Reduced white matter diffusion in glioblastoma patients after radiotherapy with photons and protons

Introduction:

Radio(chemo)therapy is standard in the (adjuvant) treatment of glioblastoma. Inevitably, brain tissue surrounding the tumor bed or residual tumor is also irradiated, which may lead to acute and late side-effects. Diffusion-weighted imaging (DWI) with magnetic resonance imaging (MRI) has been shown to be a sensitive method to detect early changes in the cerebral white matter after radiation. The aim of this work was to assess possible changes in the mean diffusivity (MD) of the white matter after radio(chemo)therapy using DWI and to compare these effects between patients treated with proton and photon irradiation.
Patients & methods:
70 patients diagnosed with glioblastoma underwent adjuvant radio(chemo)therapy with protons (n=20) or photons (n=50). MRI follow-up examinations were performed at three-monthly intervals and were evaluated until 33 months after the end of therapy. For all time points, MD maps were calculated and normal appearing white matter was segmented in T1-weighted MR images. Relative white matter MD changes between baseline and all follow-up visits were calculated in different dose regions.
Results:
We observed a significant decrease of MD (mean -4,0%, range -0,8 ¬– -7,9%, p<0.05) in white matter in areas, in which a dose of more than 20 Gy had been applied. The MD reduction was progressing with dose and time after radio(chemo)therapy. In patients treated with photons, significant reductions in white matter in the whole brain (mean -2,3%, range -0,9 – -3,1%, p<0.05) were seen at all time points. In proton patients, conversely, MD did not change significantly (mean -0,5%, range 0,5 – -2,4%).
Conclusion:
The results show that irradiation leads to measurable changes in white matter and that treatment with protons reduces this effect due to a lower total dose in the surrounding white matter. Further investigations are needed to assess whether those MD changes correlate with known radiation induced side-effects.