Tissue Ablation Dynamics During Magnetic Resonance-Guided, Laser-Induced Thermal Therapy

BACKGROUND

Magnetic resonance-guided, laser-induced thermal therapy is a real-time magnetic resonance thermometry-guided, minimally invasive procedure used in the treatment of intracranial tumors, epilepsy, and pain. Little is known about its dynamics and the effects of various pathologies on overall ablation.

OBJECTIVE

To determine the relationship between thermal energy delivery and the time to maximal estimated thermal damage and whether differences exist between various intracranial pathologies.

METHODS

We used real-time ablation data from 28 patients across 5 unique intracranial pathologies. All ablations were performed using the Visualase Thermal Therapy System (Medtronic, Inc, Minneapolis, Minnesota), which uses a 980-nm diffusing tip diode laser. The thermal damage area was plotted against time for each ablation. We then estimated the duration of time required to reach 50% (t50) and 97% (t97) of maximal damage. Comparisons were then made between different intracranial pathologies.

RESULTS

The duration required to reach maximal thermal damage estimate (TDE) among all ablations was 159 ± 62 seconds, and the t50 and t97 were 43 ± 21 and 136 ± 57 seconds, respectively, where t97 was reached at an average of 23 seconds before the maximal TDE. The t97 was shorter in the recurrent metastasis/radiation necrosis and epilepsy groups compared with the previously untreated glioblastoma multiforme group.

CONCLUSION

The optimal duration can be estimated by the t97, which can be achieved in less than 3 minutes and differs across ablation targets. TDE expansion decelerates with prolonged ablation. Future studies are needed to examine the radiographic and clinical outcomes as well as the effects of ablation power, irrigation speed, and the effect of previous therapies on ablation dynamics.

Intracranial MR-guided laser-induced thermal therapy: single-center experience with the Visualase thermal therapy system

OBJECTIVE MR-guided laser-induced thermal therapy (MRgLITT) can be used to treat intracranial tumors, epilepsy, and chronic pain syndromes. Here, the authors report their single-center experience with 102 patients, the largest series to date in which the Visualase thermal therapy system was used. METHODS A retrospective analysis of all patients who underwent MRgLITT between 2010 and 2014 was performed. Pathologies included glioma, recurrent metastasis, radiation necrosis, chronic pain, and epilepsy. Laser catheters were placed stereotactically, and ablation was performed in the MRI suite. Demographics, operative parameters, length of hospital stay, and complications were recorded. Thirty-day readmission rates were calculated by using the standard method according to America's Health Insurance Plans Center for Policy and Research guidelines. RESULTS A total of 133 lasers were placed in 102 patients who required intervention for intracranial tumors (87 patients), chronic pain syndrome (cingulotomy, 5 patients), or epilepsy (10 patients). The procedure was completed in 98% (100) of these patients. Ninety-two patients (90.2%) had undergone previous treatment for their intracranial tumors. The average (± SD) total procedural time was 170.5 ± 34.4 minutes, and the mean laser-on time was 8.7 ± 6.8 minutes. The average intensive care unit (ICU) and hospital stays were 1.8 and 3.6 days, respectively, and the median length of stay for both the ICU and the hospital was 1 day. By postoperative Day 1, 54% of the patients (n = 55) were neurologically stable for discharge. There were 27 cases of morbidity, including new-onset neurological deficits, and 2 perioperative deaths. Fourteen patients (13.7%) developed new deficits after the MRgLITT procedure, and of those 14 patients, 64.3% (n = 9) had complete resolution of deficits within 1 month, 7.1% (n = 1) had partial resolution of symptoms within 1 month, 14.3% (n = 2) had not had resolution of symptoms at the most recent follow-up, and 14.3% (n = 2) died without resolution of symptoms. The 30-day readmission rate was 5.6% CONCLUSIONS MRgLITT, although minimally invasive, must be used with caution. Thermal damage to critical and eloquent structures can occur despite MRI guidance. Once the learning curve is overcome, the overall procedural complication rate is low, and most patients can be discharged within 24 hours, with a relatively low readmission rate. In cases in which they occurred, most neurological deficits were temporary. The therapeutic role of MRgLITT in various intracranial diseases will require larger and more rigorous studies.

Accuracy of Laser Placement With Frameless Stereotaxy in Magnetic Resonance-Guided Laser-Induced Thermal Therapy

BACKGROUND

As magnetic resonance-guided laser-induced thermal therapy (MRgLITT) becomes more accepted, there needs to be an evaluation of the techniques required to achieve accurate laser placement.

OBJECTIVE

To report our experience with frameless stereotaxy and the ability to achieve accurate laser placements. We also evaluate the variables associated with proper placement.

METHODS

We performed a retrospective analysis from 3 years of MRgLITT. Demographics and operational parameters, including trajectory length, target alignment error, registration error, and radial error were recorded and compared. Blinded review was used for completeness of ablation.

RESULTS

In the study, 90 laser placements were evaluated for 72 cases. Trajectory length and target alignment error was 95.3 ± 26.0 mm and 0.7 ± 0.3 mm, respectively. Significant differences existed in registration error between 4 (0.6 ± 0.3 mm) and 5 (0.5 ± 0.2 mm) skull pins (P = .04), but no significant decreases in registration error as additional skull pins were registered. Fifteen laser placements resulted in subtotal ablations. The overall radial error using frameless stereotaxy was 0.9 ± 1.6 mm. In the study, 65% of lasers were exactly on the planned trajectory. Of the 30 that were not, the radial error = 2.6 ± 1.9 mm. Radial error of subtotal laser ablations was 0.5 ± 0.9 (range, 0-2.8 mm) and was not significantly different from 0.8 ± 1.7 (range, 0-7.1 mm) radial error of lasers with total ablations (P = .52). Lasers with radial error >0 mm resulted in an incomplete ablation in 26.7% of cases.

CONCLUSION

Skull pin-based frameless stereotaxy for MRgLITT results in consistent accuracy, with the majority of cases resulting in complete ablations. A significant proportion of lasers with RE >0 mm still result in complete ablations.

Does the real-time thermal damage estimate allow for estimation of tumor control after MRI-guided laser-induced thermal therapy? Initial experience with recurrent intracranial ependymomas

OBJECT Although control of intracranial ependymomas is highly correlated with degree of resection, it is unknown if the same is true for MRI-guided laser-induced thermal therapy (MRgLITT). The authors report their experience with MRgLITT for ependymoma and examine the utility of the real-time thermal damage estimate (TDE), a recent software advance, with respect to completeness of ablation and impact on tumor control. To the authors' knowledge, this is the largest single-center experience utilizing MRgLITT for recurrent ependymomas. METHODS Five tumors in 4 patients were treated with the Visualase Thermal Therapy System. Two tumors were treated similarly on recurrence. Ablation was performed using a 980-nm diode laser with a real-time image acquisition system. Single-plane TDEs were calculated and compared with the original lesion area to compute percentage area ablated (PAA). Volumetric analysis was performed, and percentage volume ablated (PVA) was estimated and correlated with the TDE. Tumor control was correlated with the TDE and volumetric data during treatment. RESULTS Nine ablations were performed on 5 tumors, 2 of which had multiple recurrences. The average pretreatment lesion volume was 8.4 ± 6.3 cm(3), and the average largest 2D area was 5.3 ± 2.7 cm(2). The averaged TDE was 3.9 ± 2.1 cm(2), average PAA was 80.1% ± 34.3%, and average PVA was 64.4% ± 23.5%. For subtotal ablations, average recurrence time was 4.4 ± 5.3 months; 1 adult case remains recurrence-free at 40 months. Using TDEs, the correlation between recurrence time and PAA was r = 0.93 (p = 0.01), and for PVA was r = 0.88 (p = 0.02). Furthermore, PVA and PAA were strongly correlated (r = 0.88, p = 0.02). CONCLUSIONS Through using the PAA, the real-time TDE correlated with the volume of ablation in this initial investigation. Furthermore, the TDE and volumetric data corresponded to the level of tumor control, with time to recurrence dependent on ablation completeness. MRgLITT may have a role in the management of recurrent ependymomas, especially with recent software advances.

Identifying MRI markers associated with early response following laser ablation for neurological disorders: preliminary findings

There is a renewed interest in MR-guided laser interstitial thermal therapy (LITT) as a minimally invasive alternative to craniotomy for local treatment of various brain tumors and epilepsy. LITT allows for focused delivery of laser energy monitored in real time by MRI, for precise ablation of the lesion. Although highly promising, the long-term effects of laser ablation as a viable treatment option for neurological disorders have yet to be rigorously studied and quantified. In this work, we present a quantitative framework for monitoring per-voxel thermal-induced changes post-LITT over time on multi parametric MRI. We demonstrate that voxel-by-voxel quantification of MRI markers over time can enable a careful and accurate (a) characterization of early LITT-related changes (if and when they are exaggerated and when they subside), and (b) identification and monitoring of MRI markers that potentially allow for better quantification of response to LITT therapy. The framework was evaluated on two distinct cohorts of patients (GBM, epilepsy), who were monitored post-LITT at regular time-intervals via multi-parametric MRI. On a cohort of six GBM studies we found that (a) it may be important for the initial treatment-related changes to subside to more reliably capture MRI markers relating to tumor recurrence, and (b) T1w MRI and T2-GRE may better differentiate changes that may correspond to tumor recurrence from patients with no recurrence, as compared to T2w-MRI, and FLAIR. Similarly, our preliminary analysis of four epilepsy studies suggests that (a) early LITT changes (attributed to swelling, edema) appear to subside within 4-weeks post-LITT, and (b) ADC may be more reflective of early treatment changes (up to 1 month), while T1w may be more reflective of early delayed treatment changes (1 month, 3 months), while T2-w and T2-FLAIR appeared to be more sensitive to late treatment related changes (6-months post-LITT) compared to the other MRI protocols under evaluation.