The goal of the trial is to observe the changes of 68Ga FAPI signal before and after total neoadjuvant therapy for rectal cancers, and the correlation between the image parameters, immune checkpoints expression as well as the patient outcome. The trial will recruit patients with biopsy-confirmed rectal cancer aged 18 years old or older, with WHO/ECOG Performance Status 0-1, and eligible for total neoadjuvant therapy at the clinicians' discretion. After signing the informed consent, the participants will undergo a standard staging work-up if not already done, including colonoscopy and cross-sectional images such as CT, MR, and FDG-PET. Kidney function (by serum creatinine) and liver function (by serum alanine aminotransferase) will also be assessed. Only patients with stage II-III rectal cancer will be recruited. If patients meet the inclusion and exclusion criteria, they will undergo the first 68Ga-FAPI PET within 30 days before the beginning of total neoadjuvant therapy. At 22-24 weeks into the TNT, follow-ups for response evaluation will be conducted, including colonoscopy and cross-sectional images such as CT, MR, and FDG-PET. The second 68Ga-FAPI PET will be performed within one month of these exams. Afterward, participants will either undergo surgery or have image follow-ups every 3 months. The participants will be followed up for up to 2 years after the second 68Ga-FAPI PET, and immunochemical staining with CD47, CD73, PD-L1, and FAP on the biopsy or surgical specimens will be performed in one batch to avoid batch-to-batch variation.