Brain modifications after stereotactic radiotherapy recorded by Functional MRI.

Purpose or Objective
Brain irradiation is commonly used in malignant diseases (i.e. metastases or Glioblastoma) and in benign diseases (i.e. meningioma, epilepsy, vestibular schwannoma or Parkinson disease). The use of stereotactic radiosurgery (SRS) allows the administration of very high doses in a single fraction (e.g. 120Gy), in a small brain volume. After irradiation, morphological and functional cerebral changes occur depending on the total dose, dose per fraction and the irradiated brain volume. The aim of this work is to use f-MRI to record adult normal brain tissue modification after irradiation with different radiotherapy doses and schedules and to identify new parameters of brain radio-damages.
Material and Methods
With a dedicated small animal radiotherapy device allowing IGRT (PXI, X-Rad SmART), we specifically irradiated with a 2mm-collimator, mimicking SRS, a small part of adult brain mice (n=72), known to have no impact on vital function, with dose schedules: 1X20Gy, 3X10Gy, 4X5Gy and no RT as control. We imaged brain mice longitudinally with a dedicated 9.4-T MRI (Agilent). Imaging was realized once before as reference level and after irradiation every month for the first 6 months and every 3 months during one year. For each mouse we acquired 14 slices of 1 mm thickness and 0.5 mm gap with an “in plane voxel resolution” of 0.5 mm. We performed T1-weighted, T2-weighted, T1-mapping, T2-mapping and DW-MRI. For DW-MRI, we performed Fast Spin Echo MultiSlice sequences, with 9 different B-value and B0 (from 20 to 1000). We performed IntraVoxel Incoherent Motion (IVIM) analysis to obtain information on intravascular diffusion, related to perfusion (F: perfusion factor).
Results
Only mice irradiated with 120Gy showed brain modifications in T1 and T2 anatomic images and in T1 mapping, ADC, D and F but no changes were recorded in D* or T2 mapping. All these changes started 5 weeks after SRS and then stabilized after 7 weeks. The mean values for the control group were stable during the 5 months (ADC 0,73μm²/ms; D 0,66μm²/ms; F 4,67%, T1 1,25 sec). For the 120Gy group, values were significantly higher after 5 weeks (Δ = compared to the control group) with ADC 1,66μm²/ms (Δ=151%); D 1,37μm²/ms (Δ=107%); F 18,84% (Δ=303%); T1 1,99 sec (Δ=59%). No specific behaviour changes were observed during all the experiment.
Conclusion
In this work, we studied normal brain modifications after SRS therapy with anatomical and functional MRI. SRS doses and schedules in this work reflected those used in clinic for tumor treatment or functional SRS. We showed an increase of ADC value 5 weeks after one single dose of 120Gy, compared to normal brain tissue. These results are consistent with radio-necrosis. In addition, we highlighted an increase of IVIM parameters D and F and an increase of T1 mapping in radio-necrosis area. These results increase the numbers of MRI parameters that could be used for following brain damage after radiation.