PD-L1 PET Imaging During Neoadjuvant (Chemo)Radiotherapy in Esophageal and Rectal Cancer

This imaging study tests a PET scan using a special tracer that targets a protein called PD-L1 — a key part of the immune system's interaction with cancer — to track how tumors in the rectum or esophagus respond to pre-surgery radiation and chemotherapy. **You may be eligible if...** - You are 18 or older with confirmed rectal or esophageal cancer (any sex) - You are scheduled to receive pre-surgery chemoradiation or short-course radiation - You have not had previous surgery on the tumor (biopsy is fine) - You are able to provide written consent **You may NOT be eligible if...** - Your cancer has spread in a way that cannot be treated with curative intent - You are pregnant or breastfeeding - You have previously received immunotherapy drugs (anti-PD-1, anti-PD-L1, anti-CTLA-4) - You have HIV, hepatitis B or C, or an immune deficiency - You have an active autoimmune disease that required systemic treatment in the past 2 years - You had a solid organ transplant or are on immunosuppressant medications Talk to your doctor to see if this trial is right for you.