Despite recent controversy surrounding the efficacy of mammographic screening, it remains the only screening test for breast cancer that has been extensively evaluated in randomised controlled trials (RCTs) and shown to reduce breast cancer mortality (IARC, 2002; Nystrom et al, 2002). Since evidence exists that early detection reduces mortality from breast cancer, it is reasonable to evaluate a new screening test by assessing its effect on early detection of breast cancer. While RCTs examining mortality as an outcome are the gold standard, studies assessing new tests are commonly evaluated using surrogate measures as indicators of early breast cancer detection. These surrogate measures may be measurable at the time of screening or require follow-up. Immediate indicators include the cancer detection rate and the size, stage, and nodal status of cancers detected. The measure requiring follow-up is the interval cancer rate. Ascertainment of interval breast cancers poses a number of challenges that include identification (requiring linkage to cancer registries), standardisation, and validation of review and categorisation methods, as well as access to the films taken at diagnosis. Assessing whether tests differ in their interval cancer rate is best assessed by randomising people to the different tests. In designs in which women are assessed by both tests, women in whom cancers are detected by either test would obviously be identified and treated; interval cancers that arise thereafter would be those missed by both tests. To assess whether interval cancer rates differ between the tests is therefore best assessed by randomising women to the different tests. However, all of the immediately measurable surrogates can be assessed in cross-sectional analytic studies of people in whom both of the different tests have been carried out.
New screening methods can be evaluated relative to the proven screening test (screening mammography being the comparator test for breast cancer) as a replacement for the comparator or as an incremental (additional) method, whereby the new test would only be carried out in those where the comparator test is negative. To allow estimation of both replacement and incremental accuracy, the basic study design is to screen women independently by both mammography and the new test, and assess how many cancers and false positives are detected by one test and not the other. If the objective is to assess incremental accuracy only (additional cancers detected, as well as additional false positives caused by use of the new test), this can be carried out by re-examining only those who were negative on mammography and can be carried out with or without knowledge of the mammography result.
Over the past two decades numerous breast imaging tests have been evaluated and used mainly as adjunct diagnostic methods to mammography, and some have been proposed as possible screening tests. Some of these tests, such as computer-aided detection (CAD), are rapidly evolving and continue to be evaluated. Other tests, primarily ultrasound, have an established role in the investigation of symptomatic women and in the evaluation of screen-detected findings. In this paper, we present a systematic review of published work on the accuracy of new technologies that have been proposed for breast cancer screening.