Research Objective and Principle: Through a randomized controlled study, evaluate the effectiveness of dexamethasone in improving radiation-induced rectal injury in rectal cancer patients undergoing pelvic radiotherapy, thereby providing evidence for treatment options in patients at risk of radiation-induced rectal injury and aiming for adoption in international guidelines.

Primary Objective: Improvement rate of radiation-induced rectal injury. Secondary Objectives: Severity of radiation-induced rectal injury, completion rate of pelvic radiotherapy, safety of dexamethasone enema, quality of life, pathological complete response (pCR) rate.

Study Design: Prospective, single-center, randomized controlled study. Study Population and Expected Enrollment: Patients with rectal cancer undergoing conventional pelvic radiotherapy, expecting to enroll 40 patients.

Trial Duration: From February 2026 to February 2028.

Intervention:

Experimental Group: Patients will receive enema (dexamethasone 1 mL + normal saline to total 30 mL) once daily from day 10 of radiotherapy until radiotherapy completion.

Control Group: Patients will receive enema (dexamethasone 1 mL + normal saline to total 30 mL) once daily from the occurrence of radiation-induced rectal injury until radiotherapy completion.

Statistical Hypothesis: Based on previous reports, the incidence of acute radiation-induced rectal injury is 86%, and it is expected that the experimental group can reduce it to 40%. The sample size was estimated using a formula designed to compare 2 proportions, with a set at 0.05 and a power of 80%. The study aimed to enroll at least 36 patients. Considering a dropout rate of 10%, at least 40 patients need to be included.

Inclusion Criteria:

1. Aged 18 to 75 years, regardless of gender;
2. Fully understand this study and voluntarily sign the informed consent form, able to comply with the study protocol and complete all trial procedures;
3. Patients pathologically diagnosed with rectal cancer;
4. Undergoing conventional pelvic radiotherapy (total dose of 40-50.4Gy in 25-28 fractions);
5. Eastern Cooperative Oncology Group (ECOG) score of 0-1, with an expected survival time of more than 6 months;
6. Assessment by the attending physician and researcher indicates that vital organ function can tolerate the treatment risks.

Exclusion Criteria:

1. Severe comorbidities, including uncontrolled stable medical diseases after treatment, or a history of neurological or psychiatric disorders (such as dementia or epilepsy), making them unsuitable for radiotherapy;
2. Patients who have received pelvic radiotherapy;
3. Presence of malignant pleural effusion or malignant abdominal effusion, or accompanied by bowel obstruction;
4. Pregnant or breastfeeding women;
5. Patients expected to undergo major surgery during the study period;
6. Participation in other clinical trials within 4 weeks prior to enrollment;
7. A history of alcohol abuse, drug use, or substance abuse within the past year;
8. Severe allergic constitution, or allergy to dexamethasone;
9. Inability to cooperate with the enema;
10. Subjects deemed unsuitable for participation in this trial for other reasons by the researcher.