This single-center, phase II clinical trial investigates the diagnostic efficacy of Ga-68-FAPI-46 PET scans in detecting colorectal cancer (CRC) recurrence during post-treatment surveillance, particularly in patients with elevated serum carcinoembryonic antigen (CEA). The study compares the sensitivity, specificity, and accuracy of Ga-68-FAPI-46 PET with the conventional F-18-FDG PET scans, aiming to enhance recurrence detection in CRC patients who underwent curative surgery. Thirty participants will be enrolled and monitored through imaging studies and clinical follow-up.

Colorectal cancer (CRC) is one of the leading causes of cancer-related mortality, with a significant recurrence rate following curative surgery, particularly in stages II and III. Early detection of asymptomatic recurrence is critical for initiating salvage treatment. Serum CEA elevation is a marker for recurrence, necessitating advanced imaging tools for precise localization.

This study evaluates the use of Ga-68-FAPI-46, a novel imaging agent targeting fibroblast activation protein (FAP), which is overexpressed in the tumor stroma. Ga-68-FAPI-46 demonstrates higher tumor uptake and faster clearance from normal tissues compared to F-18-FDG, offering a potential diagnostic advantage. Participants will undergo both F-18-FDG PET and Ga-68-FAPI-46 PET scans, with lesion detection validated through pathology or clinical follow-up.

The trial aims to determine the diagnostic performance of Ga-68-FAPI-46 PET in terms of sensitivity, specificity, positive predictive value, and negative predictive value, using the McNemar test for statistical comparison. The study also emphasizes participant safety and data protection, adhering to Good Clinical Practice (GCP) guidelines.

Inclusion Criteria:

• Age ≥ 20 years old
• The surgical intervention for colorectal cancer aims for curative resection, with the goal of achieving a disease-free status that persists for six months or more, as assessed clinically and through imaging studies.
• Elevated serum CEA levels:

Exceeding the laboratory's normal range (5 ng/ml) or, but not surpassing 13 ng/ml; using the individual's post-surgery +/- adjuvant chemotherapy or targeted therapy minimum value as a baseline, a consecutive increase of more than 10% for two times or as determined by the attending physician suggests suspicion of recurrence.

• Already performed or scheduled for FDG PET scanning examination. (5) Able to lie flat for at least 30 minutes.
• Signing the subject consent form.
• ECOG grade 0-2.
• The timing of F-18-FDG usage in this trial follows the " 2023 - Colorectal Cancer Treatment Principles" of our institution.

Exclusion Criteria:

• pregnant woman and breastfeeding women
• severe renal impairment (eGRF< 30ml/min)
• Known or suspected allergy to radiopharmaceuticals
• Simultaneously or previously diagnosed with malignant tumors other than colorectal cancer.
• Inability to undergo the necessary PET scan procedure
• Refusal or unwillingness to sign the informed consent form
• Severe medical conditions (severe disabilities, mental disorders)