MBC is a rare breast malignancy, constituting a small proportion (5%) of breast cancer [1,2,3,4,5]. It consists of pathologically heterogenous tumor formed of epithelial and/or mesenchymal elements [5,6,7]. Some studies reported that MBC occurs in older women (> 50 years), with a large tumor size; however, other studies report a younger age at presentation [3, 6, 13, 15]. In this study, about (45.45%) of the patients presenting with MBC were 50 years and older. The main presentation was a mass (81.82%), one tumor presented during screening and two presented with metastasis. As a result, all the patients had a diagnostic breast imaging, regardless of their age, rather than a screening as an initial investigation, excluding the two patients who presented with metastasis.
In our study, the mean size of the masses is 5.96 cm, and 12/22 of the cases (54.55%) the masses measured more than 5 cm which is higher compared to other publications [4, 8]. In view of the heterogeneous histological patterns, MBC imaging features tend to be variable. Based on the mammogram, the current study demonstrated that most masses (63.16%) were irregular in shape, with indistinct and spiculated margins in 57.89% and 10.53%, respectively. A similar observation was observed on the ultrasound with the majority (72.73%) of the masses had an irregular shape, and indistinct and microlobulated margins in 36.36% and 27.27%, respectively, with a complex echopattern in more than a third of the cases.
Apart from our own evaluation of the masses with the latest criteria or the cases referred as known cancers (BI-RADS 6), only two cases were coded as BI-RADS 3 (one presented with microcalcifications and the other with a complex echopattern) and both missed their 6-month follow-up. The overall mass description was suspicious, resulting in the ACR-BI-RADS reaching 4 and 5 in 16/18 cases (the remaining were not coded at time of reporting), similar to some previous studies where malignant features were evident, and in contrast to other studies reporting that the radiological features could be mistaken for a benign pathology [3, 5, 6, 8, 13, 15, 16, 22, 23].
Although squamous cell carcinomas are likely to be associated with irregular and spiculated margins and spindle cell with a more oval shape and circumscribed margins , this was not the case in our study (Figs. 1, 2). No characteristic radiological appearances in any of the subtypes were demonstrated.
Just more than half (54%) of the cases demonstrated posterior acoustic enhancement, which is within the 50–67% range reported in the literature [3, 13, 15, 22]. Yang et al. observed that MBC presents with posterior acoustic enhancement more frequently than invasive ductal carcinoma . Although it was present in more than half of the cases, it cannot be considered as a differentiating sign from other breast cancer, even with the absence of hormonal receptor expression.
The increased proportion of irregularly shaped masses observed in the current study could be due to the fact that we described the lobular shaped masses as irregular, according to the latest ACR criteria. However, this description did not affect the overall results of the assessment of the masses in recent studies, using the previous ACR criteria [8, 15, 22]. In addition, microcalcifications were seen only in three cases (14.29%) with suspicious patterns of pleomorphic /coarse heterogeneous morphology and a regional/segmental distribution, in which two of these were a squamous subtype of MBC. This observation is consistent with the literature demonstrating less than 25% of calcifications in their results [3, 6, 8, 13].
Regarding the additional features in the mammogram, skin changes (thickening and ulceration) were present in 52.38% of the cases. This is considerably higher than previous reports and was significantly related to lymph node involvement (p value = 0.022) [3, 6, 9, 15, 24]. Though it was associated with an increased mass size, this observation was not statistically significant (p value = 0.853).
In our study, squamous cell subtypes had cystic and solid components (Fig. 1a) similar to other studies that have classified it in a differential diagnosis of complex breast masses [6, 16, 25]. However, this finding was not statistically significant in our study (p value = 0.568).
A breast MRI was done in only four cases. The masses had different shapes and T2 signal intensity, with only one an oval-shaped mass with irregular margins, and a hyperintense signal in the T2-weighted image with rim enhancement, similar to the literature [2, 5, 8]. All four cases displayed a washout enhancement pattern in the kinetic curve (Fig. 1b, c).
Core needle biopsy is the gold standard for breast cancer diagnosis; however, in some circumstances it is difficult to differentiate MBC from other breast cancers on core needle biopsy alone. In the current study, approximately one third of the cases were initially diagnosed as a conventional infiltrating ductal carcinoma on core needle biopsy, and subsequently reclassified as MBC on the resected specimens. This is most likely related to a sampling issue [5, 6, 13].
Generally, most MBC is triple negative with high-grade morphology and high proliferation index (Ki 67%), and these findings are similar to our results [2, 8, 22]. The HER2-positive cases have a relationship with increased mammogram calcifications . However, in our study, there were only four cases (18.18%) showed HER2-positive staining (3 +) by immunohistochemistry and they did not show microcalcifications in their mammograms. This could be due to a limited HER2 expression in MBC and the rarity of calcifications within it . It is noteworthy that our results indicated that HER2-positive masses are primarily observed in the cases with squamous differentiation; however, this observation was not statistically significant (p value of 0.846). The squamous type is the most prevalent type of MBC which is consistent with the current study, with 45% of the cases [6, 9].
MBC has a potential of hematological metastasis, more than lymphatic spread [3, 11, 12]. Consequently, it presents with few lymph nodes involvement in 25–40% of the cases [3, 5, 6, 12, 13, 17]. In our study, 10/22 of cases (45.45%) presented with axillary lymph node involvement. This may be contributed to the frequent large mass size in our series; however, there was no statistical significance (p value = 1.000). The majority of the cases (12/19 cases) had advanced disease at presentation with either T3 or T4 clinical or pathological staging. More than one third of the cases already had metastasis at the initial diagnosis or diagnosed in the follow-up period (18.18% and 22.73%, respectively), most frequently to the lung. For the group who underwent surgery, a mastectomy was most frequently performed, and three cases had lumpectomies, as supported by recent studies [11, 23].
This study is limited by the small sample size, which is due to the rarity of the disease. Another limitation is that most of the documents from the hospital electronic system were missing and the ultrasound was evaluated from the PACS images which are operator dependent. We collected the BI-RADS classifications from the initial radiological report, and thus we avoided any bias in the study given the known diagnosis by the authors. The strong point of the study is the reassessment of the radiological appearance to obtain additional characteristics. In addition, this is one of the first studies of metaplastic carcinoma using the latest ACR criteria in association with the latest pathological WHO classification.