This is a retrospective analysis of 126 consecutive patients with LGGs. All underwent awake surgery with intraoperative direct electrical stimulation at Nagoya University Hospital (Nagoya, Japan) between December 2012 and May 2020. Patient data on clinical information and outcomes were collected and analyzed. The Ethics Committee at Nagoya University Hospital approved this retrospective data evaluation and the experimental design of the study (approval number: 2020-0079). We obtained written informed consent prior to the surgical mapping procedure from all participants included in the study.
Pre- and postsurgical neuroradiological examination
We performed preoperative MRI, including three-dimensional T1-weighted imaging, conventional MRI (T1-, T2-, and FLAIR-weighted imaging), and diffusion-weighted imaging, using a 3.0 Tesla scanner (Trio, Siemens, Germany). To assess the EOR, MRI (T1-weighted, T2-weighted, and FLAIR-weighted) was also conducted at 1 week and 3 months postoperatively and at every 3 months thereafter.
Intraoperative awake brain mapping technique
All 126 patients underwent awake brain mapping with direct electrical stimulation using an asleep-awake-asleep protocol, as previously described[17, 23, 26-29]. In brief, we performed a craniotomy under general anesthesia. In the first stage, the tumor margins and cerebral sulcus and gyrus were identified using a neuro-navigational system, and letter tags were placed along the tumor boundaries on the cortical surface before the brain shifts occurred. Cortical mapping with direct electrical stimulation was then performed to detect the motor and language areas. We used a bipolar stimulator with a 2-mm diameter and a 5-mm interelectrode distance (Unique Medical, Osaka, Japan) to deliver a biphasic current (pulse frequency, 60 Hz: single pulse phase duration, 0.5 ms). The Neuromaster MEE1200 (Nihon Kohden, Tokyo, Japan) was used for intraoperative neurological monitoring during the awake surgery.
The stimulation intensity used for individual patients was determined by increasing the amplitude in 0.5-mA increments from a baseline of 1.0 mA until a functional response was elicited. Maximum individual intensities ranged from 2 mA to 8 mA. After determining the optimal stimulation threshold, cortical mapping was performed by applying electrical stimulations of the same intensity over the whole exposed cortical area surface. This stimulation threshold was then also used for subcortical mapping. Using strip electrodes, continuous digital electrocorticograms (ECoGs) were monitored to detect after-discharges during direct electrical stimulation and tumor resection.
For language mapping, the patients were asked to perform counting and picture-naming tasks, with the goal to identify the cortical language sites and subcortical fibers using direct brain stimulations. The type of observed language disturbances was determined based on a detailed classification for language disorders, including speech arrest, anomia, dysarthria, anarthria, speech slowness, initiation trouble, perseveration, and paraphasia.
After the cortical mapping was complete, tumor resection was initiated using the subpial resection technique. We removed the tumor to the level of the white matter, while frequently checking the patient’s response using subcortical electrical stimulation. Subcortical mapping enabled us to determine the functional boundaries between the tumor and white matter pathways. Thus, tumor removal was accomplished according to the cortical or subcortical functional boundaries in all patients, while aiming for achieving supratotal resection whenever possible. After tumor resection, intraoperative MRI was routinely performed to assess the EOR of the tumor using a 0.4 Tesla vertical field MR scanner (Aperto Inspire, Hitachi, Tokyo, Japan) set up in the operating room of the Brain THEATER at Nagoya University Hospital.
Degree of tumor resection and volumetric analysis
Volumetric analysis was performed using the iPlan® cranial planning software included in the BrainLAB iPlan® Cranial version 2.6 and 3.0 (German HealthCare Export Group, Bonn, Germany). The pre- and postoperative tumor volumes were measured in all patients to estimate the EOR using contrast T1-weighted or FLAIR-weighted MRI data obtained before and after tumor removal. If the tumor was not enhanced or partially enhanced on MRI, the EOR was evaluated based on residual high-intensity lesions on FLAIR-weighted MRI. On the other hand, if the tumor was enhanced on MRI, the EOR of the tumor was calculated based on the residual enhanced tumor. The EOR was calculated using the difference between preoperative and postoperative tumor volumes: (preoperative tumor volume – postoperative tumor volume) / preoperative tumor volume. Volumetric EOR was categorized as follows: supratotal resection, EOR > 100%; gross total resection, EOR = 100%; subtotal resection, EOR ≥ 90% to < 100%; and partial resection, EOR < 90%. A supratotal resection was defined as tumor resection extending beyond the MRI-verified abnormal area or the complete removal of any abnormality, with the postoperative cavity volume being larger than the preoperative tumor volume[22, 25, 31]. This was also defined as a postoperative tumor cavity/preoperative tumor volume >100%.
Tumor progression was defined as newly detected areas of contrast enhancement and/or an obvious increase in the FLAIR signal abnormality on follow-up MRI relative to the baseline postoperative MRI obtained within 72 hours of the operation.
Adjuvant therapy protocol
Further treatment of grade II gliomas that underwent total or supratotal resection was based on observation. When more than 10% of the tumor was left, chemotherapy such as temozolomide (TMZ) was applied as adjuvant treatment.
For grade III gliomas, initial adjuvant treatment included focal external-beam radiation therapy by conventional radiation planning to approximately 60 Gy (±5% total dose), with daily concurrent TMZ at 75 mg/m2 throughout the course of the radiation therapy. This was followed by adjuvant temozolomide according to the Stupp protocol.
Direct DNA sequencing for IDH1 and IDH2 mutations
Direct sequencing of IDH1 and IDH2 was conducted as previously described. A 129-bp fragment spanning the sequence encoding the catalytic domain of IDH1, including codon 132, and a 150-bp fragment spanning the sequence encoding the catalytic domain of IDH2, including codon 172, were amplified for IDH sequencing.
All statistical analyses were conducted using SPSS version 27.0 (IBM Corporation, Armonk, NY, USA) for Windows (Microsoft Corporation, Redmond, WA, USA). Survival was estimated using the Kaplan–Meier method, and the log-rank test was used to assess survival differences among groups. Progression-free survival (PFS) was calculated from the day of the first surgery till the occurrence of true tumor progression, death, or the end of the follow-up. Overall survival (OS) was calculated from the day of the first surgery until death or the end of the follow-up.
Factors influencing PFS in our cohort of patients with LGGs during awake brain surgery were investigated using univariate and multivariate Cox proportional hazards models adjusted for major clinical prognostic factors, including age at diagnosis (>40 years vs. ≤40 years), histologic type (astrocytic vs. oligodendroglial), WHO grade (grade III gliomas vs. grade II gliomas), tumor location (frontal regions vs. other regions), IDH1 or IDH2 status (mutation or wild type), final EOR (>100% vs. <100%), chemotherapy (+ vs. -), and chemoradiotherapy (+ vs. -). The covariates and the independent variables that showed significant differences in the univariate analysis were used for the analysis. The remaining factors in the multivariate Cox proportional hazards model (p < 0.05) were considered to be independent predictors of PFS.