HER2-positive Apocrine Carcinoma of the Breast: A population-based Analysis of Treatment and Outcome

Abstract Purpose Apocrine carcinoma of the breast (APO) expresses HER2 in 30-50% of cases. This study explored the clinicopathological features and outcome of HER2+/APO and matched HER2+/NST cohort. Methods We used the SEER database to explore the cohorts. Univariate and multivariate analyses were used to assess the survival. Based on ER and PR [=HR] and HER2 status, we divided the cohorts to match the intrinsic molecular subtypes for comparisons. Results We retrieved 259 cases of HER2+/APO. Most HER2+/APO were HR negative (65%). HER2+/APO were more prevalent in the 80+ age group (24.7% vs. 15.7%, p<0.001). HER2+/HR-/APO had a significantly lower histological grade than the HER2+/HR-/NST (p<0.001). Breast cancer-related deaths were more prevalent in HER2+/NST (7.8% vs. 3.9%, p=0.019). This was particularly evident between HR- subgroups (10.4% in HER2+/HR-/NST vs. 4.2% in HER2+/HR-/APO, p=0.008) and was reaffirmed in breast cancer-specific survival in univariate analysis (p=0.03). Other than race and HR status, HER2+/APO subgroups did not differ in clinicopathological parameters. Conclusions Our study confirms the rarity of the APO and reveals that HR status in APO does not affect these patients' prognosis. HER2+/APO tumors tend to have a less aggressive phenotype and a more favorable outcome despite a markedly lower ER/PR positivity.