The high positive predictive value of ctDNA positivity for disease progression complements recent metastatic gastric cancer data showing that on-treatment changes in ctDNA levels correlate with response and survival20 and suggests that ctDNA can be a powerful adjunct to radiographic imaging (and non-specific serum biomarkers such as CEA) for disease monitoring in gastric cancer. This evidence concerns the gene CEACAM5 and gastric cancer.