Example of Breast Cancer Screening

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Incidence and Mortality

The American Cancer Society estimated that, in 2004, 215,990 women would be newly diagnosed with breast cancer; 40,110 would die of this disease.20 About 59,390 women will be diagnosed with carcinoma in situ of the breast, primarily by mammography.20 From 1996 to 2000, the age-adjusted incidence in nine Surveillance, Epidemiology, and End Results (SEER) registries was about 137 per 100,000 women; the age-adjusted mortality during this period was about 28 per 100,000.21 The probability at birth of being diagnosed with breast cancer in 80 years of life is about 11%; the probability at birth of dying of breast cancer by age 80 is about 2%.21 Breast cancer incidence for all women increased from 1980 to 2000, although the increase slowed considerably in the late 1990s. Between 1990 and 2000, breast cancer mortality for all women decreased by about 2.3% per year.22,68 The reasons for this decrease are not clear and may be due to a combination of screening and improved treatment.69

Screening Tests

Three primary tests are currently in use for breast cancer screening. Although still controversial, the overall evidence shows that mammography results in a reduction in breast cancer mortality by less than 20%.6 Indirect evidence suggests that clinical breast examination (CBE), when well conducted, may also lead to a small reduction in mortality, but uncertainty about this remains.70,71 Breast self-examination (BSE) has been shown in a large RCT to be ineffective in reducing mortality.72

The accuracy of mammography depends on a number of factors. One large prospective cohort study of 329,495 women of ages 40 to 89 years from seven population-based mam-mography registries found sensitivity ranged from 62.9% in women with dense breasts to 87% in women with fatty breasts. Specificity ranged from 89.1% in women with dense breasts to 96.9% in women with fatty breasts.73

The accuracy of mammography varies among radiologists and among countries.74-78 In general, North American radiologists tend to interpret a higher percentage of mammograms as positive than radiologists in other countries, without evident additional benefit. In one study of community radiologists in New England, false-positive rates ranged from 2.6% to 15.9%.76 The accuracy of CBE also varies widely among clinicians.79

Newer approaches to breast cancer screening are being studied, targeted especially to increasing sensitivity.80 Interestingly, although earlier mammography from the 1970s and 1980s was certainly less sensitive than present-day mam-mography, the Health Insurance Project (HIP) study from this era found a similar reduction in breast cancer mortality as more recent studies.81,82 Thus, it is not clear whether increasing sensitivity will provide additional reduction in breast cancer mortality.

Rationale for Screening

The primary rationale for screening comes from the RCTs of screening that have been conducted over the past 30 years.17,83 Although the overall evidence suggests that breast cancer mortality is reduced by mammographic screening, the reduction is less than 20%6; this means that 80% of the women who have potentially fatal cancers are not helped by screening and earlier treatment. Clearly, some breast cancers are aggressive and metastasize before they can be detected by mammography. Some, however, respond better to earlier treatment than to later, thus reducing mortality.

Given the relatively low reduction in mortality from breast cancer from screening, the absolute number of women whose lives would be extended is small. From one to two women in their forties and from two to six women in their fifties and sixties would have their lives extended by screening annually for 10 years.84

Harms of Screening

The two major potential harms of screening for breast cancer are false-positive tests and overtreatment of ductal carcinoma in situ (DCIS). One study estimated that 49% of women would have at least one false-positive mammogram after 10 rounds of screening; almost 19% would undergo a biopsy as a result of the false positive.12 False-positive mammograms sometimes lead to a recommendation of a short-interval follow-up (e.g., 6 months rather than a year), despite the evidence that such a policy rarely leads to increased cancer detection.85 False-positive mammograms do lead to increased anxiety, both in the short run and after 6 to 12 months, for some women.10,86

Ductal carcinoma in situ (DCIS) is a heterogeneous intermediate lesion with an uncertain prognosis. This lesion was rare before screening mammography but has increased dramatically as the number of women undergoing mammograms has increased. About 1 in 1300 mammograms detects DCIS;

from 16% to 28% of all breast "cancers" are DCIS.87 Probably less than 50% of untreated women with DCIS ever develop invasive breast cancer.88-90 Treatment is often surgical; some women have mastectomy whereas others have breast conservation surgery. Few women treated for DCIS eventually die of breast cancer.91

Because DCIS is so common (an estimated 59,000 cases in 2004)1 and because its prognosis is so uncertain, many women undergo unnecessary surgery because of its diagnosis. This is an important area of overtreatment. One modeling study found that detection of DCIS plays a minor role in the reduction in breast cancer mortality from screening.92

Balance of Benefits and Harms

Screening for breast cancer is an important example of the trade-offs involved in the decision to be screened. On the one hand, screening likely does extend some women's lives. On the other hand, screening also leads to many women having workups for false-positive screening tests, and other women having treatment for DCIS, a lesion that would never develop into invasive breast cancer for many women. It is important for women to understand these trade-offs; women should be offered the opportunity to participate in the decision about screening.

Improving screening programs should seek not only to improve sensitivity. Improved sensitivity may or may not further reduce mortality. Improved specificity should also be a priority. If the number of women with false-positive tests can be reduced, potential harms could be decreased, thus improving the balance between benefits and harms.

Another way of improving breast cancer screening programs would involve finding ways of determining which women with DCIS are truly at risk of invasive cancer, allowing some women to avoid unnecessary surgery.

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