Cancer Incidence and Mortality

In 2009, an estimated 1,479,350 people in the United States will be diagnosed with cancer, and 562,340 will die of cancer.[1] Estimates of the premature deaths that could have been avoided through screening vary from 3% to 35%, depending on a variety of assumptions. Beyond the potential for avoiding death, screening may reduce cancer morbidity since treatment for earlier-stage cancers is often less aggressive than that for more advanced-stage cancers.

Several potential harms must be considered against any potential benefit of screening for cancer.[2] Although most cancer screening tests are noninvasive or minimally invasive, some involve small risks of serious complications that may be immediate (e.g., perforation with colonoscopy) or delayed (e.g., potential carcinogenesis from radiation). Another harm is the false-positive test result, which may lead to anxiety and unnecessary invasive diagnostic procedures. These invasive diagnostic procedures carry higher risks of serious complications. A less familiar harm is overdiagnosis, i.e., the diagnosis of a condition that would not have become clinically significant had it not been detected by screening. This harm is becoming more common as screening tests become more sensitive at detecting tiny tumors. Finally, a false-negative screening test may falsely reassure an individual with subsequent clinical signs or symptoms of cancer and thereby actually delay diagnosis and effective treatment.

In developing the cancer screening summaries, the PDQ Screening and Prevention Editorial Board uses the following definitions:

• Screening is a means of detecting disease early in asymptomatic people.
• Positive results of examinations, tests, or procedures used in screening are usually not diagnostic but identify persons at increased risk for the presence of cancer who warrant further evaluation.
• Diagnosis is confirmation of disease by biopsy or tissue examination in the work-up following positive screening tests. (Following a positive screening result, cancer can often be ruled out by procedures other than biopsy or tissue examination.)

The purpose of this summary is to present an explicit evidence-based approach used in the development of the screening summaries. In reaching conclusions, evidence on the balance of risks and benefits is weighed. Cost and cost-effectiveness, however, is not taken into account. Assignment of levels of evidence associated with such screening tests is also discussed.

Summary Development

The cancer screening summaries are based on various levels of published scientific evidence and collective clinical experience. The highest level of evidence is taken as mortality reduction in controlled, randomized clinical trials. The results of clinical studies, case-control studies, cohort studies, and other information are also considered in formulating the summaries. In addition, the incidence of cancer, stage distribution, treatment, and mortality rates are considered. The summaries are subject to modification as new evidence becomes available.

The Scientific Basis