It is a single-center, retrospective, controlled study to investigate the efficacy and safety of neoadjuvant chemoradiotherapy combined with PD-1 inhibitor and thymalfasin for locally advanced mid-low rectal cancer.

Study Purpose 1. To evaluate the efficacy and safety of neoadjuvant chemoradiotherapy combined with PD-1 inhibitor and thymalfasin for locally advanced mid-low rectal cancer. 2. To explore the effects of neoadjuvant chemoradiotherapy combined with PD-1 inhibitor and thymalfasin on the immune microenvironment of locally advanced mid-low rectal cancer. Study Design: A single-center, retrospective, controlled study Subjects were divided into two groups according to whether or not they received thymalfasin: group 1 was treated with neoadjuvant chemoradiotherapy combined with PD-1 inhibitor, and group 2 was treated with neoadjuvant chemoradiotherapy combined with PD-1 inhibitor and thymalfasin. Subjects received long course radiotherapy (50 Gy/25f, 2 Gy/f, 5 days/week) for the first 5 weeks and three 21-day cycles capecitabine (1000 mg/m2, bid, po, day1-14) plus three 21-day cycles tislelizumab (200 mg, iv.gtt, day 8) for the first 9 weeks. After that, patients rested for two weeks (week 10-11)。6-8 weeks after the end of radiotherapy, patients underwent TME surgery (12-14 weeks). Thymalfasin was started on the first day of neoadjuvant chemoradiotherapy, 1.6 mg subcutaneously twice a week until the end of the last neoadjuvant treatment. Enrollment: 26 participants, 13 in each group Study Population: locally advanced mid-low rectal cancer Primary Endpoint: pathologic complete response（pCR） Exploratory endpoint: Paraffin specimens were collected from biopsies before neoadjuvant therapy and after surgery in patients meeting the inclusion criteria. The expression of CD86, CD163, CD4+T，CD8+T，PD-1 were detected.