In the present study, the investigators will evaluate the efficacy, toxicities, and of online adaptive radiation therapy (ART) for patients with rectal cancer treated with neoadjuvant chemoradiotherapy.

Inclusion Criteria:

1. ≥18 years old;
2. Diagnose with rectal cancer;
3. T3-T4 or N+, and no distant metastasis
4. Neoadjuvant chemoradiotherapy intended;
5. Surgery is expected after neoadjuvant therapy;
6. ECOG score 0-2, expected to lie in the treatment bed for at least half an hour;

Exclusion Criteria:

1. With contraindications to rectal MRI;
2. Received surgery, radiotherapy or chemotherapy for rectal cancer;
3. History of pelvic radiotherapy.