This project aims to evaluate the role of 18F-FAPI combined with 18F-FDG PET/MRI imaging in quantitatively and accurately evaluating the grading of rectal cancer after SCRT neoadjuvant therapy in patients with advanced rectal cancer as the research object, with postoperative histopathological analysis as the reference index, and to assess the ability of patients to achieve pCR. A diagnostic model and evaluation system will also be constructed.

1. Correlation analysis between the expression levels of FAP and GLUT1 and the degree of tumor regression in rectal cancer

   The study focuses on patients with advanced rectal cancer treated with SCRT neoadjuvant therapy, and conducts immunohistochemical staining of FAP and GLUT1 (glucose transporter 1) on postoperative specimens to evaluate histopathological changes related to the degree of rectal cancer tumor regression. The expression levels of FAP and GLUT1 and the grading of tumor regression are quantitatively analyzed, and their correlation is analyzed to explore the correlation between the expression levels of FAP and GLUT1 in the lesions of neoadjuvant therapy patients and the degree of tumor regression.

2. Construction and validation of a diagnostic model for the degree of tumor regression in rectal cancer after neoadjuvant therapy using 18F-FAPI combined with 18F-FDG PET/MRI imaging

   The patient underwent preoperative 18F-FAPI and 18F-FDG PET/MRI imaging, followed by immunohistochemical staining of FAP and GLUT1 on postoperative specimens and evaluation of histopathological changes related to the degree of rectal cancer tumor regression. The patient's uptake of 18F-FAPI and 18F-FDG (SUVmax, SUVmean, SUVpeak, FAPI metabolic volume, MTV, TBR, TLG), expression levels of FAP and GLUT1, and degree of tumor regression were quantitatively analyzed, and the correlation between the patient's uptake of 18F-FAPI and 18F-FDG and the degree of tumor regression was explored. A graded diagnostic model and evaluation system were constructed to verify their effectiveness.

3. Comparison of the efficacy of PET/MR-TRG and MR-TRG in assessing pTRG after neoadjuvant therapy for rectal cancer

Compare the constructed PET/MR tumor regression grading diagnostic model with the commonly used clinical imaging diagnostic method MRI in evaluating the efficacy, pTRG, and pCR of neoadjuvant therapy for rectal cancer.

Inclusion Criteria:

• Pathologically confirmed as rectal adenocarcinoma, with baseline clinical staging of T3~4 and/or N+;
• No distant metastasis;
• Not receiving chemotherapy or any other anti-tumor treatment before enrollment;
• Has not received immunotherapy before enrollment and is able to adhere to the protocol during the study period (neoadjuvant therapy of short-term radiotherapy sequential chemotherapy combined with immunotherapy);
• After neoadjuvant therapy, 18F-FAPI and 18F-FDG PET/MRI imaging were performed within 7 days before surgery, and the two scans were collected over two days;
• Patient age ≥ 18 years old

Exclusion Criteria:

• Pregnant or lactating patients;
• Patients with severe heart disease in clinical practice;
• Organ transplantation requires immunosuppressive therapy and long-term use of hormone therapy;
• Patients with autoimmune diseases;
• Serious uncontrolled recurrent infections or other serious uncontrolled comorbidities;
• The imaging quality is poor and cannot be used for diagnosis and evaluation;
• Fasting blood glucose levels are higher than 11.1 mmol/L;
• Patients with contraindications for MRI examination.

The subjects the investigators selected are adults who are not restricted by gender.