Download Efficiency of X-ray Mammography in detecting breast cancer

Efficacy of x-ray
mammography screening in
diagnosing breast cancer
Author:
Sahra Abdallah Haji
Tutors:
Dr A. Dora
Dr C. Elwell
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Contents
Executive Summary…………………………………………………………………...2
What is x-ray mammography?.......................................................................................3
How age affects the mammogram………………………………………………..……4
Alternatives to mammography………………………….……………………..…...….6
Is x-ray mammography efficient? ………………………………………………...…..7
Executive Summary
Mammography is an apparently safe procedure that is carried out with the minimum
radiation exposure possible. But, how efficient really is mammography in reducing
the death rate due to breast cancer? Is it worth using it to diagnose breast cancer
when the procedure could itself cause cancer? This report aims to evaluate the risks
of having a mammogram for women of different ages. Alternatives are also
considered. It should be used by medical specialists to assess whether it is worth
screening a female patient since breast cancer is more prevalent in women than men.
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What is X-ray mammography
In 1895, Wilhelm Roentgen took an x-ray image of his wife’s hand, Bertha. This was
only shortly after having discovered x-rays. The image was made possible because xrays are largely absorbed by materials of high densities such as bone. As a result,
these show up as white areas on x-ray film.
Mammography is a diagnostic imaging examination that exploits this property of xrays. It is significantly useful for the early detection and treatment of breast cancer
since it can spot cancers as small as a grain of table pepper (1). These can be easily
missed when taking a self examination as they are too small to sense.
The overall procedure should only take about ½ an hour or less. It involves tightly
compressing the breast between two parallel plates. A pulse of x-rays is then used to
take the image. The x-ray dose is low, usually around 2.03 mGy (2, 3). During the
procedure, the breast is flattened to evenly spread out the tissue (4). Resultantly, the xray attenuation is about the same for the overall region of the breast. In addition, the
radiation dosage absorbed by the breast is at a minimum.
The craniocaudal view of the mammogram, looking vertically down on the breast, is
the most anatomically recognisable. It is the main one used to determine whether
abnormal growths, cysts or tumours are present in the breast. These appear as areas
of abnormally high density in comparison to normal breast tissue.
(5)
Figure 2- Craniocaudal
view f right breast
Figure 2 - Mediolateral
oblique view of left
breast
(6)
Nonetheless, other views such as a medio-lateral oblique view, looking at the breast
from the opposite shoulder, are also taken. This is so that the densities of the tissue
can be compared from different angles. Comparisons of mammograms helps reduce
the probability of false positives and false negatives (7).
1
www.ehealthmd.com/library/hrt/HRT_glossary.html
http://www.ingentaconnect.com/content/rsm/jms/1998/00000005/00000002/art00006
3
http://bjr.birjournals.org/cgi/content/abstract/73/867/278
4
http://www.cancer.org/docroot/PED/content/PED_2_3X_Mammography_and_Other_Breast_Imaging
_Procedures.asp
5
http://brighamrad.harvard.edu/Cases/bwh/images/268/reillymammo1.gif
6
http://breast-cancer-research.com/content/2/6/392/figure/F1
7
http://cmic.cs.ucl.ac.uk/studentships/
2
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How age affects the mammogram
Over the past century, there have been a number of medical debates over the age at
which regular screening should commence. In the UK, most women between 50 and
70 have been invited for a mammogram every three years since 1988 (8,9). But,
younger women from about the age of 34 can be screened as well especially if these
women have first-degree relatives that have had cancer.
“Mammographic screening before age 50 years is less effective than at older ages and
the associated radiation risks are higher. (10)”
This is because young women have denser breasts (11) that contain microcalcifications
(12)
and have thick epithelial tissue. As a result, a younger woman’s breast absorbs
more of the radiation from the surface (13). Therefore, the radiation is less likely to
penetrate deeper into the breast tissue, which is the area where malignant cancers are
most likely to be found.
Microcalcifications are calcium ions, which are spread out inside the fatty tissue of
the breast in an inhomogeneous manner. As well as increasing the absorbed dosage
of radiation, they reduce the image quality of mammograms. Consequently, it is
difficult to differentiate between smaller cancers and the microcalcifications. This is
very significant since the whole point of screening is to find the cancer in its early
stages (14), before it has the chance to spread. If the internal features of the breast are
ambiguous, then there is no point of taking the mammogram in the first place.
Furthermore, it increases the probability of false positives.
However, since breast cancer takes about 20 years to develop, it is less likely that
young women have cancer anyway, Up to age 25, 1 in 15,000 (15) will develop breast
cancer. This means that screening younger women unnecessarily exposes other
organs to radiation, without contributing to the diagnosis. Since high density breasts
absorb more radiation, the breast screening of young women may actually induce
cancer. Sadly, cancers grow more rapidly in younger women (16). So, there’s an
increased risk of them developing the induced cancer.
Fortunately, breast density is not constant throughout women’s lifetimes (17). Older
women tend to have smaller breast density. Additionally, as women get older, the
thickness of the epithelial tissue decreases. On a mammogram, epithelial tissue would
normally appear white and make the internal features of the breast harder to
distinguish from the surface. But, not only do older women’s breasts have thinner
epithelial tissue and low breast density but they also have higher fat content. The
8
http://www.cancerscreening.nhs.uk/breastscreen/publications/nhsbsp-the-facts-english-2007.pdf
http://www.cancerscreening.nhs.uk/breastscreen/
10
http://www.nature.com/bjc/journal/v93/n5/full/6602683a.html
11
http://www.medicalimagingmag.com/issues/articles/2003-05_01.asp
12
http://www.medicalimagingmag.com/issues/articles/2003-05_01.asp
13
http://bjr.birjournals.org/cgi/content/abstract/73/867/278
14
http://www.netdoctor.co.uk/health_advice/examinations/mammography.htm
15
http://www.cancerscreening.nhs.uk/breastscreen/breastcancer.html
16
http://womens-health.jwatch.org/cgi/content/full/2004/1208/1
17
http://jnci.oxfordjournals.org/cgi/content/full/96/19/1432
9
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resultant effect is a higher mammographic sensitivity
mammograms that are less problematic to interpret.
(18)
, which results in clearer
Overall, this means it is more beneficial to image older women because they are more
at risk of getting breast cancer anyway. Up to the age of 70, 1 in 15 will develop
breast cancer at some time in their life (19). And so, older women are more likely to be
diagnosed with breast cancer. In addition, the chances of inducing breast cancer from
radiation, decreases with age (20). Therefore, it is unlikely to have been caused by the
procedure.
There are other factors, which are thought to affect how effective the diagnosis is.
These are the ethnicity of the patient; whether the patient smokes and whether there is
a family history of breast cancer. In addition, the patient’s menopausal status and
how often the patient is screened can also have a significant consequence on how
effective the diagnosis is (21).
18
http://jama.ama-assn.org/cgi/content/abstract/276/1/33
http://www.cancerscreening.nhs.uk/breastscreen/breastcancer.html
20
Cancers detected and induced, and associated risk and benefit, in a breast screening programme, The
British Journal of Radiology, 74 (2001), 1121–1127 © 2001 The British Institute of Radiology
21
http://jama.ama-assn.org/cgi/content/abstract/276/1/33
19
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Alternatives to mammography
There are other emerging options for detecting breast cancer and improving image
quality:
Full field digital mammography provides the high resolution (22) and contrast required
to produce high quality mammographic images (23). Clinical trials in the US have
suggested it could especially be useful for imaging ‘radiographically "dense"
breasts’(24). Nevertheless, equipment and maintenance costs would be significantly
more than the standard methods used (25). Hence, it would take too long to implement.
Magnetic resonance imaging (MRI) is more sensitive than
mammography ( 26 ). It can successfully image dense breast
tissue as well as breast implants. These are difficult to image
using traditional mammography. Furthermore, the images can
be reconstructed in 3D so the exact positions of the cancer can
be located.
This is particularly beneficial for young women and those at
high risk of getting cancer (27). It is safer because MRI does not
use radiation thus it cannot induce cancer. Nevertheless, it is
not the most cost-effective option. Currently, a yearly MRI
screening is not recommended for women who have a breast
cancer lifetime risk of less than 15% (28).
Figure 3 - 3D MRI
scan of breast from
42-year-old patient
22
http://books.google.com/books?hl=en&id=cRIEuSBBEAMC&dq=breast+imaging&printsec=frontco
ver&source=web&ots=LnRVYKC0Jx&sig=1RiVQ6nDkrLHpPsVoXgKHzBrNcE#PPA416,M1
23
http://www.cancernews.com/data/Article/210.asp
24
http://www.cancernews.com/data/Article/210.asp
25
http://en.wikipedia.org/wiki/Mammography
http://www.cancer.org/docroot/PED/content/PED_2_3X_Mammography_and_Other_Breast_Imaging
_Procedures.asp
27
http://www.radiologyinfo.org/en/info.cfm?pg=breastmr&bhcp=1
28
http://www.cancer.org/docroot/PED/content/PED_2_3X_Mammography_and_Other_Breast_Imaging
_Procedures.asp
26
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Is x-ray mammography efficient?
Breast cancer is one of the major causes of death from cancer for women and is
second only to lung cancer (29). So, doctors obviously need to have a fast, safe and
economically effective way of diagnosing the illness without making it worse. This
should also lead to a clear reduction in the mortality rate from breast cancer.
X-ray mammography is still used because most doctors believe the benefits of finding
out whether a patient has breast cancer far outweigh the risks of inducing the cancer.
Nonetheless, the main arguments against x-ray mammography are false positives,
false negatives and inducing cancer.
False positives cause patients to become stressed. They may find it hard to cope with
being told that they have cancer, only for them to be told later that it was a mistake.
However, this mostly applies to younger women since they are more likely to be
diagnosed with false positives.
False negatives, allow the cancer to develop since the cancer is not discovered straight
away. Again, this affects younger women more because a cancer lump could be
mistaken for a microcalcification.
For a two-year screening interval, the ratio between the number of cancers induced to
those prevented is 1:242 for 50-69 and 1:66 for 40:49 years of age (30).. This ratio
would decrease for a three-year screening interval. This is sufficient evidence to
show that mammography screening for women between 50 and 69 years of age, is
more beneficial than for women who are 40-49 and younger (31).
In the UK, breast cancer is being detected earlier for women aged 50 and over (32).
Furthermore, largely due to the implementation of the NHS Breast Screening Project,
there has been a reduction in the mortality rate due to breast cancer (33). This means
that breast screening in the UK seems justifiable, for now.
29
http://books.nap.edu/catalog.php?record_id=11016
http://www.ingentaconnect.com/content/rsm/jms/1998/00000005/00000002/art00006
31
http://www.iarc.fr/ENG/Press_Releases/archives/pr139a.html
32
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T1B-41CNTJ62&_user=5193083&_origUdi=B6T1B-41CNTJ6S&_fmt=high&_coverDate=01%2F08%2F2000&_rdoc=1&_orig=article&_acct=C000010182&_versi
on=1&_urlVersion=0&_userid=5193083&md5=1f85f74265d2a41539073ff13d1f229c
33
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T1B-41CNTJ62&_user=5193083&_origUdi=B6T1B-41CNTJ6S&_fmt=high&_coverDate=01%2F08%2F2000&_rdoc=1&_orig=article&_acct=C000010182&_versi
on=1&_urlVersion=0&_userid=5193083&md5=1f85f74265d2a41539073ff13d1f229c
30
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