Lobular carcinoma of the breast with hybrid myoepithelial and secretory ("myosecretory") cell differentiation.

Three cases of lobular carcinoma of the breast showing a complex morphology that included myoepithelial cell differentiation are reported. One case was a pure in situ acinar lesion, while the other 2 cases were in situ and invasive carcinomas. Three different cell types were seen in these tumors: one was the phenotype commonly seen in the garden variety of in situ lobular carcinoma (LCIS) constituted by noncohesive round to ovoid cells with round nuclei and positivity for epithelial membrane antigen (EMA), estrogen receptor (ER), and progesteron receptor (PR). E-cadherin was negative in these cells. The second type was represented by cohesive elements with irregular nuclei. These cells were immunoreactive for smooth muscle actin, calponin, keratin 14, p63, and e-cadherin. EMA, ER, and PR were consistently negative. The third type, seen in a minority of cell population of case nos. 2 and 3, consisted of cells showing at the same time EMA and smooth muscle actin in their cytoplasm. This type was defined as "hybrid myosecretory cell" to highlight contractile and secretory properties present at the same time. Cells with hybrid features probably indicate that myoepithelial and secretory cells are strictly related and the existence of a stem cell, at least for these cases, is not necessary.