Two hundred twenty-four breast cancer patients with paired tissue and plasma samples were enrolled from 3 clinical centers to evaluate sensitivity and specificity of a digital PCR HER2 amplification assay. All patients were histologically confirmed diagnosis of locally advanced and recurrent or metastatic breast cancer with stage III/IV and had tissue HER2 status determinations using IHC/FISH. For the whole 224 advanced breast cancer patients, the sensitivity between dPCR in plasma and IHC/FISH in tissue samples is 43.75% (42/96), the specificity is 84.38% (108/128) and the overall concordance is 66.96% (150/224). Interestingly, when we looked at stage III, stage IV and recurrent or metastatic breast cancer separately, compared with IHC/FISH in tissue samples, the sensitivity of dPCR in plasma increases from 37.93% (11/29) for stage III to 41.67% (15/36) for stage IV cancer. Recurrent breast cancer patient had an increased sensitivity of 51.61% (16/31). This is consistent with our expectation sensitivity would increase concordantly as tumor burden goes up. On the other hand, specificity decreased from 92.68% (38/41) for stage III to 86.44% (51/59) for stage IV cancer. Recurrent breast cancer patient had a specificity of only 67.86% (19/28). This is, in part, due to inter- and intra-tumor heterogeneity. Many patients determined to be negative for HER2 amplification in tissue biopsy could have HER2 positive tumors at other sites, which was detected by the liquid biopsy. This study suggested the necessity of liquid biopsy for HER2 amplification detection and demonstrated digital PCR can be used as a companion diagnostic tool to determine HER2 amplification status. It also suggested that a liquid biopsy should follow a negative result from tissue biopsy to avoid false negative results especially for late-stage breast cancer patients and ones who experienced relapse or became resistant to current therapy. Future studies should focus on therapeutic effects on patients determined to be HER2 positive through liquid biopsy and collecting additional tissue biopsies to identify HER2 positive tumor when the original tissue biopsy and liquid biopsy don't agree.
Keywords: HER2; digital PCR; liquid biopsy; sensitivity; tissue biopsy.
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