The NOA-26-Trial is a phase 1 study and is investigating a special form of administration of the PD-1 antibody nivolumab in the treatment of meningeosis neoplastica. In the study, the drug is administered directly into the cerebrospinal fluid. The investigational drug nivolumab is a PD-1 antibody that blocks the PD-1 receptors of T cells, which leads to activation of the T cells and promotes antitumor activity. The drug has been approved for intravenous administration for many solid tumors. Patients with meningeosis neoplastica can be included in the NOA-26 study. The underlying solid tumor should either have an increased mutational burden and/or be approved in the EU for intravenous treatment with nivolumab, pembrolizumab or atezolizumab.