Intervention/type: Ex vivo manufacturing of tumor antigen–specific cytotoxic T lymphocytes (CTLs) using G-Rex bioreactors, a cellular immunotherapy (adoptive T-cell therapy). Mechanism of action: Polyclonal, TCR-mediated recognition of pediatric tumor–associated antigens with perforin/granzyme-dependent cytotoxicity, bioreactor expansion enriches early-memory T-cell phenotypes (greater persistence and activity in vivo). Targets: Pediatric solid tumor cells (e.g., neuroblastoma, osteosarcoma, nephroblastoma, sarcomas) expressing shared tumor-associated antigens presented via HLA class I. Pathways addressed: T-cell activation and cytotoxic effector pathways, memory differentiation, antigen presentation, and overcoming immunosuppressive tumor microenvironment with poor TIL infiltration. Study is observational, optimizing and standardizing CTL expansion for future clinical ACT.