177Lu-DTPA-Omburtamab Radioimmunotherapy for Leptomeningeal Metastasis From Solid Tumors (Breast, NSCLC, Malignant Melanoma)

Inclusion Criteria: * Primary ductal or lobular breast cancer, non-small cell lung cancer, or malignant melanoma * Type I or Type II LM with a "confirmed" or "probable" diagnosis according to EANO-ESMO guidelines 2017 * Life expectancy more than 2 months, as judged by the Investigator * ECOG Performance status 0, 1, or 2 * Acceptable hematological status and liver and kidney function * Written informed consent obtained in accordance with local regulations * Presence of an intracerebroventricular access device before first dosing Exclusion Criteria: * Obstructive or symptomatic communicating hydrocephalus * Progressive systemic (extra-leptomeningeal) disease * Uncontrolled life-threatening infection * Ventriculo-peritoneal shunts without programmable valves. Ventriculo-atrial or ventriculo-pleural shunts * Received craniospinal irradiation (for intraparenchymal or dural metastases) or intrathecal cytotoxic anti-cancer therapy less than 3 weeks prior to first dose of 177Lu-DTPA-omburtamab * Severe non-hematologic organ toxicity; specifically, any renal, cardiac, hepatic, pulmonary, or gastrointestinal system toxicity Grade 3 or above prior to enrolment * Grade 4 nervous system disorder. Hearing loss or stable neurological deficits due to brain tumor are allowed * Unacceptable coagulation function prior to first dosing defined as INR Grade 2 or above * Female of childbearing potential, who are pregnant, breast-feeding, intend to become pregnant, or are not using highly effective contraceptive methods or male who is not using highly effective contraceptive method * Other significant disease or condition that in the investigator's opinion would exclude the patient from the trial. * Smallest diameter of treated or untreated nodular or linear leptomeningeal metastasis \>0.5 cm on MRI (Part 2 only)