Serum HER2 extracellular domain levels and HER2 circulating tumor cell status in patients with metastatic breast cancer

Aim: To shed light on the clinical role of HER2 status in serum as extracellular domain (ECD) and corresponding circulating tumor cells (CTCs) in metastatic breast cancer patients. Methods: 68 patients were analyzed. Serum HER2 was determined by ADVIA Centaur® Serum HER2 test. CellSearch System was performed for CTC quantification. Results: HER2 was overexpressed in 21 primary tumors. In total, 19 patients had ECD >15 ng/ml (the cut-off used), 48 patients had at least one CTC. ECD positivity was associated with CTC number (p = 0.01), HER2-positive CTC (p = 0.01) and the ratio HER2-positive CTC/total CTC (p = 0.02). ECD was not associated with survival. Conclusion: ECD in combination with HER2 CTC status would deserve further investigation in larger series for addressing its putative prognostic relevance.