ER + HER2- early-stage breast cancer: association of HER2 expression, tumor characteristics, and outcomes.

Purpose: To evaluate the association between the HER2 score as provided by the Oncotype DX Recurrence Score (RS) assay, tumor characteristics, and outcomes in early-stage, ER + HER2-negative breast cancer (BC).

Methods: All women insured by the Clalit Health Services, with early-stage, ER + HER2-negative BC who underwent RS testing between 2008 and 2011 were included. Patient/tumor characteristics and Kaplan-Meier estimates for distant recurrence-free survival (DRFS) and overall survival (OS) were compared by HER2 category, based on the HER2 score provided by the RS assay: lower HER2 score group representing the lower third of the HER2 score range (≤ 8.5); higher HER2 score group representing the upper 2 thirds of the HER2 score range (8.6-10.7).

Results: 1535 patients were included (948 node negative, 587 node positive); 330 (21.5%) were categorized as lower HER2 score and 1205 (78.5%) as higher HER2 score. Compared to the higher HER2 score group, the lower score group included a significantly higher proportion of patients with RS ≥ 26 in both node-negative (41% vs. 13.6%, P < .001) and node-positive diseases (36% vs. 19.4%, P < .001). Compared to the higher HER2 score group, the lower score group had significantly lower Oncotype ER and PR scores and lower proportion of lobular disease. Age and tumor size were comparable between the HER2 score groups. Within each RS category, DRFS and OS were not associated with the HER2 score.

Conclusion: Lower HER2 score was associated with higher RS results. Further study is desired to elucidate the role and significance of HER2 expression in early-stage, ER + HER2-negative.