PLE-5695

A 7-year-old boy presents with a 5-day history of gradually worsening headaches and intermittent vomiting, without fever. The patient was also noted to be walking with poor balance and coordination. An MRI was done, which revealed a cerebellar mass. The patient subsequently underwent excision of the tumor. Biopsy reveals that the tumor is extremely cellular, with sheets of anaplastic (small blue) cells. Individual tumor cells are small, with little cytoplasm and hyperchromatic nuclei. Mitotic figures are abundant. What is the diagnosis?