Breast MC is rare in the clinic, and preoperative diagnose is difficult to obtain due to the variety of ultrasonographic features and its morphological features overlap with those of benign tumours reported in the literature [8, 9, 13–17].
In our study, although most MCs showed malignant signs and were assessed as the medium-high-risk group (BI-RADS 4B and above), 21.1% (16/76) MCs were assessed as the low-risk group (BI-RADS category 4A), for which the malignancy likelihood is exceptionally low. By comparing the ultrasonographic features of MCs in the low-risk group and medium-high-risk group, we confirmed that the majority (81.2%) of MCs in the low-risk group were oval. Lam et al.[8]reported that the ratio of oval shape masses on US was 40.6% for all MCs, 47.4% for PMCs and 30.8% for MMCs, while Kaoku et al [15]. reported that this proportion of PMC was only 9.1%. In our research, an oval shape mass on US accounted for 32.9% (25/76) of all MCs, 34.7% of PMCs and 29.6% of MMCs. Apparently, the proportion of oval masses in PMCs was higher than in MMCs, but there was no statistical difference between both in our and previous studies. Oval shape is an ultrasonographic feature of benign breast masses, which may be suggested follow-up instead of biopsy [10, 11, 18, 19]. On US, if the shape of MC is oval, it may be underestimated. In addition to the shape, there were also statistically significant differences in marginal features between the low-risk group and the medium-risk group. However, although the percentage of MCs with a circumscribed margin in the low-risk group was higher than that in medium-high-risk group, the majority (75%) of MCs in the low-risk group had not circumscribed margins, which is an ultrasonographic feature of malignancy [6, 7, 10, 11, 19]. Overall, oval shape was the main reason why MCs were underestimated as the low-risk group which suggested benign masses.
Fibroadenomas are the most common benign tumours of the breast [20]. Compared with FA which is commonly found in young women, typically in 20 to 30 years old [21], patients with MC were older, the median age was 47, which was younger than 51 to 71 years previously reported [22, 23].
Regarding ultrasonographic features, most FAs are oval or round, well circumscribed, hypoechoic with the long axis parallel to the skin surface and have normal or increased posterior echogenicity [24], which are almost consistent with our study findings. By comparing the other features of MC and FA both with oval shape, we found that the marginal features, internal echo and posterior echo features were helpful for the differential diagnosis of them. Among internal echo, marginal and posterior features, internal echo is usually the first feature to attract the attention of the ultrasound physicians. And hypoechoic is the most common type of echo in both typical benign and typical malignant masses [6, 7, 12, 19].
Compared with FA, the internal echo of most MC was non-hypoechoic including heterogeneous, isoechoic, or complex cystic and solid, which may be related to the complex tissue composition of the MC. MCs are composed of large amounts of extracellular mucin, stroma, and clusters of cancer cells. Kaoku et al[15] reported that the internal echo of MC varied with the proportions of the different components, and the proportion of stroma tended to increase as the internal echogenicity increased.
As for the difference in marginal feature of MC and FA, unlike the circumscribed margins of most FAs (84%), the margins of most MCs (84%) were not circumscribed in our study. Previous studies reported that the ratio of MC with not circumscribed margin was between 23.5–95.0% [8, 9, 13, 14, 16]. The large ratio range may be related to the different number of cases included in the studies. Compared with previous studies with fewer cases or only including PMCs, our study was more convincing, because it had the largest number of cases and included two pathologic types of PMC and MMC. Maybe, the not circumscribed margin of MC is related to the aggressive growth pattern of malignant masses [11, 19]. Furthermore, posterior enhancement was another common ultrasonographic feature of MC. This feature is probably due to the transmission of the ultrasound beam through large amounts of extracellular mucin [8].
This study had some limitations. First, our study only included commonly used ultrasonographic features. US elastography was not extensively used in our study, although it was added to the BI-RADS for US. Second, this was a single-institution retrospective study. Multicentre studies with greater numbers of patients are needed to confirm our findings.