The purpose of this study is to study the performance of MRD monitoring in predicting the efficacy and prognosis of neoadjuvant therapy in patients with rectal cancer, and to explore the value of MRD detection in evaluating the prognosis of patients. In this prospective study, 50 patients with stage II-III rectal cancer who are planing to receive neoadjuvant chemoradiotherapy will be enrolled. The tumor tissue will be collected by colonoscopy before treatment and blood samples will be collected before treatment and during treatment.The whole blood samples will receive MRD detection. The change rate and clearance rate of MRD during treatment will be calculated, and will be associated with imaging efficacy evaluation, pathological efficacy evaluation,and prognosis, to determine the performance of MRD in predicting and judging the efficacy of neoadjuvant chemoradiotherapy and postoperative recurrence of rectal cancer.

This study is a prospective, observational study. It is expected to enroll 50 patients with newly diagnosed stage II-III rectal cancer.Before neoadjuvant chemoradiotherapy, tumor tissue will be collected by colonoscopy biopsy for WES and customized panel design, and peripheral blood samples will be collected for baseline MRD detection ( MRD1 ) and during neoadjuvant concurrent chemoradiotherapy ( MRD2-6 ) .Peripheral blood sample will be collected 3-4 weeks after radical surgery ( MRD7 ). For patients who achieved cCR after the end of neoadjuvant therapy and adopted the W & W ( watch and wait ) strategy, the sampling time point was 3 months after the start of watching and waiting ( MRD7 ).

Inclusion Criteria:

1. Age 18-75

2. Rectal adenocarcinoma confirmed by pathology

3. The clinical stage is II-III.

4. Neoadjuvant chemoradiotherapy + radical surgery is planned ( cCR patients can choose to wait for observation ).

5. ECOG 0-1;

6. No distant metastasis ;

7. Main organ function is normal ;

8. signed informed consent and willing to accept long-term follow-up；

9. No anti-tumor treatment was received within 4 weeks before baseline sampling ;

10. Adequate ' colonoscopy ' tumor tissue samples can be provided for MRD customization, and sufficient whole blood samples need to be provided for baseline MRD detection.

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Exclusion Criteria:

1. unable to provide sufficient tissue / blood samples to meet the research needs ;
2. received radiotherapy, chemotherapy, or other targeted or immunotherapy before enrollment ;
3. Patients did not receive neoadjuvant therapy according to the original plan ;
4. Patients refused to accept genetic testing. -

This study intends to include 50 patients with stage II-III rectal cancer who plan to receive neoadjuvant chemoradiotherapy between the ages of 18-75 years old.