ACT-MBC prospectively assesses the impact of CTCs on treatment decisions, response assessment and prognosis in MBC patients.

Inclusion Criteria:

1. Invasive breast cancer with radiographic and/or clinical evidence of advanced metastatic or unresectable disease
2. ER+/HER2- patients prior to starting 2nd line therapy or beyond OR ER/Progesterone Receptor (PR)/HER2-negative (Triple Negative) patients prior to starting any line of therapy
3. Measurable and/or non-measurable disease is allowed
4. Male or female breast cancer is allowed
5. Age > 18 years
6. Willingness to provide mandatory blood specimens
7. Willing to return to enrolling institution for follow up imaging at least once

Exclusion Criteria:

1. Life expectancy of ≤ 6 months
2. Inability to provide blood samples based on the judgment of the treating provider

Advanced/metastatic breast cancer of following subtypes/lines of systemic therapy in metastatic setting:

• Estrogen Receptor (ER)+/Human Epidermal growth factor Receptor 2 (HER2) - patients prior to starting 2nd line or later therapy
• Triple Negative (TN) patients prior to starting any line of systemic therapy