Serial diffusion tensor imaging to characterize radiation-induced changes in normal-appearing white matter following radiotherapy in patients with adult low-grade gliomas

Purpose The aim of this study was to ascertain whether diffusion tensor imaging (DTI) metrics—fractional anisotropy (FA), mean diffusivity (MD), linear case (CL), planar case (CP), spherical case (CS)—can characterize a threshold dose and temporal evolution of changes in normal-appearing white matter (NAWM) of adults with low-grade gliomas (LGGs) treated with radiation therapy (RT). Methods and materials Conventional and DTI imaging were performed before RT in 5 patients and subsequently, on average, at 3 months (n = 5), 8 months (n = 3), and 14 months (n = 5) following RT for a total of 18 examinations. Isodose distribution at 5-Gy intervals were visualized in all the slices of fluid attenuated inversion recovery (FLAIR) and the corresponding DTI images without diffusion sensitization (b0DTI). The latter were exported for relative quantitative analysis. Results Compared to pre-RT values, FA and CL decreased, whereas CS increased at 3 and 8 months and recovered partially at 14 months for the dose bins > 55 Gy and 50–55 Gy. For the 45–50 Gy bin, the FA and CL decreased with an increase in CS at 3 months; no further change was seen at 8 or 14 months. For the >55 Gy and 50–55 Gy bins, CP decreased and MD increased at 3 months and returned to baseline at 8 months following RT. Conclusion Radiation-induced changes in NAWM can be detected at 3 months after RT, with changes in FA, CL, and CS (but not CP or MD) values seen at a thresh-old dose of 45–50 Gy. A partial recovery was evident by 14 months to regions that received doses of 50–55 Gy and >55 Gy, thus providing an objective measure of radiation effect on NAWM.