Intrathecal Double Checkpoint Inhibition

Inclusion Criteria: * Newly diagnosed confirmed or probable leptomeningeal metastases according to European Association of Neuro-Oncology (EANO) - European Society for Medical oncology (ESMO) criteria (Le Rhun et al., 2017). * Histologically confirmed (from primary tumor or from a metastatic lesion, including in the brain) non-small cell lung cancer without actionable oncogenic driver mutation or melanoma. Programmed death-ligand 1 (PD-L1) expression status (from primary tumor or from a metastatic lesion, including brain) is optional, but should be documented if available * Requirements for patients with non-small cell lung cancer: non-small cell lung cancer without a specified targetable oncogenic driver alteration: sensitising Epidermal Growth Factor Receptor (EGFR) mutation (exon 19-del and 21-L858R), anaplastic lymphoma kinase (ALK) or ROS proto-oncogene-1 (ROS1) rearrangement. * Clinically eligible for systemic immunotherapy with nivolumab and ipilimumab at the time of enrolment as judged by the investigator. If already initiated, the systemic treatment must be well tolerated, without common terminology criteria for adverse events (CTCAE) grade 3 or more toxicity, and there must be no evidence of systemic progression and no indication for whole brain radiotherapy. Intrathecal immunotherapy alone may be acceptable for exceptional patients after discussion with the coordinators of the study. Systemic immunotherapy can be started later in these patients based on investi…