This study plans to include LARC patients receiving neoadjuvant therapy at this center, conducting PET/CT and PET/MR examinations before treatment, and PET/MR examinations before surgery after neoadjuvant therapy. The changes in the lesion before and after treatment will be compared to evaluate the efficacy of neoadjuvant therapy, including: the relief situation of the rectal primary lesion ; lymph node metastasis; local infiltration around the tumor; peritoneal and other distant metastases , etc. The pathological relief diagnosed by surgical pathology is the gold standard, and the predictive efficacy of PET/MR will be evaluated, comparing the advantages and disadvantages of 18F-FDG and 68Ga-FAPI PET/MR, and comparing with traditional PET/CT and rectal MRI to explore the value of PET/MRI in predicting the efficacy of neoadjuvant therapy in LARC patients.

Inclusion Criteria:

• (1) Age 18-75 years old; (2) ECOG score 0-2; (3) Pathologically confirmed as rectal adenocarcinoma; (4) The lower edge of the tumor is within 12cm from the anal edge; (5) Clinically staged as cT3-4N0M0 or cTanyN+M0; (6) Untreated patients who have not received radiotherapy, chemotherapy, and surgery, etc.; (7) Good liver and kidney function, can tolerate radiotherapy and surgery; (8) Patients and their families can understand the research plan, voluntarily participate in this research and sign the informed consent form.

Exclusion Criteria:

• (1) ECOG score > 2; (2) Patients with multiple primary colorectal cancers; (3) Patients with a history of other malignant tumors within the past 5 years (except for cured basal cell carcinoma, cervical carcinoma in situ, surgically treated localized prostate cancer, or surgically removed breast ductal carcinoma in situ); (4) Patients with intestinal obstruction, intestinal perforation, gastrointestinal bleeding, and other emergencies requiring emergency surgery; (5) Pregnant or lactating women; (6) Patients with a history of severe mental illness, immune diseases, and hormone medication; (7) Patients with contraindications for MRI, PET examination, radiotherapy, immunotherapy, or surgical treatment; (8) Patients who have participated in other clinical studies within the last 3 months; (9) Any other conditions deemed unsuitable for inclusion by the researcher.