Supplementary Material for: Comparison of HER2 Expression in Primary Tumor and Disseminated Tumor Cells in the Bone Marrow of Breast Cancer Patients

Objective: The aim of this study was to measure the human epidermal growth factor receptor 2 (HER2) status of disseminated tumor cells (DTCs) from bone marrow (BM) aspirates and to assess correspondence or discrepancy with the primary tumor. Methods: DTCs were isolated from the BM of 156 breast cancer patients. Cytokeratin-positive DTCs were further analyzed by the chromogenic in situ hybridization method to detect HER2 gene amplification. Results: A significant correlation (p = 0.021) was found between the HER2 status of DTCs and the primary tumors. Sixty-one (68.5%) patients had a corresponding status. However, a shift of phenotype between primary tumor and DTCs was found in the remaining patients. Conclusion: This study showed a significant grade of discordance of the HER2 status between primary tumors and DTCs in the BM of a relevant subgroup of patients. Detection of HER2 amplification on DTCs could therefore help to better stratify patients for a more tailored therapy, since they would benefit from a HER2-targeted therapy.