Washington University School of Medicine published an article in the journal, Gut, showing that taking a probiotic before radiation can protect the intestine from damage in mice. There is no reason that this would be different in humans. This suggests that taking a probiotic with Lactobacillus rhamnosus GG and other strains protects the gut when taken prior to irradiation, not after. The mechanism of action is suspected to be related to the cyclooxygenase (COX 1 and COX 2) system that regulated inflammation and cell growth.
Using additional biochemical gut support also makes perfect sense, but this article did not suggest this. I'd recommend using l-glutamine, general nutritional support for the gut, S. boulardii (to prevent yeast overgrowth), essential fatty acids (to support gut repair), and small fish peptides (which help prevent the leaky gut syndrome).
The physiology of the gut microecology are reviewed and highlight the importance of maintaining health of the 1000 different microbes that live in harmony in the normal human gut. The functions of normal gut flora include metabolism of hormones, carcinogens, and toxins, synthesis of vitamins such as vitamin K2, B5, B6 and biotin, synthesis of butyrate (the most important fuel of the colon), crowding out of pathogens such as c. diff and salmonella, and stimulation of the gut immune system.