Day: August 24, 2026

J Neurosurg 145:347–363, 2026

Insular gliomas remain among the most demanding intrinsic brain tumors because eloquent cortical and subcortical networks coexist with critical middle cerebral and lenticulostriate vessels. This large single-center experience evaluates whether a transcortical, mapping-guided strategy can achieve meaningful cytoreduction without unacceptable neurological morbidity. The series comprises 502 operations in 394 patients with newly diagnosed or recurrent low- and high-grade tumors. Greater extent of resection and smaller residual volumes were associated with longer survival, while persistent deficits were uncommon. The results support maximum safe resection, but also underline that oncological benefit depends on preserving neurological function.

Objective

To determine whether transcortical resection supported by cortical and subcortical mapping remains a safe and effective strategy for newly diagnosed and recurrent insular gliomas, and to examine the relationships among tumor characteristics, extent of resection, postoperative morbidity, progression-free survival and overall survival.

Methods

This retrospective single-center study analyzed 502 resections performed in 394 patients between September 1997 and December 2022. The cohort included 316 newly diagnosed tumors—165 low-grade and 151 high-grade—and 186 recurrent tumors—69 low-grade and 117 high-grade.

Tumors were classified according to the Berger-Sanai zones. Preoperative and postoperative contrast-enhancing and non–contrast-enhancing volumes were manually segmented on MRI to calculate extent of resection and residual volume. Neurological status was assessed preoperatively, immediately after surgery, at discharge, at 3 months and at least 6 months postoperatively.

The transcortical approach was adapted to hemispheric dominance and tumor anatomy. Dominant-hemisphere lesions underwent awake language and motor mapping; nondominant lesions were generally treated asleep with triple-modality motor mapping. Subcortical stimulation defined functional white-matter boundaries, while the lenticulostriate arteries marked the medial resection limit.

Progression-free and overall survival were studied using Kaplan-Meier and Cox regression analyses. Propensity-score weighting adjusted survival comparisons for factors including age, preoperative Karnofsky Performance Status and adjuvant temozolomide or radiotherapy.

Main results

Median extent of resection exceeded 80% across the analyzed molecular subtypes in both newly diagnosed and recurrent tumors. Grade 2 tumors were larger than grade 4 IDH-wildtype glioblastomas at presentation (43 versus 17.5 cm³; p<0.001), while their median residual volume was also greater (6.8 versus 0.5 cm³; p<0.001).

Among newly diagnosed grade 2 tumors, transient motor and language deficits occurred in 9.5% and 20% of cases, respectively. Persistent deficits were substantially less frequent: 0.6% for motor function and 3.3% for language. Across all 502 operations, only 3 patients (0.59%) had permanent hemiparesis below antigravity strength. Surgical and medical complications each occurred in fewer than 3% of cases, and more than 94% of patients were discharged home.

For newly diagnosed IDH-mutant grade 2 tumors, residual non–contrast-enhancing volume below 2.7 cm³ was associated with longer overall survival, while a residual volume below 4.3 cm³ was associated with longer progression-free survival. In recurrent grade 2 tumors, residual volumes below 0.8 cm³ and 1.8 cm³ were associated with improved overall and progression-free survival, respectively.

For newly diagnosed IDH-wildtype glioblastoma, resection of more than 88.6% of the contrast-enhancing component was associated with longer overall survival, and resection above 87.08% with longer progression-free survival. Median overall survival was 18.5 months when contrast-enhancing extent of resection exceeded 88.6%.

A new permanent arm or leg deficit was independently associated with worse overall survival (HR 2.06; 95% CI 1.14–3.74; p=0.017).

Interpretation

The study provides strong observational support for a transcortical, mapping-guided approach to insular glioma surgery. Meaningful cytoreduction was achievable across all Berger-Sanai zones and in both initial and repeat operations, with a low frequency of permanent neurological morbidity.

The survival analyses also refine the concept of “maximum safe resection.” Benefit was associated not only with percentage resection but with the absolute residual volume, particularly in IDH-mutant grade 2 glioma. Nevertheless, these thresholds should be interpreted as cohort-derived prognostic markers rather than mandatory surgical targets.

The association between permanent weakness and poorer survival is clinically crucial. It indicates that aggressive cytoreduction loses part of its oncological value when it produces lasting neurological injury. Functional preservation is therefore an integral component of oncological efficacy, not merely a secondary safety endpoint.

Limitations

This was a retrospective experience from a single, highly specialized center, which limits generalizability to teams with different mapping expertise or operative volumes. The 25-year inclusion period encompassed major changes in imaging, mapping, neuropathological classification and adjuvant treatment.

Molecular information was incomplete in earlier cases; survival analyses were consequently restricted to tumors that could be classified according to the 2021 WHO framework. Treatment selection was not randomized, and residual confounding remains despite propensity-score adjustment. The study does not directly compare transcortical surgery with transsylvian resection, biopsy, laser interstitial thermal therapy or IDH-inhibitor-based strategies.

The reported residual-volume and extent-of-resection cutoffs were derived retrospectively from this cohort and require external validation. Detailed neurocognitive and quality-of-life outcomes were also less comprehensive than the motor and language assessments.

Clinical takeaway

Maximum safe resection should remain the operative goal for appropriately selected adult patients with insular glioma, including selected recurrent tumors. A transcortical approach supported by individualized cortical and subcortical mapping can produce substantial cytoreduction with low permanent morbidity in experienced hands.

The surgeon should prioritize the smallest safely achievable residual volume rather than pursue a percentage target at the expense of functional pathways or lenticulostriate vessels. In practical terms, the oncological objective and the functional boundary are inseparable: avoid a permanent deficit, because neurological injury is itself associated with poorer survival.