In the formation and growth of a primary tumor, cells can released into the bloodstream. There are many researches for these circulating tumor cells(CTC) And recently circulating cell-free tumor DNA(ctDNA) released from apoptotic or necrotic tumor cells has developed. At one report which checked ctDNA level after operation of colon cancer patients, they suggested that ctDNA could show recurrence of cancer several months earlier than the conventional follow up imaging. And several reports suggested that ctDNA can invade host cell and change host cell biology, which can evoke cancer metastasis.