MBC is a rare disease that accounts for less than 1% of all mammary tumors . In our study, the incidence rate of MBC was only 0.73%. Interestingly, we found that an increasing number of patients with MBC were reported each year. The World Health Organization only recognized MBC as a distinct pathological entity in 2000. The increased incidence we noted may represent an actual increase in the disease. Alternatively, it may be a result of improved awareness and recognition by pathologists [4, 5], who, since the early immunohistochemical reports more than 10 years ago , have increasingly recognized and reported this distinct histologic type of breast cancer. However, because of its rarity, only relatively small series have been reported.
The prognosis of MBC had not been well delineated. Although some investigators have reported a better prognosis for MBC than IDC, others had reported that the prognosis for MBC was unfavorable compared to IDC [4, 17, 18]. We observed a dismal prognosis for patients with MBC when compared with 767 patients with IDC, including 131 patients with TN-IDC. Most previous studies have found that the tumor was large at the time of MBC diagnosis. The median tumor size in our study was 5.0 cm (range, 1.5 to 20.0 cm), larger than the size of an IDC (median 2.3 cm) or TN-IDC (median 2.1 cm). Pezziet al.  reported that the larger sizes of MBC at clinical presentation appeared to result from a more rapid growth rate.
MBC presented with axillary nodal involvement less frequently than did IDC of the breast. Only 15 of the 55 patients with MBC (27%) had nodal involvement in our study. These data were consistent with previous reports that showed incidences of axillary nodal involvement at diagnosis of MBC between 6% and 28% [19–21]. MBC was usually associated with a lower incidence of axillary nodal involvement than that of an IDC with similar size. Patients with MBC had a median tumor size of 5.0 cm. Data from patients with an adenocarcinoma of the breast without metaplasia suggested that, for tumors ranging in size from 2.0 to 4.0 cm, the expected frequency of axillary node involvement was greater than 50% . In our study, only nine of 40 patients with MBC (22%) with tumor sizes between 3.0 and 5.0 cm were axillary node-positive. SLN biopsy was adaptive in patients with MBC before axillary lymph node dissection. Despite MBC being less likely to present with positive axillary lymph nodes, the risk of developing metastatic disease was greater than in typical adenocarcinoma of the breast. It was believed that hematogenous spread was more common in MBC. The above data support the concept that MBC is an aggressive tumor with a high risk of recurrence following the primary site therapy.
In our study, there was a very low incidence of hormone receptor positivity in MBC compared to IDC. Hormonal therapy was rarely provided to patients with MBC, consistent with the low incidence of hormone receptor positivity in these patients. Rosen  noted that the lack of ER and PR might be due to the absence of a prominent glandular epithelial compartment in these tumors. Previous studies have found HER2 over-expression ranging from 4% to 17% [9, 23]. In our study, nine of 35 patients with MBC (26%) had HER2 over-expression, consistent with previous studies. Triple-negative cases accounted for 67% of MBC, within the range of previous studies, where 64% to 96% of patients with MBC showed triple-negativity [5, 9].
Notably, mastectomy was performed more often for patients with MBC. This was likely due to a larger tumor: tumor size was >5 cm in 40% of patients with MBC compared with only 5% of patients with IDC. As a consequence of the lower rate of breast conservation and less nodal involvement, postoperative radiation therapy was performed less often for patients with MBC; however, these patients underwent chemotherapy more often due to their negative hormone receptor status and larger tumor size. In our study, patients who underwent mastectomy had an unfavorable survival rate during the follow-up period.
Recently, significant progress had been made in the field of MBC biology, which could hypothetically explain its far more aggressive nature compared with other triple-negative breast cancers. Lien et al.  elucidated and validated that the epithelial-mesenchymal transition-related genes were differentially up-regulated in MBC compared with IDC, and Hennessy et al.  demonstrated that MBC was distinct from basal-like cancers. MBC showed a close relationship with basal-like cancers and a novel subgroup of receptor-negative breast cancers. The patients with the features of both MBC and basal-like cancers had enrichment for stem cell-like markers with an elevation of CD29/CD24 and CD44/CD24 ratios.
Further research on the mechanisms of carcinogenesis and the potential importance of new molecular markers (for example, P63, Zinc Finger E-box Binding Homeobox 1 (ZEB1), B-cell lymphoma 2 (Bcl-2)) are needed to develop better prognostic factors for this disease. If our data were confirmed, patients with MBC, particularly those with metastatic disease, would be appropriate candidates for clinical trials evaluating new combinations of the active chemotherapy agents for breast cancer. Considering the differences in the clinical and biologic behavior of these tumors compared with IDC and TN-IDC, clinical trials designed specifically for MBC would be very helpful to further characterize the biology and therapy of this disease. In this study, the number of MBC cases is too small to convincingly determine the prognostic factors that affect treatment outcomes of MBC compared with IDC, therefore more cases and longer follow-up may be necessary for this type of analysis.