To the Editor:We read with great enthusiasm the article regarding the impact of the extent of ablation using laser interstitial thermal therapy (LITT) in survival of patients with newly diagnosed glioblastoma published by Kaisman-Elbaz et al. 1 LITT provides a valuable alternative of treatment for patients with few surgical options, such as diagnosis of highly eloquent and/or deep-seated glioblastomas (GBM), and for patients who have a high frailty index. [2][3][4] To date, the literature has highlighted inconsistencies in the technique, indications, patient selection, and functional outcomes and impact in overall survival (OS) and progressionfree survival (PFS). 5,6 The focus in LITT has been recurrent GBM and not necessarily newly diagnosed GBM. 7 Recurrent GBM is a different disease entity which is more challenging to treat since patients have usually undergone multiple lines of therapy.Kaisman-Elbaz et al 1 show that near total ablation was statistically significant in promoting OS (P = .008) in newly diagnosed GBM. In real terms, PFS was considerably higher-10.4 months-as well as OS-22.7 months-in 10 patients who had near total ablation; this is equivalent to surgical resectionrelated OS of 15 months 1 or 17.5 months in 5-Aminolevulinic acid-assisted gross total resections. 8 Kaisman-Elbaz et al 1 also report 84% of patients with excess ablation (ExA) (between 0.25 and 35.3 cc) performed with no neurological deficit. At least 75% of this cohort had lesions which were in deep or eloquent regions. 1 Neurological function is more likely to be located beyond the margins of the lesion in GBM than within the lesion. Hence, if a significant proportion of patients had ablation outside the lesion, they are more susceptible in developing neurological deficit. It would be interesting to know whether authors would consider intraoperative neuromonitoring and preoperative mapping, particularly using tractography or connectomics, to improve or prevent neurological deficits.The literature suggests some thresholds of lesion-to-tract distance more likely to cause neurological deficit 9,10 during tumor resections that have been incorporated in risk stratification scores. The corticospinal tract has high thermal susceptibility, 11 and its damage can lead to significant disability. It is crucial to assess the impact of lesion-to-tract distance to establish the risk of neurological deficit with regard to different tracts of interest to maximize the safety of LITT ablation. It is also significant to note that the

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Brain tumour surgery in visual eloquent areas poses significant challenges to neurosurgeons and has reported inconsistent results. This is a single-centre prospective cohort study of patients admitted for asleep surgery of intra-axial lesions in visual eloquent areas. Demographic and clinical information, data from tractography and visual evoked potentials (VEPs) monitoring were recorded and correlated with visual outcomes. Thirty-nine patients were included (20 females, 19 males; mean age 52.51 ± 14.08 years). Diffuse intrinsic glioma was noted in 61.54% of patients. There was even distribution between the temporal, occipital and parietal lobes, while 55.26% were right hemispheric lesions. Postoperatively, 74.4% remained stable in terms of visual function, 23.1% deteriorated and 2.6% improved. The tumour infiltration of the optic radiation on tractography was significantly related to the visual field deficit after surgery (p = 0.016). Higher N75 (p = 0.036) and P100 (p = 0.023) amplitudes at closure on direct cortical VEP recordings were associated with no new postoperative visual deficit. A threshold of 40% deterioration of the N75 (p = 0.035) and P100 (p = 0.020) amplitudes correlated with a risk of visual field deterioration. To conclude, direct cortical VEP recordings demonstrated a strong correlation with visual outcomes, contrary to transcranial recordings. Invasion of the optic radiation is related to worse visual field outcomes.

BACKGROUND:Despite the importance of complete, gross total resection (GTR) of fourth ventricular ependymomas, significant morbidity and/or subtotal resections are reported, particularly when the ventricular floor is infiltrated. Step-by-step technique descriptions are lacking in the literature.OBJECTIVE:To describe monitoring and stimulation mapping techniques and surgical nuances in the challenging subgroup of infiltrating fourth ventricular ependymomas by a highly illustrated, step-by-step description. Superimposed outlines of cranial nerve nuclei on the surgical field demonstrate critical anatomy and facilitate understanding in a way not previously presented.METHODS:We reviewed the microanatomical and neurophysiological prerequisites of resecting a diffusively infiltrating fourth ventricular ependymoma.RESULTS:We achieved GTR with the use of reproducible stimulating mapping and accurate cranial nerve nuclei identification.CONCLUSION:Enhanced microanatomical understanding, reproducible stimulation mapping, and meticulous resection techniques can result in GTR, even in diffusively infiltrating ependymomas.

Purpose To this day there is no consensus regarding evidence of usefulness of Intraoperative Neurophysiological Monitoring (IONM). Randomized controlled trials have not been performed in the past mainly because of difficulties in recruitment control subjects. In this study, we propose the use of Bayesian Networks to assess evidence in IONM. Methods Single center retrospective study from January 2020 to January 2022. Patients admitted for cranial neurosurgery with intraoperative neuromonitoring were enrolled. We built a Bayesian network with utility calculation using expert domain knowledge based on logistic regression as potential causal inference between events in surgery that could lead to central nervous system injury and postoperative neurological function. Results A total of 267 patients were included in the study: 198 (73.9%) underwent neuro-oncology surgery and 69 (26.1%) neurovascular surgery. 50.7% of patients were female while 49.3% were male. Using the Bayesian Network´s original state probabilities, we found that among patients who presented with a reversible signal change that was acted upon, 59% of patients would wake up with no new neurological deficits, 33% with a transitory deficit and 8% with a permanent deficit. If the signal change was permanent, in 16% of the patients the deficit would be transitory and in 51% it would be permanent. 33% of patients would wake up with no new postoperative deficit. Our network also shows that utility increases when corrective actions are taken to revert a signal change. Conclusions Bayesian Networks are an effective way to audit clinical practice within IONM. We have found that IONM warnings can serve to prevent neurological deficits in patients, especially when corrective surgical action is taken to attempt to revert signals changes back to baseline properties. We show that Bayesian Networks could be used as a tool to calculate the utility of conducting IONM, which could save costs in healthcare when performed.