Lymphoblastic lymphoma: the CSF smears show many lymphoblasts, typical of meningeal involvement by this type of lymphoma/leukemia. (The boy had predominantly lymph node involvement.) TdT immunochemistry was strongly positive.
See:
Bigner SH, Johnston WW: The cytopathology of cerebrospinal fluid. I: Nonneoplastic conditions, lymphoma and leukemia. Acta Cytol 25:335-353, 1981.