Radiological Spectrum of Leptomeningeal Medulloblastoma: A Case-Based Literature Review With an Additional Case Report.

Medulloblastomas are the most common malignant brain tumors in pediatric patients, typically arising from the cerebellar vermis within the posterior fossa. These neoplasms belong to the group of small round blue cell tumors and can be subdivided based on molecular profiling. While leptomeningeal spread is well recognized in advanced disease, primary leptomeningeal medulloblastoma (PLMB) without an identifiable intracranial mass is exceptionally uncommon and poses a diagnostic challenge due to overlap with infectious, inflammatory, and other neoplastic leptomeningeal conditions. Against this background, the recognition of atypical clinical presentations and subtle imaging findings is critical. In this setting, we report a 16-year-old boy who presented with progressive lower-extremity weakness. Brain magnetic resonance imaging (MRI) demonstrated cerebellar and parahippocampal diffusion restriction with minimal leptomeningeal enhancement and no discrete parenchymal mass. Spinal MRI revealed diffuse intradural-extramedullary nodules with widespread leptomeningeal involvement. Histopathology confirmed a desmoplastic/nodular medulloblastoma, non-wingless (WNT)/non-sonic hedgehog (SHH). When considered alongside the published literature, this case reflects the substantial variability in imaging findings, including inconsistent leptomeningeal enhancement, occasional diffusion restriction, and a high frequency of spinal metastases at presentation. Notably, clinical signs of intracranial hypertension, often anticipated in leptomeningeal disease, may be absent, further complicating timely diagnosis. By presenting this additional pediatric case and synthesizing current evidence, this report aims to refine the understanding of the radiological spectrum of non-mass-forming medulloblastoma and highlight the importance of recognizing subtle neuroaxis abnormalities suggestive of this rare entity.