This study included 79 with newly diagnosed medulloblastoma treated with craniospinal RT in order to evaluate  the effect of lymphopenia during RT. Eighty-three percent of patients (62/75) had grade ≥3 lymphopenia by RT Week 3, with 95% developing grade ≥3 lymphopenia at some point during therapy. Twenty-four of 79 (30%) patients developed disease recurrence. Lymphopenia during RT Weeks 4 and 5 correlates with increased risk of tumor recurrence. However, there was no difference in incidence of lymphopenia between those who received proton beam RT (93%) versus photon (97%).