Objective: We compared the structure of specific white matter tracts and information processing speed between children treated for posterior fossa tumors with cranial-spinal radiation (n = 30), or with surgery +/− focal radiation (n = 29), and healthy children (n = 37). Method: Probabilistic diffusion tensor imaging (DTI) tractography was used to delineate the inferior longitudinal fasciculi, optic radiation, inferior frontal occipital fasciculi, and uncinate fasciculi bilaterally. Information processing speed was measured using the coding and symbol search subtests of the Wechsler Intelligence Scales, and visual matching, pair cancellation, and rapid picture naming subtests of the Woodcock-Johnson Test of Cognitive Ability, 3rd revision. We examined group differences using repeated measures MANOVAs and path analyses were used to test the relations between treatment, white matter structure of the tracts, and information processing speed. Results: DTI indices of the optic radiations, the inferior longitudinal fasciculi, and the inferior fronto-occipital fasciculi differed between children treated with cranial-spinal radiation and children treated with surgery +/− focal radiation, and healthy controls (p = .045). Children treated with cranial-spinal radiation also exhibited lower processing speed scores relative to healthy control subjects (p = .002). Notably, we observed that group differences in information processing speed were related to the structure of the right optic radiation (p = .002). Conclusion: We show that cranial-spinal radiation may have a negative impact on information processing speed via insult to the right optic radiations.