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Background Retrospective analyses have shown survival differences in patients with primary metastatic breast cancer (PMBC) in comparison to patients with secondary metastatic breast cancer (SMBC). A subgroup analysis of 2,708 patients with metastatic breast cancer from two prospective non-interventional studies (NIS) allows exploring this hypothesis. The Her2-positive-NIS investigated trastuzumab therapy for Her2 positive metastatic disease additional to predominantly first line chemotherapy. The second NIS observed bevacizumab therapy for predominantly Her2 negative disease additional to chemotherapy as first- line option. Methods Information on PMBC or SMBC was available in 2,401 patients diagnosed with metastatic breast cancer and treated with targeted therapy between 2000 and 2011. Previously endocrine therapy in palliative status was allowed. Progression free survival (PFS) was defined as the time from the start of antibody therapy to disease progression or death; overall survival (OS) was defined as the time from the start of targeted therapy with antibody to death of any cause; PFS and OS were estimated by Kaplan-Meier product limit method. Results 570 (24%) women had PMBC and 1831 (76%) had SMBC. Median follow-up was 23 months (range: 0.2 - 137). Median PFS in patients with PMBC and SMBC was 13.0 and 10.4 months, respectively (p = 0.0043); median OS for women with PMBC and SMBC was 33.5 and 28.5 months, respectively (p = 0.0023). 5 years after start of the study treatment 28% patients with primary and 23% with secondary metastatic disease were alive. Conclusion In a population of breast cancer patients treated with modern antibody therapies, woman with PMBC live significantly longer than those with SMBC. A possible explanation for this observation might be a negative selection process of patients suffering from secondary disease in spite of adequate primary treatment. Every fourth patients with metastatic breast cancer is a long survivor. The clinical-pathological characteristics and risk factors for this group are unclear. Prospective trials including translational investigations for these patient populations are needed. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P3-06-04.

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