Application of circulating tumor DNA detection in diagnosis and treatment of locally advanced rectal cancer. First, to explore the feasibility of ctDNA as a detection index for rectal cancer. Second, evaluate the accuracy of ctDNA detection in rectal cancer. Third, to explore whether ctDNA can be used in the evaluation of preoperative concurrent chemoradiotherapy, so as to provide guidance for subsequent treatment. Fourth, to explore the guidance value for the decision of postoperative adjuvant therapy and the frequency of reexamination. Fifth, search for possible recurrence related mutations.

Inclusion Criteria:

• Histologically or cytologically confirmed primary rectal cancer；
• First treatment；
• 18 years of age or older, male or female；
• According to the indications of operation, surgical treatment can be carried out；
• According to the indications of radiotherapy, radiotherapy can be carried out；
• The physical status score of ECOG was 0~1；
• Organ function is appropriate;
• Expected survival time more than 3 months；
• During the treatment period and 6 months after treatment, contraception was not in lactation；
• The patient volunteered and signed the informed consent.

Exclusion Criteria:

• Poor compliance and failure to receive medication or follow up according to plan；
• There are other serious violations of the programme;
• At present, it has been included in other clinical trials, or at the same time, it has been included in other medical research which is judged to be incompatible with this research；
• It has a history of other cancers. Unless the cancer is completely relieved, the disease is not treated for more than 3 years.

Locally advanced rectal cancer, who need to receive neoadjuvant radiotherapy and radical operation