Quiz. What is your diagnosis?

An 11 month old female presented to the pediatrician with an increased head circumference. Head CT was reviewed and subsequent MRI revealed a heterogeneous enhancing mass involving the medial aspect of both the right and left cerebellar hemispheres (Fig. 1). The mass extended through the posterior foramen magnum and effaced the 4th ventricle. The tumor was resected and appeared as a tan to white, firm, nodular mass measuring 4.2 × 1.8 × 0.7 cm in aggregate. An intraoperative smear and frozen section was performed. Microscopic sections were remarkable for a proliferation of small round blue cells with a pale, largely nodular architecture on HE (Fig. 2). Within the nodules, the neoplastic cells were smaller and streamed through the neuropil-like tissue. The majority of these pale nodular areas were Reticulin free (Fig. 3). Intra-nodular areas demonstrated prominent neuronal differentiation with Neu-N (not shown). The Ki-67 proliferation index was low in the intra-nodular areas (< 1%) and was high in the inter-nodular areas (approximately 30%) as shown in Fig. 4.