Clinical Relevance of Serum HER2 and Circulating Tumor Cell Detection in Metastatic Breast Cancer Patients

Background/Aim: Presence of circulating tumor cells (CTCs) is associated with impaired survival in metastatic breast cancer (MBC). This study was designed to evaluate whether assessment of serum HER2 (sHER2) levels provide additional prognostic information in MBC. Materials and Methods: Two hundred and fifty-three MBC patients were enrolled in this multicentre trial. CTCs were detected before the start of first-or later-line treatment using the CellSearch system. sHER2 was determined using ELISA. Results: >= 5 CTCs were detected in 122 of 245 evaluable patients (49.8%). One hundred and nineteen of 251 patients (47%) had sHER2 levels above 15 ng/ml. Median overall survival (OS) was 16.3 months in patients with elevated sHER2; median OS in patients with non-elevated sHER2 has not been reached (p=0.001). Patients with >= 5 CTCs were more likely to present with elevated sHER2 (61% vs. 33% in those with <5 CTC; p<0.001). In patients with HER2-negative tumors, elevated sHER2 was associated with shorter OS and PFS; in HER2-positive patients with OS only. Including sHER2, CTC status and established prognostic factors into a multivariate analysis, only the presence of CTCs and higher-line of therapy remained independent predictors of OS. Conclusion: Elevated levels of sHER2 are associated with worse survival, irrespective of the HER2 status of the tumor. However, sHER2 does not provide additional prognostic information in patients with known CTC status.