Incomplete inside-out growth pattern in invasive breast carcinoma: association with lymph vessel invasion and recurrence-free survival.

Invasive micropapillary carcinoma (IMPC) is a rare subtype of epithelial tumor of the breast listed in the 2003 World Health Organization histologic classification of tumors of the breast. It is characterized by inside-out micropapillary morphology, frequent lymph vessel invasion (LVI), and lymph node metastasis; however, its etiology remains unknown. This study investigated the incomplete inside-out growth pattern (IGP) in invasive ductal carcinoma, not otherwise specified (NOS), and examined the association between incomplete IGP and clinicopathologic features, including the presence of intratumoral lymph vessels (ILV), LVI, nodal metastasis, and prognosis. Tumor tissues from 166 invasive duct carcinomas NOS and 10 IMPCs were immunostained using an anti-epithelial membrane antigen antibody to detect IGP and with D2-40 antibody to determine the presence of ILV and LVI. Incomplete IGP was detected focally in 88 (53%) of 166 invasive duct carcinomas NOS. Transition areas between IMPC and invasive duct carcinoma NOS also showed prominent incomplete IGP in 9 (90%) of 10 IMPCs. Incomplete IGP in invasive duct carcinomas NOS was associated with larger tumor size, higher frequencies of ILV, LVI, nodal metastasis, and poorer recurrence-free survival by univariate analysis. Incomplete IGP, ILV, and tumor size independently affected LVI by multivariate analysis. These findings indicate that incomplete IGP of tumor cell clusters is not uncommon and is a useful tool for predicting LVI in invasive duct carcinoma NOS of the breast.