Surgery continues to be its basic therapy and plays a central role in interdisciplinary glioma management. Today's surgeon has a battery of tools in imaging and surgery.
The choice of operative approach depends on the location, size, gross characteristics (extent of demarcation, consistency, and vascularity), probable histology, and radio sensitivity of the tumor, as well as on the neurological status and general condition of the patient, and, of course, the surgeon's technical ability, and available tools.
The goal is maximal tumor removal without creation of new neurological deficits.
Recent advances have made removal of a strategically located tumor possible.
Successful excision of deep seated (thalamus and basal ganglia) gliomas through trans sylvian-trans insular approach, trans callosal approach, and trans cortical-trans ventricular approach, have been reported with image guided surgery.
The principles and techniques of surgery and adjuvant therapy are the same for all intrinsic brain tumors. The sites and types of tumor may necessitate minor modifications.
A meticulous preoperative planning is mandatory. The planning starts at bedside.
Intrinsic tumors, generally, is confined to sulcal boundaries, and do not cross pial barriers.
A proper history and clinical examination, obtained at the bedside, may give a 'clue' about the exact site where the tumor started, and help in tumor excision. A tumor, which appears to be a fronto-parietal on CT/MRI, may be actually a frontal tumor starting in the frontal lobe and growing backwards pushing the central sulcus backwards, and will have significant motor deficit with no sensory deficit at examination. Hence a generous excision may be planned, and will not damage the motor strip.
A thorough pre operative study of all radiological imaging must be reviewed, with special attention to tumor location and neighborhood 'eloquent areas', vascular channels, central sulcus, sylvian fissure ,widened sulci overlying the tumor, and other safe corridors; special reference to be made to the preferred position of the patient at surgery, and aim of the surgery.
"The brain floats in a sea of CSF, with rivers of CSF which allows access to its interior", says Yasergil.
Recent advances in neuroimaging with MRI, MR/CT angiogram, MR spectroscopy, and functional MRI have greatly helped the surgeon decide on the so called 'safe corridors'.
Preoperative corticosteroids provide rapid, often dramatic, symptomatic improvement. Therapeutic effect of steroids is limited to 6-8 weeks. The daily standard dose is 16 to 24 mg for dexamethasone and 80 to 120 mg for methylprednisolone. Dose-related symptomatic improvement with minimal side effects has been shown for up to 96 mg of dexamethasone and 500 mg of methylprednisolone daily. The effects of corticosteroids include, reduction of tumor mass, reduction of peritumoral edema, decrease in permeability of the blood-brain barrier, and increase in cell cycle time.
All patients with a history of seizures should receive anticonvulsants.
The prophylactic use of anticonvulsants is controversial similar to the use pre operative antibiotics.
Position of the patient at surgery is most important.
The site of lesion, the familiarity of the surgeon should be taken into account.
Osmodiuretic therapy is very useful in both an emergency and a peri-operative situation.
A surface marking of sagittal sinus, central sulcus and sylvian fissure will help in their protection at surgery.
Sylvian fissure –
a line drawn from nasion to lambda (approximately corresponds to EOP).
Rolandic fissure –
a line perpendicular to the midpoint of a line drawn from outer margin of the eyebrow and the tragus.
It is then possible to outline the frontal, parietal, and the temporal lobes.
Posterior to a line from the lambda to the posterior mastoid is the occipital lobe.
Individual variations are the rule.
The Procedure The scalp incision should provide sufficient exposure of the relevant cranium, maintain adequate blood supply to the scalp flap and minimize cosmetic deformity. Traditionally, various classical flaps have been described.
Infiltration of 2% xylocaine and adrenaline (1:200,000) mixture, if permitted by the anesthetist, in the connective tissue overlying the galea helps in dissection as well as controlling the blood loss.
Bone removal should preserve blood supply to the bone plate, avoid air sinuses, remove sufficient bone to permit identification of cranial and parenchymal landmarks, achieve brain retraction without compression, and avoid extensive exposure of uninvolved cortex. In the author's experience, a generous craniotomy do not add to benefits.
A dural flap is turned towards the venous sinus closest to the exposure, taking precautions to avoid the cortical veins.
Use of a quality microscope in tumor removal from this stage is the order of the day.
Neuroendoscope assisted microsurgery is becoming popular, and helps in exploring the corners with minimal brain retraction.
'En bloc resection' may be achieved in a surface (lobar) tumor an area that is either clinically silent or already non-functional. Technical refinements in surgery with subpial dissection or transsulcal approaches with brain mapping and awake craniotomy has helped for tumors at eloquent areas.
In lesions over the sulci, the transit vessels must be preserved as they may be supplying the normal brain.
Minicraniotomy and trans-sulcal excision of a low grade glioma
A subcortical tumor can be identified by distortion of neuroanatomical landmarks.
The overlying gyri may be flattened and pale due to paucity of vascularity.
Gentle palpation and /or needling with a brain cannula may help in the absence of surface changes.
A glioblastoma is softer, whereas a low grade glioma would be firmer with respect to the surrounding parenchyma.
Alternatively, delineation of the subcortical dimensions by real-time B-mode ultrasonography, or performing a cortical incision under stereotactic guidance, if available, may be carried out.
Overlying cortex may be removed in 'non eloquent' areas and the tumor may be debulked from inside out.
Piecemeal excision is preferred. The common oncologic principles of surgical excision cannot be applied to brain tumors; clearance margin of 1.5 to 5 cms around the tumor margin cannot be given in most brain tumors in view of risk of severe morbidity. The surgeon is forced to enter the tumor and debulk it before excising the capsule.
The site of the cortical incision and the angle and depth of the transparenchymal approach are determined by the location and shape of the tumor with respect to eloquent cortex, major fiber tracts and deep nuclei as well as bone prominences, dural septa, vascular structures and ventricles.
Intratumoral resection is needed to avoid eloquent areas such as,
Pre and Postcentral gyrus,
Calcarine gyri ilaterally,
Dominant posterior inferior frontal gyrus,
Posterior superior temporal gyrus, Inferior parietal lobule,and
Nondominant superior parietal lobule.
In the rare instance in which an eloquent gyrus must be incised, the incision should be made transversely to its axis. Intratumoral bleeding may be a problem, and will stop only with tumor excision.
The tumor is facilitated by suction, bipolar coagulation and microscissors, and , if available by, ultrasonic aspiration, and laser. Disruption of uninvolved tracts of white matter, or opening the ventricle must be avoided.
Lobectomy in addition to gross total tumor excision, may be useful for large temporal and frontotemporal tumors. Lobectomy with partial tumor excision has no benefit.