Background: The dynamic advancement of pediatric hemato-oncology and intensified therapeutic protocols have significantly increased survival rates while simultaneously highlighting the challenge of long-term treatment complications. Within the cohort of adolescent and young adult (AYA) survivors, delayed neurocognitive deficits, often manifesting as the chemobrain phenotype, and psychosocial disorders constitute a particularly substantial burden. Contemporary neurogastroenterological evidence indicates a fundamental role of persistent dysbiosis and gut-brain axis dysfunction in the pathogenesis of these alterations. This review aims to critically synthesize translational evidence elucidating the impact of iatrogenic gut microbiota damage and modifiable dietary patterns on the development of long-term neurocognitive sequelae in survivors of early childhood cancer. Methods: A systematized narrative review was conducted in accordance with the SANRA guidelines by integrating data from in vivo models and observational studies. A comprehensive literature search across the PubMed, Embase, Cochrane Central, Scopus, and Web of Science databases up to June 2026 was performed utilizing the Population, Exposure, and Outcomes (PEO) framework. Results: Oncological therapies, including myeloablative conditioning and broad-spectrum antibiotic therapy, induce a microbial scar phenomenon characterized by the depletion of commensal Firmicutes in favor of resistant pathobionts. The subsequent decline in the synthesis of neuroprotective short-chain fatty acids (SCFAs) alongside the pathological activation of the kynurenine pathway disrupts central nervous system homeostasis. Translocation of lipopolysaccharides (LPSs) across the compromised intestinal barrier generates systemic inflammation recognized as inflammaging, which, in turn, stimulates neurotoxic microglial hyperreactivity. This pathophysiological cascade is accelerated by a pro-inflammatory Western diet, whereas anti-inflammatory interventions such as the MIND diet and postbiotics demonstrate measurable restorative potential. Conclusions: The pathophysiology of delayed neurotoxicity is largely a consequence of systemic neuroinflammation driven by intestinal dysbiosis. Implementing individualized dietary and microbiome-targeted strategies into survivorship care protocols constitutes a crucial direction for clinical prophylaxis. Validating their clinical efficacy in the AYA population necessitates prospective randomized controlled trials integrated with shotgun metagenomic sequencing.
