Abstract
BACKGROUND: This study compared plasma cell-free DNA (cfDNA) and tumor tissue DNA (ttDNA) to explore the clinical applicability of cfDNA in patients with metastatic gastric cancer (mGC) receiving palliative second-line paclitaxel + ramucirumab treatment. METHODS: Targeted sequencing of 106 genes was conducted using germline DNA and cfDNA at baseline (baseline-cfDNA) and progressive disease (PD-cfDNA). The results were compared with those of ttDNA-based cancer panel data. RESULTS: Of 76 consecutive patients, 46 (27 males; median age 57.5 [range, 32-73] years) who had all three samples were included. Combined analysis of ttDNA and baseline-cfDNA revealed that TP53 (58.7%) was the most frequently mutated gene, followed by CDH1 (26.1%), KRAS (21.7%), and APC (13.0%). For these genes, the sensitivity and positive predictive value of baseline-cfDNA over ttDNA were 71.8% and 51.9%, respectively. When baseline-cfDNA and PD-cfDNA results were combined, 34 patients (73.9%) were found to have additional mutations compared with ttDNA results alone. PD-cfDNA analysis revealed 14 novel pathogenic mutations in ten patients (21.7%). At baseline, patients with a high circulating tumor DNA fraction concentration showed a significantly shorter progression-free survival (PFS) (P = 0.016) in univariable and multivariable analyses. High maximal variant allele frequency (VAF) (P = 0.022), high sum of VAF (P = 0.028), and high TP53 VAF (P = 0.022) were associated with worse PFS in univariable analysis. CONCLUSIONS: Although cfDNA alone cannot replace ttDNA entirely, cfDNA analysis revealed additional mutations. Notably, PD-cfDNA analysis revealed novel pathogenic mutations that emerged during treatment. Moreover, the baseline circulating tumor DNA fraction concentration and VAF values were associated with longer PFS.