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Background: 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 of MYC amplification in bodily fluids of group 3 MB patients. Methods: We identified a cohort of five MYC-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 two MYC-amplified MB cell lines as well as tumor tissue of the MYC-amplified cohort. Finally, a total of 49 longitudinal CSF samples were analyzed at multiple timepoints during the course of the disease. Results: Detection of MYC amplification 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, MYC amplification was not detectable by ddPCR in blood samples. Conclusions: ddPCR proves to be a sensitive and specific method for the detection of MYC amplification 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.

BACKGROUND: Malignant brain tumors carry a high risk for leptomeningeal dissemination, but the CSF compartment is often not affected by systemic therapy. Intraventricular therapy via an Ommaya reservoir is one possibility to increase the cytotoxic drug concentration in the CSF. Unfortunately, the number of drugs that can be administered directly into the CSF is limited. We report on our experience with topotecan administered via an Ommaya reservoir. PATIENTS AND METHODS: Between 2015 and 2021, 50 patients aged 1 to 22 years (mean and median both 8 years) with various malignant brain tumors received intraventricular topotecan via an Ommaya reservoir. Topotecan was administered at 0.4mg twice a week (>1 and 2 and