Creating evidence-based summaries on cancer genetics is challenging because the rapid evolution of new information often results in evidence that is incomplete or of limited quality. In addition, established methods for evaluating the quality of the evidence are available for some but not all aspects of cancer genetics. Varying levels of evidence are available for different topics, and PDQ summaries are subject to modification as new evidence becomes available. As in other areas of medicine, testing and treatment decisions must be based on information that sometimes falls short of the optimal level of evidence. Recognizing the limits inherent in certain observations will alter the weight given to recommendations based on that evidence and serves to keep minds open to new improved information, as it comes along.
The quality of evidence depends on the appropriateness of the study in terms of the question being evaluated and on how well the study was designed, implemented, analyzed, and interpreted. For evaluating outcomes of both medical and social interventions, the strongest evidence is obtained from well-designed and well-conducted randomized clinical trials. For evaluating other questions, particularly those related to the prevalence of gene variants and inherited syndromes and determining the clinical validity of genetic tests, the strongest evidence is obtained from well-designed descriptive studies. Particular elements of study design, such as the nature of the population studied or the duration of observation, may be crucial to assessing the quality of a study.
Note: Some citations in the text of this section are followed by a level of evidence. The PDQ editorial boards use a formal ranking system to help the reader judge the strength of evidence linked to the reported results of a therapeutic strategy. (Refer to the PDQ summary on Levels of Evidence for more information.)
Incidence and Mortality
Estimated new cases and deaths from colon cancer in the United States in 2011:
New cases: 101,340 (colon cancer only).
Deaths: 49,380 (colon...
During the early phases of research in a new area, information relevant to the needs of patients and clinicians may come from work at all levels of evidence, including well-designed quasi-experimental studies (nonrandomized, controlled single-group, pre/post, cohort, time, or matched case-control studies) or nonexperimental studies (case reports, clinical examples, qualitative or narrative studies, or theoretical work). Such research may yield information important to patients and clinicians, who must make treatment or risk management decisions before full data on the risks and benefits of cancer genetic testing are available. In addition, such work helps to inform future research using more rigorous designs.