Breast Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Breast Cancer Screening Concepts
Numerous uncontrolled trials and retrospective series have documented the ability of mammography to diagnose small, early-stage breast cancers, which have a favorable clinical course. Although several trials also show better cancer-related survival in screened versus nonscreened women, a number of important biases may explain that finding:
- Lead-time bias: Survival time for a cancer found mammographically includes the time between detection and the time when the cancer would have been detected because of clinical symptoms, but this time is not included in the survival time of cancers found because of symptoms.
- Length bias: Mammography detects a cancer while it is preclinical, and preclinical durations vary. Cancers with longer preclinical durations are, by definition, present during more opportunities for discovery and therefore are more likely to be detected by screening; these cancers tend to be slow growing and to have better prognoses, irrespective of screening.
- Overdiagnosis bias: An extreme form of length bias; screening may find cancers that are very slow growing and would never have become manifest clinically in the woman's lifetime.
- Healthy volunteer bias: The screened population may be the healthiest and/or the most health-conscious women in the general population.
Because the extent of these biases is never clear in any particular study, most groups rely on randomized controlled trials to assess the benefits of screening. (Refer to the Cancer Screening Summary Overview for more information.)
Assessment of Performance and Accuracy
Performance benchmarks for screening mammography in the United States are described on the Breast Cancer Surveillance Consortium (BCSC) Web site.
The sensitivity of mammography is the percentage of breast cancers detected in a given population, when breast cancer is present. Sensitivity depends on tumor size, conspicuity, and hormone sensitivity as well as breast tissue density, patient age, timing within the menstrual cycle, overall image quality, and interpretive skill of the radiologist. Overall sensitivity is approximately 79% but is lower in younger women and in those with dense breast tissue (see the BCSC Web site).[2,3,4] Delay in diagnosis of breast cancer is the most common cause of medical malpractice litigation and half of the cases resulting in payment to the claimant involve false-negative mammograms.