Inclusion Criteria:  From February 2021 to August 2022, patients with locally advanced rectal cancer were treated with neoadjuvant chemoradiotherapy in the Third Hospital of Peking University. There were blood routine before radiotherapy, 1 week, 2 weeks and 90 days after radiotherapy, and MRI examination including routine MRI sequence and IDEAL-IQ sequence before radiotherapy, 1-2 weeks and 90 days after radiotherapy. Each patient used the same chemotherapy regimen.  Exclusion Criteria:  Patients with metabolic bone diseases such as hyperparathyroidism and Cushing's syndrome. Patients with history of pelvic radiotherapy or systemic chemotherapy. Patients with leukocytes below 4.0×10^9/L or long-term severe anemia before radiotherapy. Patients with tumor bone metastases; patients with pelvic trauma, fracture, and infectious diseases.