Emerging studies highlight the pivotal role of the microbiota in modulating cancer immunotherapy outcomes, particularly in the context of radiotherapy. Pilot research indicates that fecal microbiota transplantation (FMT) may be safe and tolerable in Parkinson’s disease patients, extending interest to neurological and oncology arenas. Understanding the gut-brain-tumor axis could unlock new therapeutic strategies that improve immune responses to cancer treatments by manipulating microbial communities impacting tumor microenvironments.