Screening for Cancer Prevention and Early Detection
In this section you will learn:
Research identifying the biological underpinnings of cancer initiation and development has led to screening tests that can be used for cancer prevention and early detection.
There are five types of cancer for which screening tests have been developed and used in the clinic to screen generally healthy individuals.
Independent groups of experts rigorously evaluate data on the benefits and potential risks of cancer screening tests before putting forth recommendations about the use of the tests.
Every person has a unique risk for each type of cancer based on his or her genetic, molecular, cellular, and tissue makeup, as well as his or her lifetime exposures to cancer risk factors.
Some people are at increased risks for certain cancer types and may need to take measures to reduce the risks.
There are significant disparities in cancer screening rates among certain segments of the U.S. population.
Most cancers arise and progress as a result of the accumulation of genetic mutations that disrupt the orderly processes controlling the multiplication and life span of normal cells. There are numerous factors that cause cells to acquire genetic mutations (see sidebar on Sources of Genetic Mutations and Figure 4). The identity, order, and speed at which a cell acquires genetic mutations determine whether a given cancer will develop and, if a cancer does develop, the length of time it takes to happen.
Knowledge of the causes, timing, sequence, and frequency of the genetic, molecular, and cellular changes that drive cancer initiation and development provides opportunities to develop screening strategies to detect, if present, precancerous lesions or cancer at an early stage of development (see Figure 7). If precancerous lesions are found to be present they can be removed before they become cancer, something that is sometimes referred to as cancer interception. Finding cancer early, before it has spread to other parts of the body, makes it more likely that a cancer can be intercepted and a patient treated successfully.
What is Cancer Screening and How is it Done?
Cancer screening refers to checking for precancerous lesions or cancer in people who have no signs or symptoms of the cancer for which they are being checked. It has many benefits, but it can also result in unintended adverse consequences (see sidebar on Cancer Screening). Thus, population-level use of a cancer screening test must decrease deaths from the screened cancer and provide benefits that outweigh the potential risks. Determining whether broad implementation of a screening test across the population can achieve these two goals requires extensive research and careful analysis of the data generated.
There are five types of cancer for which screening tests have been developed and used in the clinic to screen generally healthy individuals (see sidebar on Cancers for Which Population-level Screening Has Been or Is Being Performed). Some of these tests can be used to prevent cancer from developing because they detect precancerous changes in a tissue that can be removed before they have a chance to develop into cancer. Others can detect cancer at an early stage of development, when it is more likely that a patient can be treated successfully.
For cancers other than breast, cervical, colorectal, lung, and prostate cancer there has never been population-level use of a screening test for individuals at average risk for disease. Thus, more research is needed to identify biomarkers and develop imaging technologies that can be used to develop new screening tests and cancer prevention therapeutics, as well as to more precisely identify those for whom current and future cancer screening and cancer prevention therapeutics are beneficial. Two areas of research that show promise in this regard involve expanding our knowledge of the genetic, molecular, and cellular characteristics of precancerous lesions (86, 87), and increasing our understanding of the inherited genetic mutations and variations that increase a person’s risk for certain types of cancer (88).
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Who Should Be Screened, When Should They Be Screened, and Why?
In the United States, an independent group of experts convened by the Agency for Healthcare Research and Quality of the U.S. Department of Health and Human Services rigorously evaluates data regarding the benefits and potential risks of cancer screening tests to make evidence-based recommendations about the routine use of these tests. These volunteer experts form the U.S. Preventive Services Task Force (USPSTF). The evidence-based USPSTF recommendations fall into several categories, most prominently recommendations for screening certain average-risk individuals at certain intervals, recommendations against screening, and deciding that there is insufficient evidence to make a recommendation (see sidebar on USPSTF Cancer Screening Recommendations). In addition to considering evidence regarding potential new screening programs, the USPSTF reevaluates existing recommendations as new research becomes available and can revise them if deemed necessary.
Many professional societies also convene panels of experts to meticulously evaluate data regarding the benefits and potential risks of cancer screening tests, and each society has made its own evidence-based recommendations about the use of these tests. Because the representatives on each panel weighing the benefits and potential risks of a given cancer screening test are often different, and different groups give more weighting to certain benefits and potential risks than other groups do, this can result in differences in recommendations from distinct groups of experts.
The existence of different cancer screening recommendations can make it challenging for individuals to ascertain which cancers to be screened for and when. Nevertheless, there is more consensus among recommendations than disagreement (see sidebar on Consensus among Cancer Screening Recommendations). The differences among the recommendations of different groups of experts highlight the areas in which more research is needed to determine more clearly the relative benefits and potential risks of screening, to develop new screening tests that have clearer benefits and/or lower potential risks, or to better identify people for whom the benefits of screening outweigh the potential risks.
Evidence-based cancer screening recommendations that apply to individuals at average risk of disease are only one consideration when a person makes decisions about which cancers he or she should be screened for and when. A person’s own unique risks for developing each type of cancer, his or her tolerance of the potential risks of a screening test, and his or her general health are also important considerations. Each person’s overall risks are determined by genetic, molecular, cellular, and tissue makeup, as well as by lifetime exposures to cancer risk factors (see Figure 3). Therefore, every individual should consult with his or her health care practitioner to develop a cancer prevention and early detection plan tailored to his or her personal cancer risks and tolerance of potential screening risks as Leon Adams did. These factors can vary over a person’s lifetime so it is important that individuals keep up a dialog with their health care practitioner and continually evaluate their cancer screening plans, updating them if necessary. One recent study found that the way in which health care practitioners put forward information can heavily influence whether older adults continue screening, even if it is unlikely to benefit them (89).
Some individuals are at increased risk of certain cancers because they inherited a cancer-predisposing genetic mutation (see Table 3). If an individual has a family or personal history of cancer and thinks that he or she is at high risk for inheriting such a mutation, he or she should consult a physician and consider genetic testing (see sidebar on How Do I Know If I Am at High Risk for Developing an Inherited Cancer?). There are genetic tests that individuals can use without a prescription from a physician, but there are many factors to weigh when considering whether to use one of these direct-to-consumer tests. As a result of the complexities of these tests, including the potential for them to detect genetic changes whose association with disease risk is unknown (see sidebar on Interpreting Genetic Tests), the FDA and Federal Trade Commission recommend involving a health care professional in any decision to use such testing, as well as to interpret the results.
Several medical conditions also increase a person’s risk for certain types of cancer. For example, individuals who have ulcerative colitis or Crohn’s disease are at increased risk for colorectal cancer, although recent research suggests that the increased risk might not be as great as previously estimated (93, 94). Thus, more comprehensive studies are needed to accurately establish the increased risk to allow individuals with these medical conditions to develop optimal cancer prevention and early-detection plans. Ulcerative colitis and Crohn’s disease are relatively rare conditions, but much more common medical conditions also increase risk for certain types of cancer. For example, diabetes, which affects 9.3 percent of U.S. adults age 18 or over (95), increases an individual’s risk for several types of cancer, including liver, pancreatic, and endometrial cancers (96).
If a person is at increased risk for developing a certain type or types of cancer, he or she should consult with his or her health care practitioner to tailor risk-reducing measures to his or her personal needs. Some people may be able to reduce their risk by modifying their behaviors, for example, by quitting smoking. Others might need to increase their use of certain cancer screening tests or use cancer screening tests that are not recommended for people who are generally healthy. Yet others may consider taking a preventive medicine or having risk-reducing surgery (see Table 5 and Supplemental Table 1).
Recent data show that about 10 percent of childhood cancers are associated with specific, inherited genetic mutations. In an effort to facilitate early detection and treatment of these cancers, the AACR convened an international group of leading pediatric cancer experts who have developed and published consensus screening surveillance recommendations for children with the most common cancer predisposition syndromes (97).
As we learn more about the genetic, molecular, and cellular characteristics of precancerous lesions and the biology of cancer, we will be able to develop and implement new strategies that pair this increased understanding with knowledge of an individual’s unique cancer risk profile, including his or her genetic makeup at birth, exposures to cancer risk factors, age, and gender. This information will allow us to better tailor cancer prevention and early detection to the individual patient, ushering in a new era of precision cancer prevention (98, 99). Importantly, we must ensure that advances are uniform for all segments of the population, which may prove challenging given that there are currently significant disparities in cancer screening rates among certain segments of the U.S. population (see sidebar on Disparities in Cancer Screening).
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