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:
WebMD senior writer Miranda Hitti interviewed breast cancer survivors as
part of a series for Breast Cancer Awareness Month. The series, called “Me
& the Girls,” explores the personal stories of these women after they were
diagnosed with breast cancer.
Breast cancer survivor Ilene Smith, MS, RD, 49, lives in the New York area.
In late October 2007, Smith felt a lump in her left breast while on a
conference call for work. "I got cold, and so I put my hand under my arm, and I
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.