Significance of HER2 in Microinvasive Breast Carcinoma.

Objectives We compared the clinicopathologic features, clinical management, and outcomes of human epidermal growth factor receptor 2 (HER2)-expressing and nonexpressing microinvasive breast carcinomas (MiBC) to explore the significance of HER2 in MiBC. Methods Clinicopathologic and follow-up information of cases with final diagnosis of MiBC with known HER2 status between 2007 and 2019 were analyzed. Results Nineteen (41.3%) HER2-positive (HER2+) and 27 (58.7%) HER2-negative (HER2-) MiBCs were identified. HER2 positivity was likely to be associated with high nuclear grade, presence of tumor-infiltrating lymphocytes, hormonal receptor negativity, and increased Ki-67 in both microinvasive and associated in situ carcinomas. Nodal metastases were found in 2 ER+/HER2- cases (5.3%). One HER2+ case was found to have isolated tumor cells in the axillary node. The majority of patients with HER2+ MiBCs (76.5%) did not receive HER2-targeted therapy. All patients with available follow-up were alive without recurrence or distant metastasis, with a median follow-up of 38 months. Conclusions Similar to the larger size of invasive breast carcinomas, HER2 positivity is associated with high-grade morphologic features in MiBCs. However, HER2 overexpression in MiBCs does not appear to be associated with nodal metastasis or worse outcome in our study cohort. The role of HER2-targeted therapy in this clinical setting merits additional study.