Preoperative Workup

Magnetic resonance imaging (MRI) with and without gadolinium is essential for preliminary differential diagnosis, decision for surgery, and operative planning. Thallium SPECT scan, PET scan, or MR spectroscopy may help in determination of high grade versus low grade tumor, although none of these studies is definitive, and differentiation between HGGs and metastases is difficult.⁴ These studies are more helpful in cases with previous surgery and radiation in determining recurrent tumor vs. radiation effect, especially MR spectroscopy and MR perfusion. For selected patients who cannot undergo MRI (e.g., patients with a cardiac pacemaker), CT with and without contrast provides similar, albeit less detailed information. CT perfusion may be an aid in better defining the tumor from cerebral edema as well as help with the potential for post-treatment radiation effects.

A Wada (intracarotid amobarbital) test is the definitive, albeit invasive, method to establish cerebral hemispheric dominance for language and memory. It is required for procedures in patients with a seizure disorder with tumor in whom a formal temporal lobectomy is planned. A Wada may be useful in selected other patients, such as patients with a dominant hemisphere temporal lobe tumor in whom tumor resection without temporal lobectomy is planned. Although both functional MRI and a Wada test offer similar information about cerebral dominance,^5,6 a Wada test does not offer anatomic localization of critical areas for language as MRI does nor truly investigates the potential bilaterality of language.^7,8 Functional MRI is more useful than a Wada test for lesions of the dominant hemisphere near the motor cortex, frontal lobe pars triangularis and opercularis (Broca’s area), or Wernicke’s area. With changes in metabolic activity and blood flow demand, an active area of the brain during a silent speech or motor task becomes infused with more oxygenated blood; this change can be detected since oxygenated blood carries a different paramagnetic signal than deoxygenated (blood oxygen level–dependent or BOLD signal) (Fig. 8-1).^9,10 One limitation of functional MRI is that it becomes less useful in patients with a recurrent tumor because of altered vascular patterns and MR artifacts from the previous surgery. Some patients require both a Wada and a functional MRI as part of preoperative planning.

FIGURE 8-1 Functional MRI during a silent speech task in a patient with a glioblastoma of the left temporal glioblastoma (left and right are reversed).

Cytoreduction

Although decompression of mass effect is a surgical goal and influences symptomatic survival, controversy exists over whether the extent of resection influences survival or time to progression for HGGs. Dandy originally proposed hemispherectomy for selected patients with malignant tumors, but there was no significant effect on mortality.¹¹ Next, there were data suggesting that for GBMs, biopsy and resection were equivalent in terms of survival, and that it was really postoperative radiation that had a meaningful effect on survival. More recently, the Glioma Outcomes Project reported a statistically significant extension of survival for patients with HGGs who undergo resection over biopsy (median survival 51.6 weeks vs. 27.1 weeks, respectively).¹² This study was limited by lack of central pathologic review, lack of quantification of amount of resection, sampling error from a biopsy, and selection bias in biopsy vs. resection. Other surgeons have also reported extension of survival for patients with 90% or better resection; resection better than 98% was associated with a median survival of 13 months versus 8.8 months with less than 98% in one study.¹³ In a recent series focused on the GBM population for patients given aggressive (but not always gross total) resection, Gliadel chemotherapy wafers, radiation, and Temodar, median survival was 20.7 months, and the 2-year survival rate was 36%.¹⁴ Additionally, subtotal resection, the volume of residual tumor at the time of first recurrence, may negatively influence response to chemotherapy in terms of time to progression and overall survival.¹⁵ Thus, it is not clear that only a biopsy should be performed if a surgeon is facing a tumor that cannot totally be resected. However, even gross total resection does not truly address the diffuse nature of malignant gliomas.

Intraoperative Imaging

Even with continual improvements with operating microscopes, some form of intraoperative imaging or navigation is useful. A high-quality intraoperative ultrasound assists in many types of surgeries, but cannot aid in incision and craniotomy planning. Also, many tumors, especially lower-grade tumors, may not be dense enough compared to the normal brain to be visualized adequately by this method. Ultrasound is more helpful when the density difference is greater, such as if there is a hematoma to evacuate in addition to the tumor, if there is a cystic component, or if there is a need for ventricular access.

Frameless stereotactic navigation is very helpful in a craniotomy for tumor resection—some would say essential. This technology incorporates a preoperatively obtained MRI with fiducial markers that are left in place on the scalp. In the operating room, these markers or contours of the face can be registered in reference to a frame that is visualized by a computer via an optical apparatus, electromagnetic waves, or mechanical arms. This technology allows the surgeon to visualize points on the scalp and skull and compare them to the MRI, aiding in planning of a small, localized incision and craniotomy as well as ensuring that the exposure of the lesion is adequate. Surgical navigation can be performed intracranially with a localizing probe or with image fusion into the operating microscope based on focus depth. Because frameless navigation is based on a preoperative set of images without updating in the operating room, the surgeon needs to account for brain shift during the procedure. Brain shift up to 2 cm can occur, and is more common with increased patient age, cortical rather than subcortical structures, larger tumor volume, and lesions far from some point of tethering such as the skull base or falx.¹⁶ Once brain shift is taken into account, resection to the imaging abnormality borders (when safe) assists in the goal of cytoreduction.

Intraoperative MRI systems are available as well. Low-field (<0.5-T) systems allow most normal operating room equipment to be used throughout the case except right at the point of imaging.^17,18 Because the imaging can be updated, the surgeon does not need to account for brain shift. For craniotomies, once resection is deemed complete by the surgeon, an MRI can be performed to assess whether there is occult residual tumor, aiding in the aim of cytoreduction. These systems offer a smaller field of view, less detailed images, longer acquisition time, and fewer types of imaging options than conventional diagnostic MRIs.

High-field (1.5-T) systems exist, which offer all the imaging capabilities of a standard, diagnostic MRI.¹⁹ A biopsy needle can be watched in near–real time as it is passed to target and verified at the target before samples are taken. With craniotomies, intraoperative imaging can confirm completeness of tumor resection, which is especially helpful in cases of low-grade gliomas when the distinction between tumor and normal brain is less apparent. At closure, an MRI can be performed to exclude hemorrhage; for patients with a biopsy or simple craniotomy, excluding the hemorrhage may allow a patient to be transferred to a step-down unit instead of the ICU. These systems, however, require construction of an operating room suite specifically designed for an intraoperative MRI to provide adequate shielding and safety measures. The high-field strength requires a larger magnet than the low-field systems, limiting access to the patient. Normal operating room equipment can be used outside the 5-gauss line (several feet from the center of the bore of the magnet); inside that line, only MRI-compatible (titanium or surgical-grade stainless steel) instruments can be used.

Motor Strip Mapping

Surgery in the parietal lobe or the posterior frontal lobe may require motor strip mapping. Short-acting muscle relaxants are used during induction, and anesthetics are lightened for the mapping, but the patient does not need to emerge from anesthesia fully. Rather than identification of the motor cortex itself, this technique relies on identification of the sensory cortex by looking for somatosensory-evoked potentials (SSEPs). A 1 × 8 or other sized subdural electrode is used. By noting the electrodes with a positive (precentral) as opposed to a negative (postcentral) amplitude and noting the two electrodes between which there is phase reversal, the primary motor cortex can be identified and protected. This technique has good correlation to magnetoencephalography when integrated into the surgical navigation system.²⁰

Awake Craniotomy

Awake craniotomy with cortical mapping affords the ultimate protection for surgery in or near language areas. In one option, patients are never intubated—they are merely sedated with an agent like Propofol, which takes effect quickly and wears off quickly; proponents of this pattern espouse that patients’ airways are less irritated and that language testing is better quality. The other way to accomplish the anesthesia is for patients to be nasotracheally intubated so that the endotracheal tube can later be withdrawn out of the vocal cords and language tasks can be performed. In addition to a local anesthetic to the incision, a field block to the scalp in the area of the incision as well as the Mayfield pins is performed with a long-acting anesthetic such as bupivicaine. Muscle relaxants may be used at induction, but they must be short-acting. Draping is per routine except that the face needs to have an unobstructed view so visual naming and interaction with the neuropsychologist are possible, so one must be careful that the head frame and subsequent retractor bars do not obscure vision. Once the craniotomy is created, anesthetics are lightened, the endotracheal tube is withdrawn from the vocal cords, and a handheld bipolar stimulator is used to stimulate areas likely to have language function or areas of planned resection to determine the effect. Continuous language testing can be performed during resection to reconfirm safety. Once resection is complete, the endotracheal tube is replaced and general anesthesia is reinstated for closure.

Frontal Lobe

High-grade gliomas are most common in the frontal lobe, as it occupies one third of the surface of the brain and is the largest lobe.²¹