventive Services Task Force recommendation
statement. Ann Intern Med 2009;151(10):716-26.
- Elmore JG, MB B, Moceri VM, et al. Ten-year risk of false positive screening mammograms and clinical breast examinations.
N Engl J Med 1998;338(16):1089-96.
- Gummersbach E, Piccoliori G, Zerbe CO, et al. Are women getting relevant information about mammography screening
for an informed consent: a critical appraisal of information brochures used for screening invitation in Germany, Italy,
Spain and France. Eur J Pub Health 2009;20(4):409-14.
- Nelson H, Tyne K, Naik A, et al. Screening for breast cancer: systematic evidence review update for the U.S. Preventive
Services Task Force. US Preventative Services Task Force Evidence Syntheses. Rockville (MD), USA: Agency for Healthcare
Research and Quality, 2009. Available from:
www.ncbi.nlm.nih.gov/books/NBK36391/#ch4.s1
(Accessed May, 2011).
- Ministry of Health. Cancer: New registrations and deaths 2007. In: Ministry of Health, editor. Wellington: Available
from: www.dhbnz.org.nz (Accessed May 2011), 2010.
- DHBNZ. National summary of PHO Performance 1 January 2010 - 30 June 2010. Wellington: DHBNZ, 2010. Available from:
www.dhbnz.org.nz (Accessed May, 2011).