For a stage iii b colon cancer removed surgically, why the chemo should be systemic rather than regional although there are no metastasis?

Several reasons: In stage iii b there is metastasis to few lymph nodes , not to the adjacent organs, it is possible to have invisible dormant undetected tumor cells in the body , adjuvant chemotherapy is given to kill or eliminate these cells , regional chemotherapy through inf. Mesenteric vessels will damage normal organs , need systemic control of the disease.

Micrometastasis: The goal of adjuvant therapy is the eradication of occult micrometastatic tumor cells before metastatic disease becomes clinically evident. Undetected micrometastases can contribute to the failure of primary treatment. Adjuvant chemotherapy for stage iii colon cancer dramatically improves both relapse-free and overall survival.