Prognostic Marker Expression In Microinvasive Breast Cancer

Introduction/Objective Microinvasive breast carcinoma (Mi) is defined as invasive carcinoma (IC) less than 1 mm in greatest dimension. Depletion of the focus during further immunohistochemical investigation is commonly observed. Studies on prognostic marker expression report high concordance between the Mi and concomitant ductal carcinoma in-situ (DCIS). Compared to IC, Mi has a higher rate of Her2 positivity, indicating aggressive phenotype and potential variation in marker expression over time. We aimed to verify the concordance between Mi and DCIS in patients with pure Mi, as well as between foci of Mi and IC in patients with both. Methods A total of 21 cases of Mi and 9 cases with both IC and Mi disease were identified and evaluated for pathologic characteristics and ER, PR, and Her2 expression by immunohistochemistry. The rate of Her2 in Mi was compared to national benchmarks for IC. Results Mi was associated with high grade DCIS. 100% concordance of Her2 expression between Mi and DCIS was present in 14/21 cases where Mi was not depleted. Discrepancy between ER/PR was seen in two cases (Mi negative/DCIS focally positive). 50% of these Mi cases were Her2 positive, compared to the 13–25% generally reported for IC. Overall, by extrapolating Mi status from the concomitant DCIS when the Mi became depleted, 38% of Mi was positive for Her2. 100% concordance between foci of Mi and IC in patients presenting with both was observed. Conclusion Mi had a more aggressive prognostic marker phenotype compared to IC based on Her2 positivity. Concordance of prognostic marker expression between Mi and concomitant DCIS supports reporting of the DCIS status when the Mi becomes depleted, with only rare ER/PR discrepancies observed. Concordance between foci of Mi and IC in patients with both does not support time-based variation in marker expression. However, further investigation is warranted to uncover potential changes.