Research Article: Ablation dynamics during laser interstitial thermal therapy for mesiotemporal epilepsy

Date Published: July 6, 2018

Publisher: Public Library of Science

Author(s): Walter J. Jermakowicz, Iahn Cajigas, Lia Dan, Santiago Guerra, Samir Sur, Pierre-Francois D’Haese, Andres M. Kanner, Jonathan R. Jagid, Jonathan H Sherman.


The recent emergence of laser interstitial thermal therapy (LITT) as a frontline surgical tool in the management of brain tumors and epilepsy is a result of advances in MRI thermal imaging. A limitation to further improving LITT is the diversity of brain tissue thermoablative properties, which hinders our ability to predict LITT treatment-related effects. Utilizing the mesiotemporal lobe as a consistent anatomic model system, the goal of this study was to use intraoperative thermal damage estimate (TDE) maps to study short- and long-term effects of LITT and to identify preoperative variables that could be helpful in predicting tissue responses to thermal energy.

For 30 patients with mesiotemporal epilepsy treated with LITT at a single institution, intraoperative TDE maps and pre-, intra- and post-operative MRIs were co-registered in a common reference space using a deformable atlas. The spatial overlap of TDE maps with manually-traced immediate (post-ablation) and delayed (6-month) ablation zones was measured using the dice similarity coefficient (DSC). Then, motivated by simple heat-transfer models, ablation dynamics were quantified at amygdala and hippocampal head from TDE pixel time series fit by first order linear dynamics, permitting analysis of the thermal time constant (τ). The relationships of these measures to 16 independent variables derived from patient demographics, mesiotemporal anatomy, preoperative imaging characteristics and the surgical procedure were examined.

TDE maps closely overlapped immediate ablation borders but were significantly larger than the ablation cavities seen on delayed imaging, particularly at the amygdala and hippocampal head. The TDEs more accurately predicted delayed LITT effects in patients with smaller perihippocampal CSF spaces. Analyses of ablation dynamics from intraoperative TDE videos showed variable patterns of lesion progression after laser activation. Ablations tended to be slower for targets with increased preoperative T2 MRI signal and in close proximity to large, surrounding CSF spaces. In addition, greater laser energy was required to ablate mesial versus lateral mesiotemporal structures, an effect associated with laser trajectory and target contrast-enhanced T1 MRI signal.

Patient-specific variations in mesiotemporal anatomy and pathology may influence the thermal coagulation of these tissues. We speculate that by incorporating demographic and imaging data into predictive models we may eventually enhance the accuracy and precision with which LITT is delivered, improving outcomes and accelerating adoption of this novel tool.

Partial Text

Laser interstitial thermal therapy (LITT) is quickly changing our management of patients with brain tumors and epilepsy by offering a viable minimally-invasive surgical option. LITT’s recent resurgence is a result of advances in MRI thermal imaging that have improved real-time intraoperative monitoring of the lesion [1–4]. This is important because under-ablation may lead to treatment failure, while over-ablation can cause cognitive or neurologic deficits [1, 3–6]. Compared to open surgery, LITT patients experience less morbidity and have shorter hospital stays. However, as with any novel technique important gaps in our knowledge still exist. In the case of mesial temporal lobe epilepsy (mTLE), the most common epilepsy syndrome, recent series suggest lower seizure freedom rates for LITT (53–57%) vs. open surgery (60–80%) [1–4]. Improving seizure outcomes will not only require better understanding of laser ablation zones and their relation to epileptogenic areas, but also an improved ability to deliver LITT treatments with a millimetric level of precision.

Of 30 patients with over 6-months follow-up included in this study, sixteen (53%) were male. Average age (± SD) was 43.5 ± 11.3 years. Twenty-one (70%) had evidence of MTS. Average duration from surgery to acquisition of postoperative MRI was 6.4 ± 3.2 months. All underlying demographic and imaging data used are shown in S1 and S2 Tables.

Here, we performed an extensive analysis of Arrhenius-derived TDE maps acquired during LITT of mesiotemporal structures in an attempt to define variables that influence the irreversible coagulation of brain tissue. While these findings are preliminary, owing to the fact that numerous independent variables were analyzed for a small and diverse patient population, we do believe the mesiotemporal model utilized here is the ideal model currently available for studying LITT ablations in humans. Because several variables from MRI, such as T2 signal, volume of surrounding CSF, and laser trajectory, were consistently shown by our analyses to be associated with the short- and long-term progression of LITT ablations, this does imply that the accuracy with which LITT is planned and delivered could one day be improved by incorporating imaging data into predictive models of LITT effects.




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