Principles of Cancer Screening

Screening is the application of a test to detect a potential cancer when no signs or symptoms of the cancer are present.5-7 Ideally, the cancer is detected before it is clinically apparent, when treatment may be more effective, less expensive, or both. If a screening test result is abnormal, a diagnostic test should be ordered and treatment pursued if cancer is discovered. The value of a screening test is compromised if symptomatic individuals are included in the target population, since those with symptoms may already have advanced disease that warrants a diagnostic evaluation. The gold standard for screening is the ability to decrease mortality from cancer. Therefore, the goal of ovarian cancer screening is to reduce mortality by detecting the cancer in earlier stages, when survival rates are improved.

There are a number of general principles of effective cancer screening that must be applied to ovarian cancer screening:

1. The disease must incur a high morbidity, high mortality, and high cost.

2. The natural history and biology of the disease process should be well characterized and understood.

3. There should be a high prevalence and incidence in the screened population.

4. The disease state should have a long preclinical phase allowing for early detection and intervention.

5. Effective treatment of early-stage disease should be superior to treatment in an advanced stage.

Box 6-1. Characteristics of a Good Cancer Screening Test

Sensitivity

75%

Specificity

99.60%

Positive predictive value

10%

Safety

Yes

Simplicity

Yes

Cost

Low

Compliance

High

The ideal outcome is for the cancer to be detected early enough to be cured.5,8 For ovarian cancer, the first and last principles apply: the disease incurs a high mortality rate, and effective treatment for early stage disease exists. However, the challenges of ovarian cancer screening lie in the remaining principles: knowledge of the natural history and biology is evolving, the disease incidence is low in the general population, and the preclinical phase can be estimated but is not definitively known.

The general characteristics of a good cancer screening test have been well described (Box 6-1). The test should have the capability to detect disease before symptoms are present. The test should have a high sensitivity (the probability of a test being positive in individuals with the disease), specificity (the probability of the test being negative in individuals who do not have the disease), and predictive values (the probability that a test is a true positive or a true negative). In reality, improving test sensitivity tends to result in a reduction of specificity. For example, a test with a lower cutoff for a tumor marker would pick up more patients with ovarian cancer (increased sensitivity), but more people who do not have ovarian cancer would also test positive (decreased specificity). The clinical consequence of the increase in false-positive tests for ovarian cancer may be more unnecessary surgeries. In addition, an effective screening test should be safe, have minimal adverse effects, and be simple and inexpensive to administer. Even if all of these criteria are met, the test would not be a good screening test if patients did not willingly submit to it. It is therefore imperative that individuals to be screened deem the test acceptable and worthwhile.

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