Telpegfilgrastim vs Filgrastim for Secondary Prevention of Chemotherapy-Induced Neutropenia in Pediatric Solid Tumors

The goal of this clinical trial is to evaluate the efficacy and safety of Telpegfilgrastim (a PEGylated recombinant human granulocyte colony-stimulating factor, PEG-rhG-CSF) compared to Filgrastim (short-acting rhG-CSF) in preventing chemotherapy-induced neutropenia (CIN) in children and adolescents aged 6-24 years with malignant solid tumors receiving high-intensity chemotherapy regimens. The main questions it aims to answer are: * Does Tuopefilgrastim reduce the incidence of febrile neutropenia (FN) in the first chemotherapy cycle (Cx+1) compared to Filgrastim? * How do the two treatments compare in terms of duration and severity of neutropenia, chemotherapy delays/dose reductions, antibiotic use, and bone pain incidence? Researchers will compare the Telpegfilgrastim group (3:1 ratio, 99 participants) with the Filgrastim group (33 participants) to determine if Telpegfilgrastim demonstrates superior efficacy and safety. Participants will: * Receive subcutaneous injections of either Telpegfilgrastim (33 μg/kg, single dose) or Filgrastim (5 μg/kg/day, multiple doses) 24 hours after each chemotherapy cycle. * Undergo blood tests, physical exams, and temperature monitoring during follow-up visits. * Be assessed for bone pain severity using age-appropriate scales (FLACC or Wong-Baker). * Complete two chemotherapy cycles with close safety and efficacy monitoring.