To investigate effects of prophylactic cranial irradiation (PCI: 25 Gy in 10 fractions) on MRI of the brain. PCI is routine clinical practice for patients with small cell lung cancer who respond to chemotherapy and thoracic radiotherapy; however effects have been shown on fractional anisotropy (FA) in normal-appearing white matter (NAWM), suggested to be due to demyelination¹. We evaluated the effects of PCI on diffusion and the magnetization transfer ratio (MTR) within ROIs drawn over 10 major white matter areas (pons, medulla, and bilateral cerebellum, corona radiata, and frontal and parietal periventricular white matter), with the hypothesis that after PCI there might be increases in apparent diffusion coefficient (ADC) and decreases in FA and MTR. We also analysed the entire white matter tracts using TBSS (Tract-Based Spatial Statistics)².Six patients with small cell lung cancer were investigated using a 3.0T MRI scanner (MR750, GE Healthcare, Waukesha, WI) at approximately 4–month intervals: at diagnosis, following carboplatin and etoposide chemotherapy, and following PCI. Scans included diffusion tensor imaging (axial EPI, TE/TR = 100/5960ms, 128x128, FOV 22cm, slice thickness/spacing = 5/1mm, b=1000 s/mm2 along 25 directions) and magnetization transfer (axial 3D fast spoiled gradient echo, 60 x 3mm slices, TE/TR = 1.1/28.5ms, FOV 22cm, 256 x 160) with and without a 8ms Fermi pulse (offset 2200 Hz, flip angle 450°). Average values of FA, ADC, and MTR in the above 10 regions of NAWM were measured in ImageJ (Bethesda, MD), and the variation over time in each region was assessed using Repeated Measures ANOVA followed by post-hoc 2-tailed t-tests performed between pairs of sessions using Graphpad Prism (San Diego, CA). For TBSS, FA images were registered using FSL nonlinear registration (FNIRT). A mean FA image was then created and skeletonised, and voxel-wise cross-subject statistics were evaluated. The 4D projected FA data was fed into GLM modelling and thresholding to find voxels that correlated with the hypothesis of alterations post-radiotherapy using a paired t test. The pixels that differed significantly were projected onto the mean FA skeleton, dilated for clarity, coded red where post-radiotherapy FA was reduced and blue where it was elevated. Clusters were evaluated for significance, corrected for multiple comparisons.Repeated measures ANOVA of the 3 parameters in the 10 ROIs showed significance only for ADC in the right frontal white matter (p=0.03). Since FA and MTR in this ROI also neared significance (p=0.18 and 0.14 respectively), paired t-tests between sessions were investigated for all 3 parameters (Table 1; Fig. 1). These showed a significant difference between session B and C in agreement with the hypothesis: the session after PCI showed higher ADC, lower FA and lower MTR. The pre-treatment measurements (session A) showed no significant difference with either of the other sessions for any of the 3 parameters. The TBSS analysis with a threshold of p<0.05 found no significant clusters of altered FA following PCI. Reducing the threshold to p<0.45 did show several clusters where the FA was reduced but none where FA was increased (Fig. 2).It has been shown that high-dose radiotherapy can have a negative impact on the brain, affecting both cognition and MR measures of white matter integrity³. Effects of lower-dose PCI are as expected less marked; however, a trend toward reduction in MTR and FA and increase in ADC were seen in the current study. These appeared significant on paired t-tests pre- and post-PCI in a frontal white matter ROI, although only the ADC measurement had been significant on repeated-measures ANOVA including the data at presentation. It is possible that the chemotherapy may have affected the brain: although the differences between sessions before and after chemotherapy were not significant, it is unclear whether the small fluctuations seen were due to variable effects of chemotherapy on the brain, random physiological variation, or to limited technical reproducibility. It would be desirable to have a 2nd session before treatment initiation, to better differentiate technical or short-term physiological variability from treatment effects. Interpretation of the ROI data is also complicated by the multiple comparisons involved. The whole-brain TBSS approach, which takes this statistical aspect into account, confirmed the qualitative trend of FA reduction following PCI, but failed to find significant clusters.Following PCI, we observed a small increase in ADC and reduction in FA and MTR in frontal white matter, and a trend toward reduction of FA on TBSS. Future work should ideally include two measurements pre-treatment to distinguish between physiological or technical variability and the subtle effects of PCI and chemotherapy.