br Most of previous studies
Most of previous studies showed that the tumor was large at the time of MpBC diagnosis. The median MpBC tumor size in our study was 2.78 cm (range, 1.2e5.2 cm), larger then median size of other invasive breast cancers (1.91 cm). Pezzi et al. (2007) reported that the larger size of MpBC at clinical presentation appeared to result from a more rapid growth . In our study only 3 MpBCs (24%) were found to be < 2 cm in size. In the population based study by Pezzi et al. (2007) 29.5% of MpBCs were found to be < 2 cm in size compared with 65.2% of invasive ductal breast carcinomas . A connection between tumor size, recurrence and survival rate in MpBC has been suggested , although there are studies that indicate that there is no such a relationship [15,20].
MpBC is characterized by the presence of divergent cellular differentiation including spindle, squamous, adenosquamous, myoepithelial, sarcomatoid and rarely chondroid or osseous ele-ments with or without an accompanying conventional in situ or invasive mammary carcinoma elements (Table I) . The spindle cell carcinoma contains G-418 forming poorly cohesive sheets of predominant spindle cell morphology and often resembles a low grade sarcoma or reactive process. The squamous cell subtype contains polygonal cells with eosinophilic cytoplasm and possible keratin pearl formation. The adenosquamous carcinoma contains glands which display varying degrees of squamous differentiation, characterized by pavement-like architecture, prominent intracel-lular bridges and, to a lesser extent, keratin formation. The carci-nosarcoma is built of both malignant epithelium and malignant stroma. MpBC with mesenchymal differentiation contains overt carcinoma with a transition to osseous and/or cartilaginous stromal matrix. The myoepithelial carcinoma of the breast is extremely rare tumor, composed entirely or almost entirely of malignant spindle cells with myoepithelial differentiation [19,22].
Molecularly, MpBC is distinct from other breast cancers. It might arise from earlier, more chemoresistant breast epithelial precursor than basal-like or luminal breast cancers . Weigelt et al. (2009), suggested that MpBC are part of the spectrum of basal-like breast cancers and demonstrate a myoepithelial and epithelial to mesenchymal transition molecular make up . Further immu-nohistochemical and genomic studies characterized MpBC as a member of either the basal-like or the claudin-low molecular subtype with features of an epithelial to mesenchymal transition (EMT) and stem cell-like characteristics, including increased pres-ence of CD44þ/CD24-tumor stem cells [24,25]. According to some reports MpBC tends to originate from cancer stem cells or myoe-pithelial progenitors , but others adopted the theory of trans-formation of the carcinomatous component into the sarcomatous component through epithelial to mesenchymal transition . This
theory is supported by the overexpression of genes linked to motility, adhesion, migration and extracellular matrix formation . On the other hand, stem cell theory may be supported by high CD44/CD24 and CD29/CD24 expression in MpBCs, consistent with a high level of stem-like cells in these tumors . Despite its mo-lecular classification, MpBC shows inferior clinical outcome including poorer response to chemotherapy  compared with other invasive breast cancer types. The observation that MpBCs represent tumors enriched in EMT and stem-like cells may account for their resistance to therapy and propensity to metastasize .
Due to its heterogeneity, accurate diagnosis of MpBC may be a challenge in preoperative core needle biopsies . Therefore, surgical excision is the necessary procedure to achieve proper final diagnosis. Rakha et al. (2014) found that the most common subtype of MpBC in Western countries was spindle cell carcinoma (34%) , while squamous cell carcinoma (34%) was the most common subtype in patients from Singapore and Hong Kong. Lai et al. (2013) reported that squamous cell carcinoma (35.6%) was the most common subtype in Taiwan . Luini et al. (2007) showed that MpBC with matrix-production was the leading subtype (45.9%) in European patients, followed by carcinosarcoma (24.3%) and squa-mous cell carcinoma (18.9%) . Study by Zhang et al. (2015) showed that spindle cell carcinoma (34.4%) is a major MpBC sub-type in Chinese population . Using the current WHO classifi-cation, we found that squamous cell carcinoma (77%) is the most common subtype of MpBC in Polish population. The different fre-quencies of thoughtful MpBC subtypes in different populations may have resulted from a small number of patients in most studies and variation in classification [19,32].
Lymph node metastasis was observed in the minority of cases, and no difference in the rate of lymph node metastasis was observed with respect to other histologies (38% versus 37%) (p ¼ 0.963). In previous studies lymph node metastasis has been shown to be less frequent in MpBC [10,31], but not all of them have confirmed this remark [7,33]. MpBC presented with axillary nodal involvement with similar frequency to other invasive breast can-cers. 5 of the 13 patients with MpBC (38%) had nodal involvement in our study, what reports slightly higher incidence of axillary nodal involvement than in previous studies, where incidences at diag-nosis of MpBC were between 6% and 28%. MpBC is observed to metastasize to the bones and lungs with hematogenous rather than lymphatic spread .