Purpose To compare the performance of digital breast tomosynthesis (DBT) and two-dimensional synthetic mammography (SM) with that of digital mammography (DM) in a population-based mammographic screening program. Materials and Methods In this prospective cohort study, data from 37 185 women screened with DBT and SM and from 61 742 women screened with DM as part of a population-based screening program in 2014 and 2015 were included. Early performance measures, including recall rate due to abnormal mammographic findings, rate of screen-detected breast cancer, positive predictive value of recall, positive predictive value of needle biopsy, histopathologic type, tumor size, tumor grade, lymph node involvement, hormonal status, Ki-67 level, and human epidermal growth factor receptor 2 status were compared in women who underwent DBT and SM screening and in those who underwent DM screening by using χ tests, two-sample unpaired t tests, and tests of proportions. Results Recall rates were 3.4% for DBT and SM screening and 3.3% for DM screening (P = .563). DBT and SM screening showed a significantly higher rate of screen-detected cancer compared with DM screening (9.4 vs 6.1 cancers per 1000 patients screened, respectively; P < .001). The rate of detection of tumors 10 mm or smaller was 3.2 per 1000 patients screened with DBT and SM and 1.8 per 1000 patients screened with DM (P < .001), and the rate of grade 1 tumors was 3.3 per 1000 patients screened with DBT and SM versus 1.4 per 1000 patients screened with DM (P < .001). On the basis of immunohistochemical analyses, rates of lymph node involvement and tumor subtypes did not differ between women who underwent DBT and SM screening and those who underwent DM screening. Conclusion DBT and SM screening increased the detection rate of histologically favorable tumors compared with that attained with DM screening. RSNA, 2018 Online supplemental material is available for this article.

Tomosynthesis is a promising new technique. Our preliminary clinical experience shows that there is a potential for increasing the sensitivity using this new technique, especially for cancers manifesting as spiculated masses and distortions.

The percentage of interval cancers classified as missed ranged from 1.3% to 35.9% according to review design. To encourage learning, a review protocol should include both blinded and informed designs.

B reast cancer is the most common type of cancer among women worldwide, with almost 2.1 million new cases in 2018, and early detection with mammographic screening is considered beneficial to reduce mortality from this disease (1,2). Studies using a paired design have demonstrated a higher rate of screen-detected breast cancer when using standard two-dimensional (2D) digital mammography (DM) in combination with digital breast tomosynthesis (DBT) versus DM alone (3,4), and a randomized controlled trial from Italy reported a 90% increase in breast cancer detection when using DM and DBT together versus DM alone (5).However, the radiation dose of DBT and DM is nearly double that of DM. Thus, radiation concerns make this technique less suitable for population-based screening. Reconstruction algorithms creating 2D synthetic mammograms (SMs) from DBT image volume have become available, making the radiation dose acceptable for . A higher rate of screen-detected breast cancer also has been observed for DBT and SM when compared with DM in nonrandomized studies (9-12), while a randomized controlled trial from Norway reported no statistical difference in detection rate for

Key words: mammography screening; interval cancer; breast density; HRTAs a result of the mortality reduction found in several randomized controlled clinical trials, 1,2 breast cancer screening programs have been implemented in several western countries. 3 The Norwegian Breast Cancer Screening Program is a population-based program that offers women aged 50 -69 years biannual mammography screening. The program started as a pilot project in 4 counties in 1996, and the results from the first screening round were reported earlier. 4 Interval cancers can be defined as breast cancer diagnosed in screened women during the interval between 2 screening rounds, where the diagnosis does not follow from the screening examination. 5 The benefit of attending a screening program is reduced for women diagnosed with interval cancer. Thus the effect of a screening program in terms of mortality reduction can diminish if the frequency of interval cancer is high. This frequency depends, among other factors, on sensitivity of the test and length of the screening interval.Most interval cancers are diagnosed because of symptoms, hence interval and screening-detected cancers might be expected to have different characteristics. High breast density and use of hormone replacement therapy (HRT) have been discussed as possible contributors to reduction of screening sensitivity, and this may be related to occurrence of interval cancers. 6 -11 The purpose of our study was to quantify the interval cancers in the pilot project and to explore characteristics and factors that may be associated with interval cancer. To study the relations between use of HRT, breast density and interval cancer, data from a blinded review (breast density) and a questionnaire (use of HRT) were linked. MATERIAL AND METHODS Screening populationThe Norwegian Breast Cancer Screening Program is population based and covers women aged 50 -69 years. The screening population is identified through the Central Population Register, in which each inhabitant in Norway has a unique 11-digit number. Women are invited into the Norwegian Breast Cancer Screening Program irrespective of previous breast cancer history. The screening interval is 2 years. Two-view mammograms and independent double reading are standard. The pilot project included 4 counties with about 40% of the Norwegian female population aged 50 -69 years. During the first round (1996-97), 159,887 women born from 1927-46 were invited and 127,064 women were examined (79.5%). 4 Study subjectsWomen who had their first screening examination within the pilot project after July 31, 1998 (n ϭ 411) and women who experienced recurrence in the same breast as a previous lesion (n ϭ 6) were excluded from the study. Women who received a cancer diagnosis in the first screening round (n ϭ 854) were not included as being at risk of interval cancer. However, women who were screen-positive but cleared after assessment were included. This left 125,793 women at risk of developing interval cancer. The women were followed until the date an in...