Molecular alterations differentiate microinvasive carcinoma from ductal carcinoma in situ and invasive breast carcinoma: retrospective analysis of a large single-center series.

Microinvasive carcinoma (MIC) of the breast is a rare lesion. The clinicopathologic features and biologic behavior of MIC are unclear. Whether MIC is a distinct entity or an interim stage in the progression from ductal carcinoma in situ (DCIS) to invasive breast carcinoma (IBC) remains to be determined. A retrospective review of clinicopathologic features and analysis of the expression of estrogen receptor (ER), progesterone receptor (PR), human epidermal growth factor receptor 2 (HER-2), and Ki-67 in patients with MIC (90 cases), DCIS (268 cases) and IBC (1504 cases) was performed. Most MICs (93.3%) exhibited an intermediate to high nuclear grade, and this proportion was larger than that of DCIS (62.7%, < 0.001) or IBC (85.4%, = 0.036). The incidence of sentinel lymph node metastasis in MIC (12.5%) was higher than that of DCIS (1.6%, < 0.001), but much lower than that of IBC (39.7%, < 0.001). MICs had higher expression of HER-2 and lower expression of ER and PR compared to DCIS and IBC; and MIC was more likely to present with a HER-2+ subtype. Furthermore, DCIS exhibited greater HER-2 overexpression or gene amplification (P < 0.001) levels and lower proliferation index of Ki-67 ( < 0.001) compared to IBC. Our results suggest that the clinicopathologic and molecular phenotype of MIC are different from DCIS and IBC. Thus, MIC may be a distinct entity rather than an interim stage in the progression from DCIS to IBC. The prognosis of MIC and the biologic behavior of this uncommon subset need to be further explored.