Figure 6b. Medulloblastoma in a 3-year-old boy with a 1-month history of progressively worsening clumsiness, ataxia, headache, nausea, and vomiting. Developmental delay in speech and motor skills was also present. Papilledema was noted on physical examination. (a) Axial CT image shows a nearly homogeneous hyperattenuated mass in the posterior fossa midline. A thin crescent of the fourth ventricle (arrowheads) is noted along the anterior margin of the mass. (b) On an axial T1-weighted MR image, the mass is hypointense compared with the surrounding normal cerebellum. (c) On an axial T2-weighted MR image, the mass shows mild hyperintensity compared with surrounding normal brain tissue. (d) Contrast-enhanced axial T1-weighted MR image shows intense but mildly heterogeneous enhancement of the mass. (e) Photograph of the resected specimen highlights the soft friable nature of the mass, characteristic of a medulloblastoma.