Cancer Screening
The goal of cancer screening is a very practical one to detect cancer at an early state when it is treatable and curable. How ever, the reality is quite complex. For a screening test to be useful, the test or procedure should detect cancer earlier than would occur otherwise, and there should be evidence that ear lier diagnosis results in improved outcomes. Advances in genetics and molecular biology undoubtedly will make it possible to detect cancer at earlier and earlier stages along the car cinogenesis pathway. Thus, the lien between prevention and screening may narrow further, as it has for colorectal and cervical cancers. In spite of the potential of molecular diagnostics, screening must be evaluated according to its present reality, which is to detect asymptomatic disease when it is potentially curable and can reduce death from cancers. The potential of screening still has not been fully realized. A report by the National Cancer Policy Board and the Institute of Medicine estimated that a 19% decline in the rate at which new cancers occur and a 29% decline in the rate of cancer deaths could be achieved by 2015 through changes in behavior and greater dissemination of proven technologies, including cancer screening. The National Cancer Policy Board also estimated that appropriate use of screening among person aged 50 and older could reduce the mortality from colorectal cancer by 30% to 80%; screening among women aged 50 and older could reduce mortality from breast cancer by 25% to 30%, and screening women age 18 and older could reduce the rate of cervical cancer mortality by 20% to 60%.
This is the overview of cancer screening what it is, key terms, measures of effectiveness, and consequences. We also review briefly the status of screening for several prevalent cancers (breast, cervical, skin, prostate, colorectal, and lung). The recommendations are for the general population, and not for people with identified mutations in cancer susceptibility genes. High-risk populations are the subjects of other chapters. Generally, it is recommended that people with cancer susceptibility mutations be screened at earlier ages and more frequently for the cancers to which they are predisposed.
This overview cannot provide comprehensive information on any of the cancer sites reviewed, because the literature for each is vast. Rather, the chapter provides a succinct summary of the field and a perspective for clinicians to use in determining the efficacy of particular screening tests. Several texts and on line resources provide good overviews of the issues related to screening for specific kinds of cancer. The National Cancer Institute Physician Data Query is an excellent source for the latest data on the specific screening tests discussed here. Similarly, the American Cancer Society (ACS) provides current information on cancer rates and ACS screening guidelines. Many other good sites are available as well.
What is Cancer Screening?
Appropriate cancer screening should lead to early detection of asymptomatic or unrecognized disease by the application of acceptable, inexpensive tests or examinations in a large number of persons. The results of a screening test then should be applied expeditiously to separate apparently well persons who probably have disease from those who probably do not. The main objective of cancer screening is to reduce morbidity and mortality from a particular cancer among persons screened.
Several characteristics make particular cancers suitable for screening. These include substantial morbidity and mortality, high prevalence in a detectable pre-clinical state, possibility of effective and improved treatment because of early detection, and availability of a good screening test with high sensitivity and specificity, low cost, and little inconvenience and discomfort. Only breast, cervical, and colorectal screening have met the rigorous criteria of the U.S. Preventive Services Task Force and have sufficient high-quality evidence to justify population based screening.
Evaluation of a screening Test
In evaluating a screening test, it is essential to answer questions concerning its ability to accurately predict a presence of absence of disease. If the test result is abnormal, what are the chances that disease is present? If the test result is normal, what are the chances that disease is absent? The validity of a screening test is measured by whether it correctly classifies those persons who have disease as test-positive and those without it as test-negative.
Sensitivity and specificity address the validity of screening tests. Sensitivity is the probability of testing positive if the disease is truly present. As sensitivity increases, the number of persons with the disease who are classified as test-negative (false-negative) decreases. Specificity is the probability of screening negative if the disease is truly absent. A highly specific test rarely is positive in the absence of disease and therefore results in a lower proportion of persons without disease who are incorrectly classified as test-positive (false-positive). PV+ is an estimate of test accuracy in predicting presence of disease; PV – is an estimate of the accuracy of the test in predicting absence of disease. Predictive value is a function of sensitivity, specificity, and prevalence of disease. Accuracy is a measure of the percent of all results that are true results, whether positive or negative, that is total correct test results. A screening test that results in many false-positive findings is inefficient and potentially dangerous, because it subjects people to follow-up procedures that are often costly, with a range of risks. Similarly, false-negative results can be life threatening, because they miss cancers that could possibly be identified and treated. Every organized screening program must balance the potential for false positive and false-negatives when establishing criteria for follow-up.
Sources of bias are of particular importance in evaluating screening programs. People who choose to participate in screening programs (volunteers) are likely to be different from the general population in ways that affect survival; thus, volunteer bias is a concern. Lead-time bias is defined as the internal between diagnosis of disease at screening and when it would have been detected due to the development of symptoms. If lead-time bias is not taken into account, survival erroneously may appear to be increased among, screen-detected cases as compared to unscreened cases. Finally, length bias is the over-representation among screen-detected cases of those with a long pre-clinical period (thus, less rapidly fatal), leading to the incorrect conclusion that screening was beneficial.
The goal of cancer screening is a very practical one to detect cancer at an early state when it is treatable and curable. How ever, the reality is quite complex. For a screening test to be useful, the test or procedure should detect cancer earlier than would occur otherwise, and there should be evidence that ear lier diagnosis results in improved outcomes. Advances in genetics and molecular biology undoubtedly will make it possible to detect cancer at earlier and earlier stages along the car cinogenesis pathway. Thus, the lien between prevention and screening may narrow further, as it has for colorectal and cervical cancers. In spite of the potential of molecular diagnostics, screening must be evaluated according to its present reality, which is to detect asymptomatic disease when it is potentially curable and can reduce death from cancers. The potential of screening still has not been fully realized. A report by the National Cancer Policy Board and the Institute of Medicine estimated that a 19% decline in the rate at which new cancers occur and a 29% decline in the rate of cancer deaths could be achieved by 2015 through changes in behavior and greater dissemination of proven technologies, including cancer screening. The National Cancer Policy Board also estimated that appropriate use of screening among person aged 50 and older could reduce the mortality from colorectal cancer by 30% to 80%; screening among women aged 50 and older could reduce mortality from breast cancer by 25% to 30%, and screening women age 18 and older could reduce the rate of cervical cancer mortality by 20% to 60%.
This is the overview of cancer screening what it is, key terms, measures of effectiveness, and consequences. We also review briefly the status of screening for several prevalent cancers (breast, cervical, skin, prostate, colorectal, and lung). The recommendations are for the general population, and not for people with identified mutations in cancer susceptibility genes. High-risk populations are the subjects of other chapters. Generally, it is recommended that people with cancer susceptibility mutations be screened at earlier ages and more frequently for the cancers to which they are predisposed.
This overview cannot provide comprehensive information on any of the cancer sites reviewed, because the literature for each is vast. Rather, the chapter provides a succinct summary of the field and a perspective for clinicians to use in determining the efficacy of particular screening tests. Several texts and on line resources provide good overviews of the issues related to screening for specific kinds of cancer. The National Cancer Institute Physician Data Query is an excellent source for the latest data on the specific screening tests discussed here. Similarly, the American Cancer Society (ACS) provides current information on cancer rates and ACS screening guidelines. Many other good sites are available as well.
What is Cancer Screening?
Appropriate cancer screening should lead to early detection of asymptomatic or unrecognized disease by the application of acceptable, inexpensive tests or examinations in a large number of persons. The results of a screening test then should be applied expeditiously to separate apparently well persons who probably have disease from those who probably do not. The main objective of cancer screening is to reduce morbidity and mortality from a particular cancer among persons screened.
Several characteristics make particular cancers suitable for screening. These include substantial morbidity and mortality, high prevalence in a detectable pre-clinical state, possibility of effective and improved treatment because of early detection, and availability of a good screening test with high sensitivity and specificity, low cost, and little inconvenience and discomfort. Only breast, cervical, and colorectal screening have met the rigorous criteria of the U.S. Preventive Services Task Force and have sufficient high-quality evidence to justify population based screening.
Evaluation of a screening Test
In evaluating a screening test, it is essential to answer questions concerning its ability to accurately predict a presence of absence of disease. If the test result is abnormal, what are the chances that disease is present? If the test result is normal, what are the chances that disease is absent? The validity of a screening test is measured by whether it correctly classifies those persons who have disease as test-positive and those without it as test-negative.
Sensitivity and specificity address the validity of screening tests. Sensitivity is the probability of testing positive if the disease is truly present. As sensitivity increases, the number of persons with the disease who are classified as test-negative (false-negative) decreases. Specificity is the probability of screening negative if the disease is truly absent. A highly specific test rarely is positive in the absence of disease and therefore results in a lower proportion of persons without disease who are incorrectly classified as test-positive (false-positive). PV+ is an estimate of test accuracy in predicting presence of disease; PV – is an estimate of the accuracy of the test in predicting absence of disease. Predictive value is a function of sensitivity, specificity, and prevalence of disease. Accuracy is a measure of the percent of all results that are true results, whether positive or negative, that is total correct test results. A screening test that results in many false-positive findings is inefficient and potentially dangerous, because it subjects people to follow-up procedures that are often costly, with a range of risks. Similarly, false-negative results can be life threatening, because they miss cancers that could possibly be identified and treated. Every organized screening program must balance the potential for false positive and false-negatives when establishing criteria for follow-up.
Sources of bias are of particular importance in evaluating screening programs. People who choose to participate in screening programs (volunteers) are likely to be different from the general population in ways that affect survival; thus, volunteer bias is a concern. Lead-time bias is defined as the internal between diagnosis of disease at screening and when it would have been detected due to the development of symptoms. If lead-time bias is not taken into account, survival erroneously may appear to be increased among, screen-detected cases as compared to unscreened cases. Finally, length bias is the over-representation among screen-detected cases of those with a long pre-clinical period (thus, less rapidly fatal), leading to the incorrect conclusion that screening was beneficial.
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