Impact of molecular subtypes on metastatic breast cancer patients: a SEER population-based study.

To investigate the significance and impact of molecular subtyping stratification on metastatic breast cancer patients, we identified 159,344 female breast cancer patients in the Surveillance, Epidemiology and End Results (SEER) database with known hormone receptor (HoR) and human epidermal growth factor receptor 2 (HER2) status. 4.8% of patients were identified as having stage IV disease, and were more likely to be HER2+/HoR-, HER2+/HoR+, or HER2-/HoR-. Stage IV breast cancer patients with a HER2+/HoR+ status exhibited the highest median overall survival (OS) (44.0 months) and those with a HER2-/HoR- status exhibited the lowest median OS (13.0 months). Patients with a HER2-/HoR+ status had more bone metastasis, whereas patients with a HER2+/HoR- status had an increased incidence of liver metastasis. Brain and lung metastasis were more likely to occur in women with a HER2-/HoR- status. The multivariable analysis revealed a significant interaction between single metastasis and molecular subtype. No matter which molecular subtype, women who did not undergo primary tumour surgery had worse survival than those who experienced primary tumour surgery. Collectively, our findings advanced the understanding that molecular subtype might lead to more tailored and effective therapies in metastatic breast cancer patients.