A Study of Response to Standard Treatment Before Surgery in People With Rectal Cancer

This study looks at how well standard treatments work before surgery in people with rectal cancer. Researchers will track tumor response using MRI scans and tumor biopsies to better understand which patients respond best to pre-surgery chemoradiation. **You may be eligible if...** - You are 18 or older with a biopsy-confirmed rectal cancer - Your cancer has no known mismatch repair deficiency or HER2 amplification - You are eligible for and planning to start standard chemoradiation (using capecitabine or 5-FU) with consolidation chemotherapy before surgery - You are not pregnant and agree to use effective contraception **You may NOT be eligible if...** - You have had prior radiation to the pelvis - You are pregnant or breastfeeding - You have another cancer that could interfere with treatment assessment - You take blood thinners that would make tumor biopsies unsafe - You have a contraindication to MRI Talk to your doctor to see if this trial is right for you.