Study cohort and technical function
One hundred fifty-two included patients received tomosynthesis using the prototype (for cohort description, see Table 1). In 51 of 152 cases (33.6%), the ABUS scan could not be completed due to software errors and hardware problems. The most frequent software errors were a failure to start the scan, early abort of the scan, and failure to transmit or save the ABUS data. Hardware problems, which could be identified and partly corrected in the course of the study, included defective electrical contacts at the interface of the ABUS system and the prototype hardware. If the scan could not be initiated right away, ABUS was not performed to avoid waiting time or prolonged compression for the patient. No scan had to be cancelled by the examiner for reasons of patient safety or tolerability.
Table 1 Cohort description In total, 101 patients received both tomosynthesis and ABUS scans, 58 in mediolateral-oblique (MLO), 42 in mediolateral (ML), and one in craniocaudal (CC) projection, and were included in the analysis. The applied compression force was on average 106.30 ± 22.47 N and resulted in an averaged compression thickness of 51.83 ± 12.71 mm. The scanning time of ABUS ranged between 40 and 60 s, depending on the breast volume. The total time of performing ABUS and tomosynthesis ranged from 90 to 130 s. The average time for the interpretation of both ABUS and tomosynthesis images was 277 ± 113 s.
Coverage
The median breast area measured in tomosynthesis was 162.54 cm2 (125.25; 198.90) and 123.37 cm2 (92.36; 169.42) in ABUS with a median coverage in ABUS of 80.0% (75.0; 86.0) of the tomosynthesis coverage. The area covered in tomosynthesis was significantly larger than that in ABUS with a median difference of 31.57 cm2 (23.63; 42.89) (p < 0.001, 95% CI [17.74; 21.19]).
Image quality
All cases were rated as at least category 2, so tissue structures were identifiable in all cases with a median quality of 3 (lower quality compared to HHUS, but well-distinguishable tissue structures). In 86 of 101 cases (85.1%), ABUS image quality was rated as diagnostically useful. In 18 of 101 scans (17.8%), image quality was described as being close to HHUS (category 4). None of the scans was rated as 5 (equal or higher quality than HHUS). There was no significant difference in the rating of image quality over the three projections (p = 0.29, see Table 2).
Table 2 Image quality per projection Detection of breast lesions using the FUSION-X-US-II prototype
After the standard diagnostic workup in the breast unit, 53 of 101 cases (52.5%) were classified as likely benign (BI-RADS® 1–3). Forty-eight of 101 cases (47.5%) were classified as unclear or suspicious (BI-RADS® 4–5) with the recommendation for histological confirmation by core-cut biopsy, which was performed in 47 of 101 patients (46.5%). Thirty-four carcinomas (34/101, 33.7%) were diagnosed in the study population (for tumor characteristics, see Table 3).
Table 3 Tumor characteristics In the study setting using only the prototype ABUS and tomosynthesis images, 33 of the 34 carcinomas (97.1%) were identified and classified as suspicious (BI-RADS® 4 or 5) or unclear with the need for further diagnostic workup (BI-RADS® 0). Twenty-six of these 33 carcinomas (78.8%) were described in tomosynthesis and ABUS (see Fig. 1), six (18.2%) only in tomosynthesis, and one carcinoma (3.0%) only in ABUS. Descriptively, there seems to be no significant correlation between the mammographic breast density and missed cancers at tomosynthesis or ABUS (see Table 4), but the subgroups were too small for statistically meaningful results.
Table 4 Cancer detection through ABUS and tomosynthesis by subgroups of mammographic breast density according to ACR Twenty-two of 27 carcinomas (81.5%) detected in ABUS were completely visualized, two of 27 carcinomas (7.4%) were partly visualized due to proximity to the thoracic wall. Three of 27 carcinomas (11.1%) were classified as BI-RADS® 0 due to low image quality (category 2). One lesion was described in ABUS as suspicious (BI-RADS® 4) and was not seen in tomosynthesis. A core-cut biopsy confirmed the diagnosis of an invasive lobular carcinoma in the lesion not seen in tomosynthesis. In three cases, which were histologically confirmed as non-special type carcinoma (NST), ABUS led to a correct upgrading of the BI-RADS® classification of tomosynthesis: twice from BI-RADS® 4b to BI-RADS® 5 and once from BI-RADS® 4a to BI-RADS® 4b.
Retrospective evaluation (by the unblinded physician in knowledge of the standard diagnostic workup) of the seven carcinomas not detected in ABUS revealed that five carcinomas (71.4%) were located outside of the area covered by the ABUS images. One of these carcinomas was not detected, by neither ABUS nor tomosynthesis. Two of seven carcinomas (28.6%) were not described in the interpretation of ABUS images in the study setting but could be localized in a retrospective second-look evaluation as discreet architectural distortions.
Sixty-seven of 101 cases (66.3%) were classified as unsuspicious in the standard diagnostic workup, containing no or surely benign lesions (BI-RADS® 1–2, n = 41), morphologically likely benign lesions with recommendation for follow-up (BI-RADS® 3, n = 12), or were histologically benign after core-cut biopsy had been recommended based on imaging (BI-RADS® 4, n = 14). One patient declined the biopsy. Ten of the twelve patients with recommendation for follow-up showed no evidence for malignant disease in the 12 months follow-up at our breast unit. Two patients failed to follow-up.
Forty-two of the unsuspicious lesions (BI-RADS® 1–3) were rated concordantly in the standard diagnostic workup and the study setting. In six cases, lesions were classified as unclear in tomosynthesis (BI-RADS® 0) in the study setting, but could be correctly downgraded through ABUS (BI-RADS® 2). Further, 19 lesions were rated as BI-RADS® 3 or 4 in both the standard diagnostic workup and the study setting. In this group, four cases were classified as BI-RADS® 4 in tomosynthesis and as BI-RADS® 3 in ABUS in the study setting. Formally, ABUS led to a correct downgrading of these cases in the study setting.
Twelve cases were described as unclear or suspicious (BI-RADS® 0 or 4) in the study setting, but were unsuspicious (BI-RADS® 1–3) in the standard diagnostic workup.
Overall, the combined performance of tomosynthesis and ABUS led to a sensitivity of 97.1% (95% CI [91.4; 100]) and a specificity of 59.7% (95% CI [48.0; 71.4]) in the study setting (see Table 5).
Table 5 Classification of detected lesions with the FUSION-X-US-II prototype compared with the standard diagnostic workup (gold standard)