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Genetic studies to identify alterations that are directly associated with certain kinds of tumour.
These tests offer people the chance to detect cancer early on, depending on their sex, age and risk of developing a specific kind of cancer.
The main objective of screening is to detect the cancer in the early stages, before the patient notices any symptoms and when it is easier to treat and cure. In other cases, the tests highlight changes to cells or noncancerous precursor lesions that may eventually evolve into cancer if they go untreated.
Screening programmes are applied to breast cancer, cervical cancer and colorectal cancer, as scientific evidence has shown that when these cancers are detected early enough there is a reduction in mortality.
Although the primary goal of screening is to discover the potentially most aggressive tumours, it is true that the results of the test increase the total number of tumours diagnosed as many of them would go unnoticed if it weren’t for these programmes. However, there is no certainty that the tumours would have developed sufficiently to produce symptoms.
Screening programmes employ accurate procedures and diagnostic tests (particularly in the field of medical imaging). Medical imaging studies, for example, revolve around making comparisons against previous images, waiting to see if further changes occur, or proceeding directly to a biopsy (if possible).
With regard to the specific tests, their reliability is an important consideration that depends on the percentage of random errors they produce. No test is 100% reliable. That means none of the tests are guaranteed to be totally free from random errors. In practice, the tests are so reliable that any errors that occur have a minimal impact.
However, in the case of screening, the reliability of the tests generates two types of error: false positives (positive test results even though the individual does not have the disease), and false negatives (the test result is negative despite the fact that the person does have cancer). False positives not only give rise to psychological stress but are also a cause of overdiagnosis (more tests are required to confirm the result) or overtreatment (unnecessary treatments administered to a person who does have the disease). Overdiagnosis and overtreatment are not without risks.
False negatives have a lesser impact given the reliability of most of the tests, but they highlight the need to clarify that there is no such thing as “zero risk” despite the implementation of good screening programmes. Screening programmes increase the detection of tumours in early stages of their development and minimise the risk, but they do not eliminate it completely.