MDB-90. Primary Leptomeningeal Medulloblastoma: A Single-Site Retrospective Study of 5 Cases

Abstract BACKGROUND Primary leptomeningeal medulloblastoma without mass is a rare entity with fewer than 10 reported paediatric cases. Establishing an accurate diagnosis is challenging and reported outcomes are very poor. METHODS We conducted a retrospective analysis of cases presenting to the Royal Children’s Hospital Melbourne between 2013 and 2023. RESULTS Between 2013 and 2023 five patients, all male, were identified, with an average age of 6.6 years 1.9 - 12.2 years. The presenting history was short in all cases (2-3 weeks) and all had symptoms of raised intracranial pressure. Diagnostic imaging (MRI brain and spine) showed non-specific leptomeningeal thickening and enhancement with no primary posterior fossa mass seen; interestingly only two imaging reports considered malignancy in the differential diagnosis. Surgical biopsy confirmed the diagnosis of medulloblastoma on histology; 3 anaplastic and 2 with classic histology. Subgrouping by IHC, available for 4 patients, showed non-WNT/non-SHH, with one reclassified as SHH by DNA methylation (performed in 2 patients). MYC was amplified in one patient (assessed by FISH in all tumours). Four patients were treated with craniospinal irradiation (36-39.6Gy with 18Gy boost), followed by chemotherapy as per SJMB12 stratum N3 (n=3) or HeadStart III (n=1). One case, an infant, was treated according to COG ACNS0334 with concurrent intraventricular topotecan, followed by maintenance with irinotecan/temozolomide/IVent topotecan. At follow up (average 54 months; 11 – 126 months), four children with residual disease on end of treatment MRI are alive without disease progression. One patient with classical SHH medulloblastoma and MYCN amplification progressed on treatment and died of disease 11 months after diagnosis. CONCLUSIONS Contrary to the historical literature, our case series suggests a more favourable outcome if this disease is recognised early. Further studies are warranted to better understand the molecular profile of these tumours.