Cancer screening

What is cancer screening?

The aim of cancer screening programs is to pick up very early cancers (or conditions that may lead to cancer) in healthy individuals, before symptoms appear. The screening test helps to decide if a person is at high or low risk for developing the cancer, but it does not diagnose the cancer. Further tests will be needed to decide if the person does or does not have cancer.

What is the screening pathway?

Cancer screening includes the initial test as well as what happens after the test. This is called the screening pathway. The pathway includes any further testing needed to see if there is cancer (the diagnosis), appropriate treatment at the right time, and evaluation of the process to make sure the pathway is working correctly.

There are national screening programmes for some cancers, but some cancer screening can occur outside of these programmes. This is called opportunistic or unorganised screening. A common example of opportunistic or unorganised 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.    

Who is screening for?

Screening programmes are for those people most at risk of developing the specific cancer. This might mean a 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 risks of screening. Screening tests are for people who have no signs or symptoms of the cancer being screened for. If a person does have symptoms they will need diagnostic tests rather than 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. Individuals are considered high risk if they have either a genetic abnormality or 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 

Why don’t we screen everybody?

Screening everybody would be cost prohibitive. Screening helps with an early diagnosis is but it is not 100 percent accurate, and may indicate further testing is required for many people who do not actually have cancer. Only those people most at risk, who will gain the most benefits, are offered screening.    

Why don’t we screen for every type of cancer?

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, which means some people have to have further investigations to find 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:

 Criteria for Assessment, endorsement and identification of Action Relating to Cancer Screening

What are the benefits and risks of screening?

The main aim of cancer screening is to reduce the impact and incidence of cancer. Screening is like using a sieve. The purpose 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. If someone is told they have an abnormal test result they will need to have further tests to find if it is really cancer.

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).

Some screening tests pick up early cell changes that may or may not develop into a cancer so treatment is offered to prevent this from happening. An example of this is cervical screening, where abnormal cells are treated to prevent them from becoming cancerous. Because there is no way to know which abnormal cells will become cancerous and which won’t, they are all treated.

Some people who get caught in the ‘sieve’ with a positive result for cancer don’t actually 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’, with results that indicate they are fine when they may actually have the cancer being screened for. This is called a missed case or a false negative.

Some people will have cancers detected that may never have caused them any harm (sometimes called indolent cancers) but because there is no way to truly know this they still receive treatment for them. Some cancer treatments have serious side effects and 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. 

What does screening tell you?

Screening is not able to provide a definite ‘yes’ or ‘no’ answer to the question, “do I have cancer?” A positive result does not mean you definitely have cancer; rather your risk is higher and you need further investigation to determine whether you may or may not have cancer. 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.     

We know that early detection is the best policy and that there are screening tests available to help an early diagnosis. Talk to your health professional about the benefits and possible risks associated with these procedures.