Thanks for the question!There is evidence that radiation therapy leads to changes in the gut microbiome. One study found a decrease in bacterial diversity and a few specific bacteria in cancer patients following radiation therapy. In addition, another study found that patients who developed diarrhea after radiation had a different microbiome composition prior to beginning treatment. This suggests that some the microbiome may be protective against the development of side effects to radiation.

Several randomized clinical trials have been performed to determine if probiotics prior to radiation can provide the microbiome-mediated protective effects to adverse side effects. The results of these trials are inconsistent due to the use of different probiotic mixtures. In the trials that reported a reduced incidence of diarrhea, the probiotics consisted of Lactobacillus acidophilus, VSL#3 (which consists of 8 different probiotic bacteria), or a combination of Lactobacillus acidophilus with Bifidobacterium bifidum (https://pubmed.ncbi.nlm.nih.gov/25040088/).

-RM