To demonstrate as a proof-of-concept that PPS-ctDNA can distinguish this situation from a metastatic case, we identified a patient who had undergone synchronous surgery of the primary CRC and liver metastasis, then subsequently developed a liver mass and elevation of CEA that was initially thought to be due to recurrent CRC, but later revealed to be a cholangiocarcinoma on histopathology upon surgical resection. This evidence concerns the gene CEACAM5 and cholangiocarcinoma.