Simple SummaryMYCis a well-described oncogene across multiple cancer types. In medulloblastoma (MB), significance is subtype-dependent and associated with a particularly dismal outcome inMYC-amplified group 3 MBs. In our study, we established a highly sensitive and specific method for the detection ofMYCamplification in cerebrospinal fluid (CSF) enabling its use as a liquid biopsy biomarker. We show preliminary results of its potential as a marker for early diagnosis, disease staging and monitoring. The fast and cost-effective method could substantially improve means of diagnosis and therapy monitoring in high-risk MBs.AbstractBackground: Liquid biopsy diagnostic methods are an emerging complementary tool to imaging and pathology techniques across various cancer types. However, there is still no established method for the detection of molecular alterations and disease monitoring in MB, the most common malignant CNS tumor in the pediatric population. In the presented study, we investigated droplet digital polymerase chain reaction (ddPCR) as a highly sensitive method for the detection ofMYCamplification in bodily fluids of group 3 MB patients. Methods: We identified a cohort of fiveMYC-amplified MBs by methylation array and FISH. Predesigned and wet-lab validated probes for ddPCR were used to establish the detection method and were validated in twoMYC-amplified MB cell lines as well as tumor tissue of theMYC-amplified cohort. Finally, a total of 49 longitudinal CSF samples were analyzed at multiple timepoints during the course of the disease. Results: Detection ofMYCamplification by ddPCR in CSF showed a sensitivity and specificity of 90% and 100%, respectively. We observed a steep increase in amplification rate (AR) at disease progression in 3/5 cases. ddPCR was proven to be more sensitive than cytology for the detection of residual disease. In contrast to CSF,MYCamplification was not detectable by ddPCR in blood samples. Conclusions: ddPCR proves to be a sensitive and specific method for the detection ofMYCamplification in the CSF of MB patients. These results warrant implementation of liquid biopsy in future prospective clinical trials to validate the potential for improved diagnosis, disease staging and monitoring.