Synchrotron X-ray microtransections: a non invasive approach for epileptic seizures arising from eloquent cortical areas

Abstract

Synchrotron-generated X-ray (SRX) microbeams deposit high radiation doses to submillimetric targets whilst minimizing irradiation of neighboring healthy tissue. We developed a new radiosurgical method which demonstrably transects cortical brain tissue without affecting adjacent regions. We made such image-guided SRX microtransections in the left somatosensory cortex in a rat model of generalized epilepsy using high radiation doses (820 Gy) in thin (200 μm) parallel slices of tissue. This procedure, targeting the brain volume from which seizures arose, altered the abnormal neuronal activities for at least 9 weeks, as evidenced by a decrease of seizure power and coherence between tissue slices in comparison to the contralateral cortex. The brain tissue located between transections stayed histologically normal, while the irradiated micro-slices remained devoid of myelin and neurons two months after irradiation. This pre-clinical proof of concept highlights the translational potential of non-invasive SRX transections for treating epilepsies that are not eligible for resective surgery.

Figure 1. Synchrotron X-ray transections.

Irradiation parameters and tissular effects. (a,b) 3D Microbeam interlacement; transparent arrows denote the trajectory of each microbeam (MB). Numbers represent the sequence of their delivery. The interlaced volume (transection) is shown in red. (c) X-ray image of a GAERS brain with two rows of 5 implanted electrodes. Transections are indicated by red bars. (d) The scan of a Gafchromic™ film shows 4 microbeams, interlaced to create a 200 μm wide, octahedral irradiated transection. (e) Radiation dose deposition profile for the 4 transections. Estimated peak dose in the transection 820 Gy, entrance dose of a single microbeam 800 Gy. (f) Gadolinium contrasted T₁ MR image of a rat after 4 transections. (g–t) Horizontal brain sections 2 months post-irradiation, haematoxylin & eosin (g), myelin stain (h); scale: 2 mm, white/grey matter limit is represented as a dashed line. Microphotographs of cortical transections (i,k) show a tissue gap (i), and myelin loss (k), while the path of a single microbeam in the contralateral hemisphere shows only loss of cell nuclei (j,l; scale: 200 μm). Immunolabelling of brain vasculature (RECA and Type IV Collagen, m,n) and neurons (NeuN, o,p) in transected (m,o) and contralateral (n,p) hemispheres (scale: 50 μm). Ultrastructural changes, as described in the text, within (q,s) and between transections (r,t). Within transection (q) extracellular space is filled with extra cellular matrix (ecm) secreted by fibroblasts (fib) and phagocytosis of dead cells by microglial cells (mic) are observed. Capillaries were intact (s) and the junctions (j) of endothelial cells (end) are closed but some vacuoles can be observed (j). Extracellular matrix and collagen fibrils (col) are observed around the capillary. Between transections (r,t), the endothelial cells of the capillaries were not damaged (r), nor the surrounding tissue, including synapses (syn) and axons (ax) (t). (TEM, magnification 12,000).

Figure 2. Pre- and post-irradiation records and statistics of seizures, seizure power, coherence and background signals.

(a) Average (n = 4) normalized time/frequency maps of seizure power (time rescaled: start = 0; end = 1) recorded from the 10 electrodes (black points), before (left panel) and 1 week post-irradiation (right panel). The 4 lower maps represent the average coherence computed from the left and the right electrodes. (b) Difference of power between pre- and post-irradiation seizures: the t values are color- coded. Mediolateral coordinates are displayed by the grid overlying the brain. Time/frequency maps are normalized against the background signal preceding the seizures (duration: 5% of the seizure length). Red bars indicate the location of the transections. (c) Example of seizures recorded from the two symmetrical electrodes implanted in the transected (left) and contralateral (right) somatosensory cortex barrel fields (S1BF2) before (upper grey seizures) and 1 week after (lower red and green seizures) the transections. (d) Quantification of the mean seizure normalized power (left panel) and coherence (right panel) for the 3 central left and right electrodes before irradiation and 1, 3, 9 weeks post-irradiation. Data are expressed as percent of the baseline power/coherence of the 6–8 Hz band (*p < 0.05; **p < 0.01; Two-way repeated measure Anova, Sidak post-hoc). (e) Average fast-Fourier transforms (FFT) of background signals recorded from the left (left panel) and right electrodes (right panel) before (black histograms) and 1 week following irradiation (colored histograms). FFT amplitudes are displayed as frequency bands (delta, theta, alpha, beta, low gamma, high gamma and epsilon). There was no significant statistical differences between pre- and post-irradiation for any band (Two-way repeated measure Anova, p > 0.05).