Umbilical Cord Blood Infusion to Promote Hematopoietic and Immune Reconstitution in Children With Relapsed or Refractory Malignant Solid Tumors

Pediatric malignant solid tumors (including neuroblastoma, Wilms tumor, hepatoblastoma, rhabdomyosarcoma, osteosarcoma, Ewing sarcoma, and germ cell tumors) account for approximately 60% of childhood malignancies. Despite multimodal therapy, children with relapsed or refractory disease have poor outcomes. Repeated lines of intensive salvage chemotherapy cause cumulative damage to the hematopoietic system, resulting in profound and prolonged myelosuppression (absolute neutrophil count \<0.5×10\^9/L or platelet count \<20×10\^9/L) that often fails to recover despite standard supportive care with short-acting recombinant human granulocyte colony-stimulating factor (rhG-CSF) and recombinant human thrombopoietin (rhTPO). This leads to chemotherapy delays that compromise treatment continuity and intensity. Umbilical cord blood (UCB) contains hematopoietic progenitor cells, NK cells, T lymphocytes, and various hematopoietic and immunomodulatory factors. UCB infusion has been shown to accelerate hematopoietic recovery after chemotherapy and may enhance anti-tumor immune function. This is a single-center, single-arm, prospective exploratory study. Approximately 10 children (aged 1-18 years) with relapsed or refractory malignant solid tumors who develop severe post-chemotherapy myelosuppression with delayed hematopoietic recovery (persistent ANC \<1.0×10\^9/L and platelets \<50×10\^9/L despite ≥7 days of rhG-CSF/rhTPO support, resulting in chemotherapy delay) will receive UCB infusion in addition to continuing background anti-tumor therapy and standardized hematopoietic supportive care. Each patient may receive up to 3 UCB infusions. The follow-up period is 12 weeks from the first infusion. The study aims to: (1) evaluate the effect of UCB infusion on promoting hematopoietic recovery and on-schedule chemotherapy completion (compared with institutional historical controls); (2) evaluate immune reconstitution after UCB infusion, including dynamics of T-lymphocyte subsets, NK