Effects of stereotactic radiosurgery on an animal model of hippocampal epilepsy

Stereotactic radiosurgery has been shown in small clinical series to reduce or abolish seizures in patients with lesion-related or idiopathic epilepsy. The radiation dose necessary to eliminate epileptogenesis is unknown, and the histological and metabolic effects of radiosurgery remain undefined. We hypothesized that in a rat model of kainic acid-induced hippocampal epilepsy, radiosurgery could provide a significant reduction in seizure frequency while limiting biochemical and structural histological damage to the brain.

Kainic acid (8 g) was injected into the rat hippocampus using stereotactic targeting. Focal seizures so generated were identified with scalp and depth electroencephalography (EEG). Epileptic rats were randomized to a control group (n = 20) and to radiosurgery groups in which maximum doses of 20, 40, 60, or 100 Gy (8-9 animals per group) were administered. Over a 42-day period, seizure frequency was determined by direct observation for 8 hours per week. Scalp EEG was performed weekly in all animals. Magnetic resonance imaging (MRI) studies (T1- and T2-weighted water-proton and quantitative sodium images) were obtained on Days 7, 21, and 42.

As compared with the control group, treated animals showed significant reductions in the number of seizures during each successive week after 20-Gy radiosurgery (P = 0.01-0.002). When we combined the number of seizures observed in the latter half of the study (Weeks 4-6), we found a significant reduction in seizures after 20-Gy (P = 0.007), 40-Gy (P = 0.03), 60-Gy (P = 0.03), and 100-Gy (P = 0.03) radiosurgery as compared with control animals. Increasing doses of radiosurgery correlated with higher percentages of rats that became seizure-free by EEG criteria. MRI-determined total sodium concentration in the injected hippocampus was 49.8+/-3 mmol/L, compared with 42.8 mmol/L on the contralateral side (within normal limits). This significant increase in sodium concentration was present in control rats (because of the kainic acid) and did not change with increasing radiosurgery dose. No parenchymal effects from radiosurgery were identified after 20, 40, and 60 Gy, and only two rats had necrosis at 100 Gy. All animals showed hippocampal injury from kainic acid by proton MRI and histological examination.

In this rat hippocampal epilepsy model, stereotactic radiosurgery was followed by a significant dose-dependent reduction in the frequency of observed and EEG-defined seizures. These effects were not accompanied by increased radiation-induced structural or metabolic brain injury as assessed by proton and sodium MRI or histological examination. The role of radiosurgery as a new, nondestructive surgical therapy for idiopathic epilepsy warrants further investigation.