This study aims to evaluate the expression characteristics of PHGDH in tumor tissues based on the clinical data and pathological tissue samples of colorectal cancer patients who have previously received immunotherapy, and to analyze its correlation with patients' clinical features, immunotherapy efficacy, and survival prognosis, providing a basis for clarifying the clinical relevance of PHGDH.

Inclusion Criteria:

1. Patients with histologically confirmed colorectal adenocarcinoma.
2. Initiated treatment with immune checkpoint inhibitors (at least one cycle) between June 1, 2018 and December 31, 2025.
3. Availability of pre-treatment archival tumor tissue (FFPE block or slides) for PHGDH immunohistochemistry testing.
4. Have evaluable baseline radiographic images (CT or MRI) prior to immunotherapy.
5. Sufficient medical records for survival follow-up data extraction.

Exclusion Criteria:

1. Unavailability of adequate tumor tissue for PHGDH expression analysis.
2. Permanently discontinued immunotherapy prior to the first scheduled radiological response assessment.
3. History of other active malignancies within the past 5 years (except for adequately treated basal cell carcinoma of the skin or carcinoma in situ of the cervix).
4. Insufficient clinical documentation to determine the primary outcome (e.g., progression-free survival).

This is a single-center, retrospective cohort study. The study population consists of patients with histologically confirmed colorectal adenocarcinoma who initiated immune checkpoint inhibitor therapy at [Your Hospital Name] between June 2018 and December 2025. Participants will be categorized into PHGDH-high and PHGDH-low groups based on the expression level of PHGDH protein in pre-treatment tumor tissue (assessed by immunohistochemistry). The primary objective is to compare the effectiveness of immunotherapy (e.g., progression-free survival) between the two groups.