This study aims to explore how well the usual ways of checking if rectal cancer patients are free of disease signs after treatment work for a new treatment method. The new method combines chemotherapy, radiation, and immune-boosting drugs before surgery. We know these usual ways work for the standard treatment, but we're not sure if they're good enough for this new combo treatment. We'll look at patients with a specific type of rectal cancer that doesn't respond well to just immune-boosting drugs alone. By comparing how patients treated with the new method and the old method respond, we hope to find better ways to tell if the treatment is really working, which could help improve treatment plans for these patients.

Inclusion Criteria:

1. histopathologically confirmed adenocarcinoma;
2. pMMR/MSS tumors;
3. clinical staging of T3-T4 or any lymph node-positive T (N+);
4. no evidence of distant metastasis based on radiological examinations;
5. treatment with long-course chemoradiotherapy (LCRT) with or without immune checkpoint inhibitors (ICI);
6. post-neoadjuvant therapy evaluation with endoscopy and pelvic MRI.

Exclusion Criteria:

1. Presence of other active malignancies;
2. Having severe comorbidities, such as heart failure, severe hepatic or renal insufficiency, etc;
3. Lack of imaging and endoscopic examinations;
4. Treatment plan does not meet the study requirements.

This retrospective study evaluated medical records, endoscopic findings, and MRI images of rectal cancer patients who underwent neoadjuvant chemoradiotherapy (with or without immunotherapy) followed by surgery at Peking University Cancer Hospital between January 2019 and October 2024.