Adaptive Radiation for Locally Advanced Rectal Adenocarcinoma

Capecitabine 825 mg/m\^2 twice daily orally 5 days per week on days of planned radiation therapy.

Participants will receive varying doses of radiotherapy based on MRI guidance, beginning at 28-33 fractions (1.8 Gray units) .

Standard of Care FOLFOX regimen consists of 8 cycles (each cycle is 14 days) of 5-fluorouracil, leucovorin, and oxaliplatin. Dosage is as follows: Oxaliplatin 85 mg/m\^2 IV over 2 hours day 1 each cycle. Leucovorin (optional) 400 mg/m\^2 IV over 2 hours day 1 each cycle. 5-fluorouracil bolus\* (optional) 400 mg/m\^2 IV push day 1 of each cycle, and 5-fluorouracil infusion 2400 mg/m\^2 IV continuous infusion over 46 hours day 1 each cycle.