HER2-positive apocrine carcinoma of the breast: a population-based analysis of treatment and outcome

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 [steroid receptors/SR/] 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 SR negative (65%). HER2+/APO were more prevalent in the 80+ age group (24.7% vs. 15.7%, p < 0.001). HER2+/SR−/APO had a significantly lower histological grade than the HER2+/SR−/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 SR− subgroups (10.4% in HER2+/SR−/NST vs. 4.2% in HER2+/SR−/APO, p = 0.008) and was reaffirmed in breast cancer-specific survival in univariate analysis (p = 0.03). Other than race and SR status, HER2+/APO subgroups did not differ in clinicopathological parameters. CONCLUSIONS: Our study confirms the rarity of the APO and reveals that SR 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.