principles of screening programmes Flashcards
What is the aim of a screening programme?
Aims to identify asymptomatic people who may be at risk of a disease/condition. This is because it saves lives and money by catching the disease early.
What are the programme schedules for breast cancer and bowel cancer screening?
For breast cancer, women between the ages 50-70 are invited for screening every 3 years. This has demonstrated a 29% reduction in mortality. For bowel cancer, males and females between the ages of 60-70 are invited for screening.
What are the characteristics that make a perfect test for screening?
- Simple and Cheap
- Reproducible
- High sensitivity (true positive) and specificity (true negative).
- Minimal Complications
What are the Wilson and Jungner criteria that should be followed for a screening programme?
- Condition should be an important health problem
- The natural history of the condition should be known
- There should be a recognisable latent or early symptomatic stage.
- There should be a test that is easy to perform and interpret, acceptable, accurate, reliable, sensitive and specific.
- There should be an accepted treatment for the disease.
- Treatment should be more effective is started early.
- There should be a policy on who can be treated.
- Diagnosis and treatment should be cost-effective.
- Case-finding should be a continuous process.
What are the issues of breast cancer screening regarding pre-invasive cancers?
Although 2 randomised controlled trials demonstrated a 30% reduction in breast cancer mortality when a screening programme was initiated, the potential harms were not considered. We know of early/latent phases in breast cancer, but they may never develop into invasive cancers, e.g. low grade DCIS. For example, a trial looked at the number of women who developed IDC up to 42 years later. Only 11/28 women did. For a 60 year old woman, it may be considered unethical to treat her if there is only a small chance her cancer will spread to IDC 42 years after.
There is currently the LORIS trial which is assessing whether patients with low grade DCIS should undergo continual monitoring or standard treatment. This is a phase III trial with expected finish to occur in July 2020. The results of this trial may result in a complete shifting of the way breast screening programmes are carried out.
What are the issues of breast cancer screening programmes regarding false positive results and costings?
5% of women who go for mammography are recalled. Of these 1 in 6 women will be found to have cancer, therefore 5/6 women have undergo unnecessary stress. Although programmes have demonstrated a reduced mortality and an increase in 10-year survival rates from 40%-70% in the last 30 years (with survival being best in the screening age group), the programme itself costs £96 million. Is this the best way we can use the money? Maybe we could use this money to discover more targeted therapies, that would prevent mortality in even advanced cases.
There is also over diagnoses occurring causing a severe increase in psychological morbidity. Better screening tests would prevent this, as well as a reduced time for results to come back so that patients are waiting for months to find out.
A meta-analysis demonstrated that for every 2000 women screened, 10% will be false-positives and 30% will be over diagnosed. The MARMOT report showed that there is mortality benefit (1 death is prevented for every 250 people invited, however for every death prevented, 3 women are overdiagnosed. Is this worth it?
How does bowel cancer fit the criteria for screening?
- CRC contributes to ~10% of female cancer and 11% of male cancer, acting as the 4th commonest cancer worldwide.
2/3. We know that there is an early phase, e.g. polyps. - The current tests available to screen for this is ‘Foecal Occult Blood’ (FOB) test, which is simply, cheap, and practical. However this is combined with colonoscopy, which is used for patients who are considered at risk, from the FOB test. Combination of these tests results in a reduced number of people receiving the invasive colonoscopy.
No long term data is yet available due to the programme being implemented in 2009.
What are the predicted outcomes for bowel cancer screening?
Out of 16 patients called for colonoscopy, 8 will not have an abnormality, therefore they will have undergone an invasive procedure for no reason (also subjected to psychological stress). Around 6 of these patients will have polyps which can be removed. But again, these may have never become cancer, therefore we are potentially over-treating patients. Around 2 people will have bowel cancer detected.
In other words, from a positive FOB result, only ~11% of people will actually have cancer. Another 35% of people could have adenoma, therefore 46% of patients would be detected. However, adenoma patients may never progress to cancer, therefore we may be over-treating them and causing unnecessary stress.
Data prior to full roll out of CRC screening identified fantastic evidence for the efficacy of CRC screening. 10% of cancers were detected and 37% of high risk adenomas were detected. Of the 10% of cancers, 71% were early stage cancers. This is a fantastic result, as if left these patients may not have presented until very late stage cases (as often happens with CRC), therefore identifying these patients early improves their overall survival (based on Dukes staging) by over 80%.
Why is lung cancer not applicable for a screening programme?
Although an important clinical issue, as well as evidence to suggest if treatment was implemented earlier, survival would improve, there is no sufficient test which may be used to screen for lung cancer. Although chest X-rays are used for diagnosis, it would not be feasible to use these as the diagnostic screening test, as continuous X-rays can be harmful to individuals. Moreover, it is not a practical test. If further tests were developed using specific biomarkers, etc, then lung cancer screening programmes may be considered, however currently it does not meet the criteria outlined.
