177Lu-DTPA-Omburtamab Radioimmunotherapy for Recurrent or Refractory Medulloblastoma

Inclusion Criteria: * Histologically confirmed diagnosis of medulloblastoma. * SHH, Group 3, or Group 4 according to World Health Organisation (WHO) 2016 classification. * Recurrent (maximum of 2 recurrences for Part 1 and 1 recurrence for Part 2) or refractory to frontline therapy. Prior frontline or second line therapy may involve surgery, craniospinal irradiation, stereotactic radiosurgery, and multi-agent chemotherapy regimens. * Have refractory disease, focal or multifocal recurrent disease, or pure leptomeningeal disease. Cytological or radiographic remission is allowed; however, not simultaneously. * Performance status score of 50 to 100 on Lansky (less than 16 years) or Karnofsky (16 years or older) scales. * Life expectancy of at least 3 months, as judged by the Investigator. * Acceptable hematological status and liver and kidney function. Exclusion Criteria: * Obstructive or symptomatic communicating hydrocephalus as determined by Ommaya patency/cerebrospinal fluid (CSF) flow study. * Residual disease (nodular or linear) measuring \> 15 mm in the smallest diameter. * Ventriculoperitoneal shunts without programmable valves. Ventriculo-atrial or ventriculo-pleural shunts. * Grade 4 nervous system disorder. Stable neurological deficits (due to brain tumor or surgery) or hearing loss are allowed. * Uncontrolled life-threatening infection. * Received radiation therapy less than 3 weeks prior to the screening visit. * Received systemic or intrathecal cytotoxic chemotherapy or intrathecal immunotherapy (corticosteroids not included) less than 3 weeks prior to the screening visit. * Received any prior anti-B7-H3 treatment. * Non-hematologic organ toxicity Grade 3 or above; specifically, any renal, cardiac, hepatic, pulmonary, and gastrointestinal system toxicity. * Other significant disease or condition that in the investigator's opinion would exclude the patient from the trial.