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)