The main findings of this explorative analysis suggest that one-view DBT may be feasible as a stand-alone technique for breast cancer screening. Compared with the current gold standard (DM), DBT was superior in terms of detection rate and equal in terms of PPV. The DM CC-view did not contribute to cancer detection but did help in ruling out cancer in a few cases with stable, benign lesions, as did previous DM examinations.
The DBT cancer detection rate may seem remarkably high. However, the current intervention should be regarded as prevalence DBT screening. If a new screening technique, which is superior to DM in terms of cancer detection, is applied, an effect similar to the one observed in the randomised mammography screening trials will be seen, i.e., a higher detection rate in the intervention arm compared to the control arm . In addition, in the current study, approximately 20 % of the population was invited to mammography screening for the first time, either because of turning 40 or being a newly arrived immigrant. As a consequence, it is reasonable to expect a lower detection rate if subsequent screening rounds were performed.
In the present study there is not enough data for a statistical comparison of the biological characteristics of the DBT and DM detected cancers. Even though the rate of detected DCIS was similar in the two reading arms, the cancers detected by DBT alone tended to be of lower grade, smaller size and lymph node negative, compared with DM-detected cancers. Also, these women were slightly younger at diagnosis. It is not clear whether this represents earlier diagnosis and/or overdiagnosis from DBT screening, since the present study was not designed to address these issues. This question will be further analysed when the entire study is completed. Our impression in the present study, as well as from a previous study, is that DBT is particularly sensitive to the detection of small spiculated lesions . Such lesions tend to be either low-grade cancers or benign radial scars, while small high-grade cancers tend to be less specific in appearance, i.e., non-specific densities with diffuse borders or well circumscribed with benign appearance . Furthermore, the cancers detected by DBT alone were found in both dense and fatty breasts, which mean that the increased cancer detection rate with DBT is not solely due to the reduction of the negative effect of superimposed tissue in dense breasts but also to better lesion conspicuity in more fatty breasts. These results are in accordance with findings in the two earlier prospective screening trials [11, 12]. Accordingly, DBT will also enhance benign lesions and sometimes islands of normal breast parenchyma. This probably contributed to the significant increase in recall rate with DBT compared to DM. Even if the recall rate was higher with DBT, it was still low and in agreement with the recommendation from the European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnostic Services . Several studies have reported a reduced false positive rate with DBT. However, the baseline recall rate was much higher in these studies compared with MBTST [11, 13, 14]. Furthermore, the increased recall rate observed in this study was higher than assumed in the sample size calculation. It is reasonable to expect that the increase in recall rate will be somewhat lower in the second half of the study population in line with increased experience of the readers. Further analysis on recall rates will be presented after the completion of the MBTST.
Implications for practice
In general, a mass screening technique should be fast, with a high sensitivity and specificity, reasonably inexpensive, and, in the case of an X-ray examination, at a low radiation dose. Accordingly, in the design of the MBTST we chose one-view DBT in the MLO-projection. This projection has been shown to be the most effective in mammography screening . We estimate that the reading time for one-view DBT in screening is about doubled compared to DM. We did not register the reading time in our study, since the reading and scoring procedures were specific to the trial and would not reflect the true time consumption in a normal screening workflow. In a previous study from our group, we found that the reading time for one-view DBT (MLO) was roughly 30 s (in an enriched population of clinical and screening cases) . This is about one third of the reported reading time for the so-called combination mode — two-view DBT in combination with two-view DM — used in the Oslo Tomosynthesis Screening Trial . In our study, the radiation dose for one-view DBT examination was lower than an ordinary two-view DM screen, which is satisfactory from a mass screening perspective. Especially considering that the use of a combination mode, evaluated in the other screening trials, gives additional radiation dose compared to a single modality approach [11, 12]. Still, the use of a synthetic mammogram derived from the DBT volume can provide means to sustain a low radiation dose even with the use of a combination mode.
The reduced compression force used in this study was much appreciated by the women, according to our examining technologists. However we did not collect any qualitative data to verify this statement. The high cancer detection rate in the current study supports previous results showing that reduced compression does not significantly compromise image quality . We believe this is an important advantage of DBT.
Breast cancer mammography screening has been considered cost-effective . The cost-effectiveness of DBT compared to DM in screening for breast cancer has yet to be evaluated, and is important for future decisions on the introduction of DBT in screening. So far, it is reasonable to assume that DBT will be more expensive, but this has to be related to the benefit of increased cancer detection, and probably earlier detection.
The ultimate measure of screening efficacy is the reduction of breast cancer mortality. Our study was not designed to assess differences in breast cancer mortality, for that purpose a very large randomised screening trial would be needed. An alternative could be to analyse the relative reduction of interval cancers as a surrogate end-point for screening efficacy. An estimation of interval cancer rates will be included in future reports from the MBTST, after the inclusion and follow-up of the whole study population.
A limitation of this study is that it was performed on an urban Swedish population, and might, therefore, not be representative for other populations. In addition, the examinations were performed using one particular brand of tomosynthesis equipment that differs from other commercially available systems. Also, the readers involved had participated in previous clinical studies on the accuracy of DBT, and might, therefore, be biased in favour of DBT. As a consequence, this explorative analysis will not introduce additional bias to the readers. Furthermore, the limitation of an explorative analysis on half of the study population is that it does not have an 80 % power in the statistical analyses.