Quantitative assessment of HER2 gene amplification of breast cancer using droplet digital PCR

We previously reported the usefulness of droplet digital polymerase chain reaction (ddPCR) for the assessment of Human epithelial growth factor receptor 2 (HER2) gene amplification in breast cancer using formalin-fixed and paraffin-embedded sections. In our previous study, we combined HER2/CEP17 ratio (HER2 gene signals to chromosome 17 signals) with ddPCR and tumor content ratio (TCR) of each sample and determined the HER2 status by adopting a two-dimensional chart. This "ddPCR-TCR method" showed a high concordance with conventional HER2 status. In this study, we updated our method to assess the HER2 status of breast cancer in a more quantitative manner. We combined obtained data of the ddPCR ratio [R_x ] and TCR [x]; we calculated "(R_x  - 1)/x + 1" for 41 samples with primary breast cancer and named the value led by this formula as "eHER2 (estimated HER2/CEP17 ratio of a tumor cell)". eHER2 was equivalent to conventional in situ hybridization (ISH) HER2/CEP17 ratio in most cases. eHER2 and ISH ratio showed a strong correlation (Spearman rank correlation ρ = 0.70, p < 0.0001). The obtained results indicated that eHER2 is a potential tool for HER2 status diagnosis in breast cancer.