TYMS and cancer: 5‐FU, an antimetabolite medication that act as a anti‐pyrimidine by impeding DNA synthesis by reticence of enzyme thymidylate synthase (TS),23 could induce and aggravate OM during CCRT,24, 25, 26 and would result in extension of treatment plan along with decrease in drug dosage, restriction on the success of cancer chemotherapy, suspension of the treatment, and finally a decrease in survival rate.25, 26