- Positioning for surgery: The most useful position for operating on hemispheric and thalamic tumors is the supine position, although a lateral or prone position may better facilitate access to the occipital and parieto-occipital areas.
- Body protection during positioning: Pad all pressure points. A well-padded donut or horseshoe device for head positioning must be used with caution, particularly for long operations. The ear must be free of pressure, and the orbits must sustain no direct pressure. Prolonged surgery may necessitate help from the anesthesia team to check the position of the head during the surgery as the head may move, particularly while turning the scalp and craniotomy flaps.
- DVT prophylaxis: Children younger than 16 years who are not obese and have no risk factors for deep venous thrombosis do not require pneumatic compression devices.
- Advantages to pin fixation: The use of pin fixation avoids direct pressure problems from padded devices and maintains the head’s position for surgery. The need to use rigid fixation for retractor systems, image-guided navigation, or positioning the head in a direction not perpendicular with the bed should be tempered by the risks of the head slipping, skull fracture, and even epidural hematoma that are associated with the use of pin fixation in a child.
- Distribution pressure: Placement of the pins depends on age, calvarial thickness, and closure of the sutures. In the very young, or in children of any age with long-standing hydrocephalus and a thin skull, the parieto-occipital area and the superior temporal line tend to be the thickest areas of corresponding perpendicular calvarium to accommodate pins. Certain rigid fixation devices allow up to six pins, adequately distributing pressure to hold an infant.
The surgical approach is dictated by tumor size, location, surrounding structures, and notwithstanding, the comfort of the surgeon.
- Haircut minimal: After positioning, a minimal amount of hair, if any, is shaved with a surgical clipper, and the incision is prepared for surgery.
- Skin incision and craniotomy: Skin incision and craniotomy should be based on tumor location and size, surrounding vascular structures, and eloquent cortex. Pre- and intraoperative images as well as image guidance may aid in localizing the lesion. Many HGGs, particularly in younger children, are very large, necessitating adequate exposure for potential vascular control and vigilance of potential brain collapse.
- Incision with cautery avoids use of hemostatic clips: After the patient and table are draped, an incision is started with a 15 blade knife through the dermis. A Colorado needle set at 15 is used to open the incision through the galea. A straight or “lazy-S” incision may be reflected with a self-retaining retractor, and a curvilinear incision may be held open with hooks. Hemostatic clips are not necessary.
Approach to tumor
- Pericranium elevated: If a free bone flap is desired not bearing the temporalis muscle, the pericranium is scored with a knife and reflected off of the skull, maintaining a vascularized pedicle. This saved pericranium may be used as either a free or vascularized graft for any dural defects at the end of surgery.
- Appropriate bur holes made for the planned craniotomy: Typically only one is needed, but particularly in younger children, a bur hole should be placed wherever the craniotome must cross a cranial suture. As the dura is adherent to the suture, these bur holes allow for adequate dissection of the dura from the bone.
- Dural tack-ups considered: Holes can be made circumferentially around the craniotomy to accommodate dural tack-up sutures (4-0 Neurolon or monofilament absorbable sutures). These same holes can be used for securing the bone flap at the end of surgery if a plating system is not used.
- Ultrasound and/or neuronavigation used to locate tumor: An ultrasound is performed prior to opening the dura to verify location. An ultrasound is again performed after the dura is opened and the images saved for comparison during and after resection. The navigation system may also aid in finding the appropriate spot to perform a corticectomy if the tumor is deep to the cortical surface or not approachable through the sylvian fissure, subfrontally, or interhemispherically. If the tumor is deep or not very large, intermittent ultrasound with a small piece of Gelfoam at the base of the dissection or guide using frameless navigation will allow for a direct route to the tumor.
- Mapping of cortical surface used if indicated: If eloquent cortex is involved or surface ECoG is necessary, monitoring and/or stimulation for mapping of cortical function is now performed.
- Brain relaxation performed: Should transcortical dissection or retraction during interhemispheric or subfrontal approaches be difficult because of concomitant hydrocephalus, an EVD proves helpful in relaxing and protecting the brain. Interhemispheric, subfrontal, or subtemporal approaches proceed with aspiration of CSF, relaxation of the brain, and progression with cottonoid patties or nonadherent dressing strips along the surface of the brain toward the target. If the cortex must be opened, an appropriate sulcus is identified and the arachnoid is opened sharply with microscissors or an 11 blade. If the gyrus is the point of entry, the arachnoid/pia are cauterized and opened sharply.
- Transcortical approaches may require transgression of normal tissue. Image guidance and intraoperative imaging may allow for a direct path to the tumor, depending on surrounding eloquent cortex. IOM may be utilized to determine the location of sensorimotor centers, and preoperative fMRI and DTI may aid in avoiding injury to eloquent cortex. Although awake craniotomy may be helpful in the older child with a HGG in eloquent cortex, staged invasive monitoring using cortical grid placement may be helpful in precisely locating speech and motor areas in the younger child. Although HGGs are not typically intraventricular, AAs and GBMs of the thalamus may well protrude into the ventricle, making a transcortical approach advantageous when hydrocephalus is present.
- Fronto-temporal approaches, with the craniotomy at the pterion and appropriate dissection of the sylvian fissure, may allow for avoidance of cortical incisions, giving excellent exposure of tumors involving the optic apparatus and hypothalamus.
- Anterior interhemispheric and subfrontal approaches avoid intentional violation of the cortical surface and may give access to optic pathway or hypothalamic tumors, as well as thalamic gliomas with intraventricular presentation.
- Posterior interhemispheric approaches may also access thalamic tumors, and a craniotomy at and below the lambda offers exposure to the posterior third ventricle and pineal region through a small callosotomy, with the avoidance of many draining veins into the sagittal sinus.
- Transcortical approaches: Proceed with Penfield dissectors or microdissectors, using patties or nonadherent dressing strips to protect the brain, and an atraumatic tip on the suction to aid in gentle retraction.
- Decide on appropriate degree of magnification: For cortical tumors that come to the surface, resection may proceed with operating loupes. For deep tumors or those potentially involved with critical structures or vessels, the operating microscope can be considered.
- Prepare for hemostasis: Given the high-grade nature of HGGs, they may be quite vascular. Even a biopsy may induce significant bleeding. In addition to bipolar electrocautery other hemostatic agents such as thrombin-soaked Gelfoam and oxycellulose can be used.
- Biopsy done prior to tumor resection: Once the tumor is encountered, a biopsy is obtained with appropriate forceps for preliminary histology. Care should be taken to obtain enough tumor specimen for permanent histology as well as for any special stains or protocols available. Depending on location and size, a self-retaining retractor system may now be placed.
- Central debulking: If the tumor is very large, and depending on the intentions of surgery and involvement of important adjacent structures, debulking the tumor may be helpful. GBMs are oftentimes necrotic, and simple aspiration is effective in decreasing the bulk of the tumor. In more firm AAs, bipolar cautery with suction, or use of the CUSA may facilitate resection.
- Separate tumor margins from surrounding brain: For smaller tumors, or once a large tumor has been debulked, using Penfield or microdissectors to develop the gliotic plane surrounding the tumor can help maintain its margin. While the infiltrative nature of these tumors does not allow for a clear margin circumferentially, in most cases there is some demonstrable delineation between normal cortex or white matter and the tumor.
- Caution with large vessels: Caution near large veins and arteries should used, but remaining subpial, a task performed more easily under the microscope, typically protects these structures as well as the optic nerves and brainstem.
- Reorient during dissection with ultrasound or neuronavigation: Again, serial images with the ultrasound and updates with the navigation system are helpful in gauging location as well as degree of resection.
- Maximal resection and then hemostasis: If the goals of surgery is complete resection, normal white matter, cortex, or ependyma should be visible circumferentially. If any of these surfaces bleeds despite cautery, oxycellulose may be left behind on that specific surgery. Any such material that floats out at the conclusion of surgery with irrigation is not effective.
- Irrigate clear: At the conclusion of resection and removal of any patties, Telfa strips, and retractors, a final ultrasound should be performed and images saved. Irrigation of the cavity/ventricle with Ringer’s lactate aids in diluting out blood or debris from surgery.
- A ventricular catheter may be left behind if there is concern about CSF leak, the ventricle has been opened, or there is concern for postoperative ventricular obstruction and the development of acute hydrocephalus.
- Subdural hematomas: Resection of large tumors is accompanied by brain collapse. As such, careful inspection of the subdural space for any subtle bleeding or significant tension on draining veins is prudent.
- Dural closure is important, but watertight closure is not mandatory if the ventricle has not been entered or hydrocephalus is not present. Autologous pericranium works well as a dural substitute. In the very young infant, no brain should be exposed upon closure with a dural covering that is secure, particularly at the edge of the craniotomy, as in the rapidly growing brain, a leptomeningeal cyst may develop.
- The bone flap may be secured by any number of methods, including monofilament absorbable suture, resorbable plates, or titanium plates and screws. The latter are typically avoided in infants younger than 2 years.
- The skin is closed using absorbable monofilament suture. Drains and dressings are used as necessary.