Differential diagnosis of breast mucinous carcinoma with an oval shape from fibroadenoma based on ultrasonographic features

Breast MC is rare in clinical practice, and a preoperative diagnosis is difficult to obtain due to the variability in ultrasonographic features and the overlap of its morphological features 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 being medium-high-risk (BI-RADS 4B and above), 21.1% (16/76) MCs were assessed as low-risk (BI-RADS category 4 A), for which the malignancy likelihood is exceptionally low. By comparing the ultrasonographic features of MCs in the low-risk and medium-high-risk groups, we confirmed that the majority (81.2%) of MCs in the low-risk group were oval shape. 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 only 9.1% of PMCs were oval shape. 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 significant difference between the two in the current study and in previous studies. An oval shape is an ultrasonographic feature of benign breast masses, which may be suggested for follow-up instead of biopsy [10, 11, 18, 19]. On US, if the shape of MC is oval, its risk category may be underestimated. In addition to shape, there were also statistically significant differences in margin features between the low-risk group and the medium-high-risk group. However, although the percentage of MCs with a circumscribed margin in the low-risk group was higher than that in the 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 low-risk which typically suggests a benign mass.

Fibroadenomas are the most common benign tumours of the breast [20]. Compared with patients with FA which is commonly found in young women (typically in 20 to 30 years old) [21], patients with MC in this study were older, with a median age 47 years, which is younger than the 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 an oval shape, we found that the margin features, internal echo patterns and posterior echo features were helpful for their differential diagnosis. Among internal echo pattern, margin and posterior features, the internal echo pattern is usually the first feature to attract the attention of ultrasound physicians. Hypoechogenicity is the most common type of echo in both typical benign and typical malignant masses [6, 7, 12, 19].

Compared with that of the FAs, the internal echo of most MCs was non-hypoechoic which included heterogeneous, isoechoic, or complex cystic and solid, which may be related to the complex tissue composition of MCs. 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, particularly, the proportion of stroma tended to increase as the internal echogenicity increased.

Regarding the difference in the margin features of MCs and FAs, unlike the circumscribed margins of most FAs (84%), the margins of most MCs (84%) were not circumscribed in our study. Previous studies have reported that the percentage of MC with not circumscribed margins was between 23.5% and 95.0% [8, 9, 13, 14, 16]. The size of this range may be related to the different number of patients included in the studies. Compared with previous studies with fewer cases or only including PMCs, our study is more convincing, because it includes the largest number of patients and included two pathologic types, PMC and MMC. The not circumscribed margins of MC are related to the aggressive growth pattern of malignant masses [11, 19]. Furthermore, the posterior enhancement was another common ultrasonographic feature of MC. This is probably due to the transmission of the ultrasound beam through large amounts of extracellular mucin in MC [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, the advantages of artificial intelligence in mucinous carcinoma diagnosis are becoming increasingly prominent [25, 26]. However, we had a small number of patients. We will collect more cases and attempt to use artificial intelligence to reduce the underestimation of mucinous carcinoma with an oval shape. Finally, this was a single-institution retrospective study. Multicentre studies with greater numbers of patients are needed to confirm our findings.