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1. What is screening?
The concept of screening for cancer is simple. Instead of waiting for
disease to become obvious, i.e. until symptoms occur, healthy
individuals are tested to see if they have disease at an earlier
stage.Screening tests are available for many types of cancer. In general
terms, a screening test must meet several criteria to be of practical
use. These ideal requirements include:
* It must be specific
This means that it only detects the disease it is looking for. For
example, if a test is positive for a number of different diseases, then
you cannot tell which is present. In other words, we don't want any
"false positives".
* It must be sensitive
This means that if the disease is present, the test must detect it. For
example, if a test is carried out on a patient who has the disease but
the result is negative, then the test is false. In other words, we don't
want any "false negatives".
* The disease must be common
There is no point (economically and socially) screening for rare
diseases.
* The natural history of the disease must be known
If a disease is curable when detected at an early stage, then screening
may be worthwhile. On the other hand, if a disease is incurable even
when detected early, then screening is of no value. In such a case,
early diagnosis without the prospect of treatment, would simply make a
patient aware of a terminal illness possibly years before it would have
surfaced by itself.
* Treatments must be available
There is no point detecting a disease if there is no treatment
available.
* The test must be acceptable to patients
Convincing healthy individuals who have no symptoms that they should
have a medical test could be a problem if the test is painful,
embarrassing, and so on. A good example of this is colonoscopy for bowel
cancer. Healthy people may not agree to have such a test as a routine
and this appears to be the case in practice.
* The test should be cost-effective
Society as a whole must decide where money is to be spent on health
allocation. Screening usually involves testing many thousands of
patients and the cost is often high.
2Who is eligible?
In the UK, the prevelence and incidence of breast cancer increase after
the age of 50 so women between the ages of 50 and 64 are sent an
appointment every three years for mammography (two-views on the first
occasion). There is no compulsion for them to attend, but obviously it
is strongly advisable that they do.There is some debate as to whether
screening should be performed every two years rather than three. At
present some women develop a cancer in the third year shortly before
they are due to have another mammogram. Having mammograms every two
years may detect these tumours earlier, but would obviously increase the
overall cost dramatically.
Over the age of 65, women can also have screening mammography, but they
normally have to make an appointment for themselves.Similarly, for women
under the age of 50, a screening mammogram can be done without the
presence of any symptoms, but this is usually when there is a strong
family history of breast cancer. See risk factors for further details.
3. Mammography
Screening for breast cancer is done using X-ray mammography. As a
screening test it fulfils many of the above criteria. i.e., it is very
specific and sensitive, showing abnormalities in the breast only, breast
cancer is common, early treatment of early disease is likely to be of
benefit, treatments are readily available and it is a reasonably cheap
test that is not too uncomfortable to patients.A two-view mammogram
consists of a craniocaudal view and an oblique (side-on) view. This
allows accurate determination of the position of any abnormality that is
seen. It also reduces the chance of "false-positives" because sometimes
a dense region can be seen on one view that may be considered
suspicious, but on the second view it is seen to be just normal breast
tissue.For more information on this test, see the mammography page.
4. Importance of screening
Screening is important because breast cancer is common and can affect
any woman. The test is quick and easy, and for the majority of women no
abnormality will be found. About 8% of women may be recalled for a
further mammogram, usually to clarify an area seen on the first two
X-rays. Even in these women, most will not have a serious problem such
as cancer. When a cancerous abnormality is detected, it is often ductal
carcinoma in situ. Early treatment of this disease is much better than
waiting for it to become ductal cancer. |
