Benefits and Harms of Mammography Screening

Magnus Løberg; Mette Lise Lousdal; Michael Bretthauer; Mette Kalager


Breast Cancer Res. 2015;17(63) 

In This Article

Screening Mammography

Attendance Rates

Mammography screening is recommended (and in Europe offered through organized programs) in most Western countries. However, in Switzerland an independent panel of experts (the Swiss Medical Board) reviewed the evidence on mammography screening and concluded that harms outweighed the benefits and recommended against mammography screening;[17] that is, that screening programs should not be implemented in areas where such programs do not exist and that the ongoing programs should be phased out. When screening is recommended, the eligible age range differs in different countries from 40 to 74 years.[4,18,19] The recommended interval between two screens varies from 1 to 3 years.[18] Mammography screening is well accepted; on average, more than half of eligible women attend screening mammography. In most countries, attendance rates are higher than 70%. Women aged 50 to 69 years have the highest attendance rate.[18,19] The attendance rate varies between countries (19.4% to 88.9%), and in different age groups. Most women who have participated once continue to participate.

False Positive Tests

As with every diagnostic test the sensitivity and specificity of mammography screening are not perfect; various levels of sensitivity and specificity for detecting breast cancer have been published.[20,21] The risk of experiencing a false positive mammogram for women undergoing biennial screening from age 50 to 69 years in Europe is about 20%,[21] and the risk of experiencing a biopsy due to a false positive test is 3%.[21] Based on data from the UK, 2.3% of all women with a false positive test had a lumpectomy, representing 76 out of 100,000 women screened in one screening round.[22] The risk is even higher in the US, where the 10-year false positive rate is 30%, and 50% of all women will experience a false positive mammogram at one time.[23,24] The challenges with a false positive test, apart from the monetary costs, are impaired psychological well-being and changes in health behavior among women with the false positive test. After 6 months, only 64% of those recalled due to a false positive test were declared cancer-free; after 1 year approximately 90% were declared cancer-free, and only after 2 years were all those who were in fact free of cancer declared cancer-free.[25] Research has shown that false positive results negatively influence women's psychological well-being during the period immediately after the tests, and a recent study showed that women with false positive findings experience psychological harm for at least 3 years after screening.[26] Women with false positive findings had higher use of health care services; 55% of women who experienced a positive recall returned to the outpatient clinic in the first year after screening, some up to eight times,[27] and reported lower quality of life than those without.[27,28] Some women may also have altered health behavior and trust in the health care system.[28]

False Negative Tests

Interval cancers are cancers detected after a normal screening mammogram and before the next scheduled mammogram. Interval cancers either were overlooked at the last mammogram or are rapidly growing cancers that become apparent in the screening interval.[29] In a re-interpretation of interval cancers, around 35% were overlooked,[30] while 65% were not visible at the latest mammogram and appeared in the interval between screening mammograms. Of all breast cancers detected among women who participate in screening, 28 to 33% are interval cancers,[20] and this proportion seems to be stable in the different screening rounds.[29] Use of digital mammography is increasing, and detection rates of ductal carcinoma in situ (DCIS) and invasive cancers are higher. Whether this will decrease the proportion of interval cancers is unknown, but the rate of missed cancers seems to be similar to that of analogue, screen-film mammography.[31] One might anticipate, therefore, that the proportion of interval cancers with digital mammography will be comparable to that with analogue screen-film mammography. However, the increasing detection rates with digital mammography might increase the amount of overdiagnosis.

Women diagnosed with interval cancer do not benefit from early detection, but could be falsely reassured by their last normal mammogram and delay seeking medical care. However, this might not seem to be the case as women with interval cancer do not have poorer prognosis than women who chose not to utilize mammography screening.[29]

For 1,000 women invited to mammography screening every second year for 20 years from age 50, 200 will experience a false positive mammogram, 30 will undergo a biopsy due to a false positive mammogram, and 3 will be diagnosed with interval cancer[32,33] (Figure 1).

Figure 1.

Summary of benefits and harms when 1,000 women are screened every second years for 20 years starting at age 50. Number of women with false positive mammograms and false positive biopsies are based on a review [32]. Number of interval cancers are based on reported number of interval cancer in the National Health Service breast screening programme [33]. The numbers of overdiagnosed and prevented breast cancer deaths are estimated based on 31% overdiagnosis [19] and 13 to 17% reduction in mortality from breast cancer [35]. These relative numbers are applied to the observed incidence of invasive breast cancer (women aged 50 to 69 years) and mortality (women aged 55 to 74 years) in the UK in 2007 [32]; this resulted in 15 overdiagnosed women and 2 to 3 prevented breast cancer deaths per 1,000 women. No deaths are prevented overall [9].