A Study on the Accurate Evaluation of Pseudoprogression of Rectal Cancer Immunotherapy Based on Multitemporal and Multiparameter MRI

Inclusion Criteria: 1. Rectal cancer confirmed by biopsy pathology; 2. Clinical stage II-IV determined by imaging examinations; 3. Decided by the MDT team to receive immunotherapy (monotherapy with PD-1/PD-L1 inhibitors or in combination with CTLA-4 inhibitors), without prior treatment of any other kind; 4. Multiparametric MRI images of the rectum obtained within 2 weeks before immunotherapy, and at 6 and 12 weeks after treatment; 5. Follow-up CT/MRI data available; evaluation conducted according to the iRECEST criteria, or surgical resection performed to obtain postoperative pathological results, which are necessary for assessing the efficacy of immunotherapy and diagnosing pseudoprogression versus true progression; 6. Blood test parameters (IL-8, NLR, LDH, S100) and clinical indicators (KPS score, CEA level, T-stage) measured within 2 weeks before immunotherapy, and at 6 and 12 weeks after treatment; 7. In cases where imaging suggests disease progression, pathological biopsy or surgical pathology results must be available. 8. The overall survival period is more than 3 months, and the patient does not have any other uncontrollable diseases; 9. The patient has been fully informed and has signed the informed consent form. Exclusion Criteria: 1. The primary tumor lesion has been surgically removed at the time of initial diagnosis; 2. History of other malignant tumors; 3. Unable to sign informed consent or complete the trial due to physical or psychological illness; 4. Pregnant or lactating women; 5. Those who are allergic to gadolinium-containing contrast agents or contraindicated to undergo enhanced MRI examination, or those who are claustrophobic or have other contraindications to MRI examination and cannot undergo MRI examination; 6. History of autoimmune diseases such as inflammatory bowel disease; 7. Treatment interruption due to serious complications related to immunotherapy.