This study is a single-cohort study. Pediatric patients with advanced, relapsed/refractory hepatoblastoma who have previously failed first-line or higher systemic therapy will receive 2 cycles of treatment with sintilimab combined with lenvatinib and chemotherapy (irinotecan). Patients may discontinue treatment due to disease progression, death, intolerable toxicity, withdrawal of informed consent, initiation of new anti-tumor therapy, or other reasons specified in the protocol, whichever occurs first.
* Dose setting:
* Sintilimab: The recommended dose is 3 mg/kg (maximum dose not exceeding 200 mg per administration), administered by intravenous infusion (60±15 minutes) once daily on Day 1.
* Lenvatinib: 7 mg/m² (maximum 12 mg), taken orally once daily on Days 1-21.
* Chemotherapy (irinotecan hydrochloride injection):
* Irinotecan hydrochloride injection: 50 mg/m², administered by intravenous infusion once daily on Days 1-5.
* Each 3 weeks constitutes one cycle. For patients who discontinue the study drug without radiological progression, follow-up and tumor assessment will continue until progressive disease (PD) occurs, new anti-tumor therapy is initiated, or death.
* Tumor tissues and peripheral blood of patients will be collected for relevant tests.