Pelvic/abdominal radiotherapy carries a risk of complications. Acute complications include diarrhea, abdominal pain, inflammatory change in the small intestine. Radiation creates changes in bacterial microbiome, the vascular permeability of the mucosal cells and in intestinal motility. Probiotics were known to improve gastrointestinal function. This is a randomized, double-blind, placebo-controlled study involving 26 patients designed to evaluate the effect of probiotics to change the intestinal microbiome in in patients undergoing concurrent pelvic/abdominal RT.