Cancer screening is designed to lower the risk of dying from cancer in the general population. It is not for people who may have some symptoms. Screening is for people who are, as far as they know, completely well. A screening test is used to help identify those people who:
- have some early cell changes that may become cancer
- or have a cancer but are unaware of it
This enables those people to be offered treatment while the cancer is at an early stage or treat pre-cancerous changes that may, if left, develop into cancer.
Cancer screening is not just the screening test. Screening also includes what happens after the test. This is called the screening pathway. The pathway includes the further testing needed to see if there is cancer (diagnosis), appropriate treatment at the right time and evaluation of the process to make sure the pathway is working correctly. It is also important that after the screening test, each step of the pathway is able to cope with the extra people needing follow-up or treatment.
Some cancer screening can occur outside of a national screening programme. This is called opportunistic or unorganised screening. A common example of this type of screening is screening for prostate cancer. This type of screening is usually either offered by the health professional or requested by the person. Opportunistic or unorganised screening does not have the assurance of a monitored, consistent and best practice screening pathway. However it may still be quality health care.
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Screening programmes target those people most at risk of developing the specific cancer. This might mean the programme is just for men, just for women or just for people in a specific age group. This ensures that the people most likely to benefit are offered screening without exposing large numbers of extra people to the possible harms of screening. However all the people who go through a screening test are well. They do not have any signs or symptoms of the cancer being screened for. If a person has any symptoms they will need diagnostic tests not screening.
Some screening is particularly targeted at people who have a very high risk of developing cancer. Such programmes are sometimes called active surveillance programmes and offer different screening programmes to the general public ones. An example of this is the active surveillance programme offered to people with very high risk of developing bowel cancer. Their risk is very high because of either a genetic abnormality or they have a very strong family history of bowel cancer with many relatives diagnosed. (For more information see the New Zealand Guidelines website www.nzgg.org.nz Information for people at increased risk of bowel cancer)
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If everyone was screened for cancer not only would it cost a lot of money, but many people would need further tests most of which would be unnecessary as they will not have cancer. Some of these people may suffer from harms associated with the follow up tests. This is why only those people most at risk, who will gain the most benefits, are offered screening.
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To have an effective screening programme there needs to be a test that will act as the ‘sieve’. The test needs to be accurate enough to be able to detect as many true positive results as possible (the test’s sensitivity) but at the same time making sure that as many negative results as possible are true negatives (the test’s specificity).
Many tests are good at picking up the true positive cases but also give many false positives. So many more people have to have further investigations to decide if they do or do not have cancer. For most cancers there is no suitable screening test available.
For more information on screening criteria see:
The main aim of cancer screening is to prevent people dying from cancer. The benefit from screening is a reduction in the number of people dying from a specific cancer. However screening is like using a sieve to sort people. Most people will pass through the ‘sieve’ correctly being reassured that they do not have any signs of cancer at that time (a true negative result). The main aim of any screening test is to catch people in the ‘sieve’ that are showing early signs of cancer (a true positive result) allowing treatment to be offered when it is most likely to have a benefit.
Some screening tests pick up early cell changes that may or may not develop into a cancer but treatment is offered to prevent this from happening. An example of this is cervical screening- abnormal cells are treated to prevent them from becoming cancerous but many of these changes would not have become cancerous if left. The problem is that there is no way to know which will become cancerous and which won’t so all are treated. Some people who get caught in the ‘sieve’ don’t have cancer- these are called false alarms or false positives. Further tests will show that they do not have signs of cancer at that time.
Others will pass through the ‘sieve’, thinking they are fine but they actually have the cancer being screened for. This is called a missed case or a false negative.
Some people will have cancers detected that would never have caused them any harm (sometimes called indolent cancers) but they still get treated for them. Many cancer treatments have serious side effects that can cause long term harm. This is called over diagnosis and over treatment.
False positives, false negatives, over diagnosis and over-treatment are all possible harms of screening.
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Unfortunately a definite ‘yes’ or ‘no’ answer is not what you get from screening. A positive result only means that you need further investigation to determine whether you may or may not have cancer. It does not mean you definitely have cancer; rather your risk is higher and needs to be further checked. A negative result does not mean you are definitely cancer free. It just means that cancer has not been detected at that time, but does not mean that it may not develop in the future.
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There are several possible outcomes for someone having a screening test. They can be reassured that at that time they do not have any sign of cancer. If they are told they have an abnormal test result they will need to have further tests to decide if it is really cancer. If cancer is diagnosed then there are four possible outcomes:
1. Treatment is successful; screening made a difference to the patient’s life
2. Treatment is unsuccessful, patient dies; screening made no difference to the patient’s life
3. Treatment is successful but would have been if the patient had waited until symptomatic- screening made no difference to the patient’s life
4. Cancer may not have caused any problems throughout the patient’s life; screening caused the patient to be over diagnosed and the treatment was unnecessary.
Anybody thinking about taking part in any screening programme should be aware of the benefits and possible harms of that screening test, and make an informed choice as to whether screening is right for them. Everyone has the right to make a choice to participate or not.
Other information on screening
Last Updated: Tuesday 16 October, 2012