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Children and adolescents diagnosed with medullablastoma and with recurrent or refractory to frontline therapy will be treated with 177Lu-DTPA-omburtamab, which is a radioactive labelling of a murine monoclonal antibody targeting B7-H3.

Part 1 is a dose-escalation phase with a 3+3 sequential-group design in which patients will receive a dosimetry dose followed by maximum of two 5-week cycles of treatment doses of intracerebroventricular 177Lu-DTPA-omburtamab.

Part 2 is a cohort-expansion phase in which patients will receive a maximum of five 5-week cycles of intracerebroventricular 177Lu-DTPA-omburtamab at the recommended dose determined in Part 1.

End of treatment will take place within 5 weeks after the last cycle and thereafter the patients will be enter the follow-up period. The patients will be followed for up to 2 years after last dose.

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.