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 fl uid 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 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 threshold 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.