White matter (WM) injury is considered as a major contributory factor of treatment-induced neurotoxicity prevalent among childhood cancer survivors who have undergone whole brain irradiation or chemotherapy (1). The identification of imaging markers of WM neurotoxicity associated to the brain tumor and its treatment is necessary to modify treatment strategies that may prevent or minimize brain injury. Diffusion tensor imaging (DTI) can be used to characterize microstructural changes in WM and the underlying pathology ; for example, decreased λ_// reflects axonal degeneration (2) whereas increased radial diffusivity (λ_⊥) was related to myelin damage. In this study, we aim to investigate the DTI measurement of white matter injury of childhood cancer survivors using Tract-based spatial statistics (TBSS) (3,4) analysis. Participants: Twenty-seven adult survivors of childhood brain tumors and twenty-six demographically matched healthy controls (mean age: 22.7 ± 4.5 vs. 22.9 ± 4.3, p>0.05) were enrolled in the study. Among the survivor group, the average age of diagnosis was 9.0±5.1 years old, and the average time between diagnosis and testing was 13.7±5.4 years. Survivors had tumors located in posterior fossa (n=21), temporal lobe (n=2), occipital lobe (n=1), hypothalamus (n=1) or third ventricle (n=2). Fourteen survivors were received radiotherapy (RT) with median radiation dose of 5400 cGY (range: 5040–5940 cGy). The control group (n = 27) consisted of age- and gender-matched neurotypical-control participants. The study was approved by the Institutional Review Board. MRI Data Acquisition: Anatomical MRI and DTI were performed on all participants using a 3T MRI scanner (Siemens Tim/Trio) and a 12-channel head coil. T1 weighted images (T1WI) were performed with TR/TE = 2250/3.98 ms, degree of flip angle = 9, inversion time = 850 ms, slice thickness = 1 mm, FOV =256 mm² and matrix of 256× 256. DTI were acquired by using single-shot spin EPI sequence with 30 gradient directions. The images were acquired with TR/TE = 7700/90 ms, flip angle = 90°, slice thickness=2mm, FOV=256mm², b-values =0 or 1,000 s/mm². Image Data Processing and Analysis: DTI were processed and analyzed using the FMRIB Software (FSL, 5.0.1, Oxford University, UK). TBSS was used to find differences in FA, axial diffusivity (λ_//) and radial diffusivity (λ_⊥) between survivors and controls throughout the white matter. A statistical threshold-free cluster enhancement (TFCE) analysis was implemented from randomize with a cluster P < 0.05 (with Family Wise Error Correction). The white matter tracts were smoothed with a 3 mm Gaussian kernel, while maintaining the maximum statistic values and limiting the extent of the spread to voxels in the white matter (defined as FA > 0.2). Then, specific ROIs were created using the mask of the clustered voxels to perform quantitative analyses of FA values for individual participants. Significant regions were defined by a cluster of voxels (n > 100). TBSS analysis indicates significant changes of FA, λ_// and λ_⊥ in survivors compared to controls in some major brain structures. Specifically, FA and λ_// were significantly lower in white matter tracts of survivor group (orange and red area) (Figure 1). In contrast, significantly higher λ_⊥ was found in survivor’s white matter tracts (blue area) (Figure 1). (2) ROI analysis showed lower FA in survivors was found in the anterior corpus callosum, bilateral frontal medial, frontal pole, and middle temporal regions, also the left superior frontal, right inferior frontal and right frontal orbital regions. However, a broad range of white matter tracts injury was revealed by λ_// including external and internal capsule (1.16±0.20 vs.1.40±0.17 μm²/ms, p<0.05) cortical spinal tract (1.39 ± 0.13 vs. 1.43 ± 0.25 μm²/ms, p<0.05). In regarding of λ⊥, only frontal pole and whole corpus callosum show significantly higher values (0.779 ± 0.068 vs 0.877± 0.054 μm²/ms, p<0.01) (Figure 2). While previous studies have shown that WM damage occurs in brain tumor patients treated with radiation and chemotherapy and reduction of FA can be used to measure the WM changes in this population, the current study demonstrated that λ_// is more sensitive than FA to detect microstructure changes in white matter (2) Since decreased λ_// is considered to be associated to the axonal degeneration and increased λ_⊥ is related to myelin damage, white matter injury in childhood cancer survivors may mostly involve in diffuse axonal degeneration. On the other hand, the myelin damage appears to be localized in corpus callosum. Although pathological evaluations are needed to correlate the DTI findings, our results indicate that combining analysis of FA, λ_// and λ_⊥ provides more sensitive and specific diagnostic information to assess white matter injury in the adult survivors of childhood brain tumors.