This study involves testing if adding a cancer drug called irinotecan to a chemotherapy mix called FOLFOX, after a type of radiation therapy, can help treat advanced-stage rectal cancer better. FOLFOX is a standard treatment made up of drugs like fluorouracil, leucovorin, and oxaliplatin. This study compares FOLFOX with a different mix called FOLFIRINOX, which also includes irinotecan. These drugs aim to stop cancer cells from growing or spreading. In this trial, participants will receive one of two treatment plans after radiation: FOLFOX or FOLFIRINOX. They will undergo regular scans and tests, including CT and MRI scans, as well as biopsies. The goal is to see if the FOLFIRINOX treatment improves cancer response and helps preserve organs better than FOLFOX.

• Participants must be at least 18 years old and have specific types of rectal cancer.
• The study involves multiple visits for treatments and tests.
• Women of childbearing age must use effective birth control during the study.