Following reports from Swedish randomized trials [1
], breast cancer screening programs with mammography have been established in recent decades in many countries [5
]. The age range of women invited to screening varies between countries. The Swedish National Board of Health and Welfare recommends mammography screening at regular intervals to all women between 40 and 74
years. The initial results from the randomized trials, showing a reduction in mortality in breast cancer, have been confirmed by long-term follow-up [6
] Similar results have been obtained in established population-based service screening programs [8
]. However, the pros and cons of mammography screening and how the results should be interpreted [10
] are still matters for debate.
Besides the primary aim of detecting breast cancers in screening programs, it is important that recall rates are kept as low as possible without impairing detection rates. In this respect, the recommended recall rate in Sweden and in the rest of Europe should not exceed five per cent [11
]. The reasons for recall are several, such as suspicious findings suggesting malignancy, indeterminate findings that need further work-up, and occasionally for technical reasons or if the woman reports clinical symptoms at the time of the screening examination.
As the radiological image of breast tissue is complex, mammograms need to be interpreted by highly specialized radiologists. Figure
shows an example of mammography images.
Figure 1 Figurea shows a rather hard to detect breast cancer in the left breast (arrow); the right breast is normal. Figureb shows an easily detected cancer in the right breast (arrow); the left breast is normal.
Factors that affect the ability to detect a breast cancer (sensitivity) are e.g. the prevalence of breast cancer in the target population, dense breast tissue, the frequency of tumours with subtle mammographic signs, and suboptimal technical quality. These factors, combined with high daily volumes (each Swedish screening centre usually screens more than 20,000 women annually), makes accurate screening a challenging task. Sensitivity levels of 70–85% and specificity levels of 82–98% at mammography screening have been reported [5
]. In order to maintain high sensitivity and specificity, resulting in high cancer detection rates and low false-positive rates, Swedish and European guidelines recommend double reading, i.e. that the breast images are reviewed by two specially trained radiologists (breast radiologists). Double reading has been shown to increase cancer detection rates by 5–17% [12
Computer-aided detection (CAD) is a computerized method for analysing images in mammography screening. Although the method has existed for approximately 10
years, its contribution to routine screening is still debatable [13
]. The program used in CAD identifies and marks areas which the software identifies as abnormal breast tissue. The CAD program is not intended to be the sole method for analysing mammography images. Rather, it is designed to alert the radiologist to possibly suspicious areas. Hence, a radiologist must interpret and make a decision to act upon (accept or dismiss) each CAD mark. On average, each screening examination generates two false positive marks; CAD gives 400 false positive marks for each true positive mark [16
Lack of an adequate number of trained breast radiologists has led to a growing interest in computerized analysis of mammography images. There has been a discussion as to whether CAD in conjunction with mammography screening could replace one of the breast radiologists. A prerequisite would be that diagnostic accuracy and patient benefit are at least equivalent to what is achieved when the mammographic images are read by two breast radiologists. Another important prerequisite is that not too many women need to be recalled for further diagnostic work-up.
The value of CAD in mammography screening has been questioned in earlier reviews [17
]. The literature is scarce on studies performed in authentic screening situations. As the performance of CAD systems has improved considerably, it was considered appropriate to reassess the performance of CAD in population-based screening programs.
This review is part of a comprehensive systematic review, published in Swedish by SBU (Swedish Council on Health Technology Assessment), of computer-aided detection (CAD) as a diagnostic method in mammography screening [19
]. SBU is an independent government agency for the critical evaluation of methods for preventing, diagnosing and treating health problems.
The objective of the present is systematic review is to address the following question: Is the reading of mammographic images by a single breast radiologist plus CAD at least as accurate as readings by two breast radiologists (current practice) in terms of:
· sensitivity (probability that a person with the disease has a positive test result);
· specificity (probability that a healthy person has a negative test result);
· cancer detection rate (number of cancer cases detected per 1,000 women examined);
· recall rate (proportion of women who are recalled for further investigation); and