In this prospective, open, multicenter, multi-arm, observational controlled study, the investigators investigated the impact of different pelvic radiation doses of anal cancer and rectal cancer patients on the dynamic changes of anal function, and analyzed the correlation between the changes of anal function and the score of anal incontinence in the radiation dose≤50Gy group and >50Gy group. The study population includes 124 patients with anal canal cancer or rectal cancer who have received sphincter preservation treatments (including radiochemotherapy), aged from 18 to 75 years old undergoing sphincter preservation treatments at The Sixth Affiliated Hospital, Sun Yat-sen University and Nanfang Hospital, Southern Medical University. The primary outcome is the average resting pressure of anal sphincter. The secondary outcomes are maximum anal sphincter contraction pressure and Wexner continence score. Evaluations will be carried out at the following stages: baseline (T1), after radiotherapy or chemotherapy (before surgery, T2), after surgery (before closing the temporary stoma, T3), and at follow-up visits (every 3 to 6 months, T4, T5……). Follow-up for each patient will be at least 2 years.

Inclusion criteria 18-75 years of age. Anal cancer and rectal cancer with pathological diagnosis. Patient will undergo sphincter-preserving therapies. Eastern Cooperative Oncology Group (ECOG) score for performance status is 0-2. Written informed consent. Exclusion criteria Patients who have undergone pelvic surgery, such as rectal cancer surgery and gynaecological procedures. Patients who have received pelvic radiotherapy. Patients with other active malignant tumors. Patients with a history of prior peri-anal abscess, anorectal trauma, and inflammatory bowel disease (IBD).

Recently (less than 4 weeks) received surgery or patients with recent severe trauma.

Significant cardiac disease: congestive heart failure of New York Heart Association class ≥2; patients with recent (less than 12 months) active coronary artery disease (unstable angina or myocardial infarction).

Recent (less than 6 months) thrombosis or embolism events, such as cerebrovascular accident (including transient ischemic attack), pulmonary embolism and deep vein thrombosis.

Patients with toxicity (Common Terminology Criteria for Adverse Events (CTCAE) Grade ≥ 2) caused by previous treatment that has not subsided.

Women who suffered anal sphincter tear after vaginal delivery. Pregnant or lactating women.

Patients diagnosed with anal and rectal cancer at The Sixth Affiliated Hospital, Sun Yat-sen University and Nanfang Hospital, Southern Medical University will be assessed for suitability for inclusion.