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:
For many women, the diagnosis of breast cancer represents not only a major
physical battle, but also the ultimate emotional challenge -- one that affects
every relationship in our life.
Indeed, from friendships to romance, from being a parent to being a
daughter, the way you relate to everyone -- and the way they relate to you --
can be affected.
"I do think cancer has more impact on emotions and emotional
relationships than other catastrophic diseases, because with cancer, death is
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.
Specificity and false-positive rate
The specificity of mammography is the likelihood of the test being normal when cancer is absent, whereas the false-positive rate is the likelihood of the test being abnormal when cancer is absent. If specificity is low, many false-positive examinations result in unnecessary follow-up examinations and procedures. (Refer to the subsection on Harms in the Screening With Mammography section of the Overview section of this summary for more information.)
Interval cancers are cancers that are diagnosed in the interval after a normal screening examination and before the subsequent screen. Some of these cancers were present at the time of mammography (false-negatives), and others grew rapidly in the interval between mammography and detection. As a general rule, interval cancers have characteristics of rapid growth [6,7] and are frequently of advanced stage at the time of discovery/diagnosis.