We thank Dr. Piper and colleagues for the opportunity to discuss the findings from our study. The data used in this study were collected during the U.S. Multicenter Pilot Study examining radiosurgery (RS) for patients with mesial temporal lobe epilepsy (MTLE) (Barbaro et al., 2009). The main focus of this study was to establish safety and side effects of RS as an alternative to conventional open surgery for epilepsy.
Because visual field defect correlated with seizure remission, we proposed that the dose of radiation delivered to the penumbra of the target was sufficiently severe to damage the primary surgical target including nearby optic radiations, a similar result to standard surgery.
As RS for MTLE becomes more prevalent, a study examining preoperative and postoperative tractography or diffusion tensor imaging, as suggested, would be of interest to determine the extent of optic radiation damage sustained during the procedure. Unfortunately, at the time of our study, diffusion tensor imaging was not routinely available among consortium members. Magnetic spectroscopy, however, was and, along with standard magnetic resonance imaging (MRI), demonstrated the following: that (1) larger transient radiation-induced changes were the main neuroimaging predictors of subsequent seizure remission, and (2) evidence of ischemic neuronal loss within the RS target was present (Chang et al., 2010). These findings support our main conjecture that the mechanism of RS involves at least some degree of radiodestruction, rather than purely neuromodulatory effects, as has been theorized (Quigg et al., 2012).