High-risk neuroblastoma accounts for 15% of cancer related-deaths in children. Half of the >1500 patients yearly diagnosed with neuroblastoma in the EU have high-risk disease, which will relapse or progress in half these cases after first-line treatment. Relapsed neuroblastoma is aggressive and often therapy-resistant. Monitoring for disease relapse and therapy response is crucial for thesurvival chance of these patients. The current standard-of-care for monitoring are imaging technologies and bone marrow assessment, which are costly, invasive and a burden for children, who often require anesthesia. These drawbacks limit how often is monitored. More sensitive, less invasive and less toxic monitoring techniques are needed. The mutational spectrum often changes inrecurring tumors, which may explain therapy resistance and provide additional druggable targets. Imaging, however, provides no information about molecular characteristics. Liquid biopsy tests are minimally invasive, allow frequent sampling and sensitively detect tumor molecular markers in tumor-derived DNA and messenger RNA circulating in peripheral blood. MONALISA aims to closeexisting gaps and establish liquid biopsies as standard-of-care to monitor relapsed/refractory neuroblastoma, as a blueprint for other pediatric cancers. Reliable, early assessment of molecular progression or relapse is the main aim of the pragmatic randomized clinical trial proposed in MONALISA. We develop a digital decision support tool to help oncologists use the new monitoring and apply patient-reported outcomes to integrate patient viewpoints and assess the effect of minimally invasive, liquid biopsy diagnostics on quality of life. We will establish whether events can be detected earlier using liquid biopsy monitoring, and whether better overall survival is enabled by earlier diagnosis and treatment interventions. This essential step towards personalized medicine will support reliable disease monitoring under treatment.