Advances in Surgical Technique for Childhood Brain Tumors

Michael Medlock, MD FAAP, Georgetown Children's Medical Center

Introduction

I will divide advances in surgical technique over the past several decades into 4 major categories: greater visual magnification, improved aids to navigation, better functional brain mapping, and more precise tumor destruction.

Magnification

The operating neurosurgical microscope has been used since the 1950's when Dr. RMP Donaghy of the University of Vermont was active in the development of microsurgical technique and instrumentation. One of his protégé's, Dr. MG Yasargil of the University of Zurich, Switzerland, further advanced the use of the operating microscope and emphasized the need for careful dissection of the pia arachnoid membranes on the surface of the brain to achieve safer retraction of normal brain and gain better access to pathological tissues.

Endoscopy is the transmission of light to and visual information from the operative field through a small tube. The tube my be flexible or rigid. Endoscopy has also been available since the 1920's when Dr. Walter Dandy of the Johns Hopkins School of Medicine first peered into the cerebral ventricles of his patients. Endoscopic surgery requires the addition of channels for dissection tools, suction, and irrigation. Video images are quickly obscured if the operative field has even a small quantity of blood that cannot be irrigated and removed.

Within the past decade endoscopes have been developed that are much smaller and have more working channels for the passage of surgical instruments. The endoscope is currently used routinely for work inside the cerebral ventricles. Hypo-vascular tumors, i.e. those with fewer blood vessels, such as colloid cysts of the third ventricle, are best approached with this technique. Vascular tumors may be better approached with conventional techniques because of the ability to stop bleeding from the tumor more quickly. The use of the endoscope in more extensive procedures has also been limited by inadequate tools for navigation during internal movement of the endoscope.

Navigation

Stereotaxis is the use of a coordinate system or map to plan a surgical approach. Stereotaxis was initially based on atlases of cadaver brains and assumed a close correlation between the distances in the cadaver brain and all patients for whom the atlas was used. Corrections for the patient's head size was based on ratios of distances between standard anatomical landmarks that could be seen on x-ray studies.

Conventional x-rays are similar to photography in that they both record images on film from a single angle and from a single moment in time. Computerized tomography (CT) involves the mathematical construction of images using many images of the same object taken from many different angles (usually a circle) over a short period of time (usually seconds). This technique allows us to see "planes of section" as if we had cut through the body. Dr. G.N. Hounsfield of England published his classic article "Computerized transverse axial tomography" in the British Journal of Radiology in February 1973, where reported the development of a mathematical technique to construct these planes of section. His first diagnosis was frontal lobe brain tumor. The first whole body CT scanner was constructed by a team led by Dr. Robert Ledley at Georgetown University. This machine now resides in the Smithsonian Institution in Washington, DC.

The accuracy of stereotaxis improved considerably when CT scanners became widely available allowing targets to be chosen based more closely on the individual patients anatomy. The first generation of CT-era stereotactic equipment required the placement of pins in the patient's skull. This was acceptable in children over 3-4 years old whose skulls had hardened but was unacceptable for infants and toddlers whose skulls would deform with rigid fixation.

In the past 10 years we have seen the development of frameless stereotactic systems that do not require rigid frame fixation during imaging. Fiducial or reference marks on the patient's skin are used to align the image with the patient at the time of surgery. These systems are less cumbersome to use and add less time to the surgical procedure than the older systems.

The main problem with current stereotactic systems based on pre-operative imaging is that they represent the patients anatomy before, not during surgery. They cannot account for changes in brain position that occur during surgery with ventricular drainage, osmotic diuresis, brain retraction, and tumor removal. Thus the pre-operative images become useless when making the crucial decisions every surgeon must make at the tumor margin where entry into normal tissue can be catastrophic.

Ultrasound has shown some promise as a real time imaging technique that may better define the tumor margin. Two dimensional techniques are in common use. We at Georgetown University and the National Biomedical Research Foundation are developing 3-dimensional ultrasound techniques for intra-operative use.

Functional Brain Mapping

Single photon emission computed tomography (SPECT), positron emission tomography (PET), and functional magnetic resonance imaging (MRI) all have the proven ability to distinguish metabolically active areas of brain. The resolution of these techniques is on the order of 5-10 mm. They can be helpful in distinguishing between radiation necrosis and tumor recurrence.

Electroencephalography (EEG) is the recording of the brain's electrical activity from scalp electrodes. Electrocorticography (ECoG) is the mapping of cerebral cortical electrical activity directly from the surface of the brain. This permits more precision in identifying cortex adjacent to brain tumors that may be causing seizures and identifying eloquent cortex.

Eloquent cortex is cortex that, if removed, will result in a deficit, i.e. handicap. The most common areas of eloquent cortex are in the left temporal and frontal lobes for speech, bilateral occipital lobes for vision and bilateral parietal lobes for movement and sensation in the arms and legs. Cortical regions that are not eloquent are probably association cortex, i.e. cortex that is repository or library for linkages between ideas. Demonstrating a deficit after removal of association cortex can be difficult if not impossible.

If the child is older and more mature, usually over 9-10 years old, brain surgery can be performed using local anesthesia and intravenous sedation so that speech areas can be mapped with the cortex exposed. This permits a safer tumor resection near eloquent cortex. Electrically evoked responses can also be used map motor cortex or to temporarily produce speech arrest.

Tumor destruction

Unipolar electrical cautery is the destruction tissue using a single electrode with the patient electrically grounded via a grounding pad. The electrical current radiates outward into the surrounding tissue from a single point producing a controlled burn. This was first used by Dr. Harvey Cushing of Harvard Medical School in the 1920's. Bipolar cautery was first used by Dr. James Greenwood of Baylor College of Medicine in 1940. Bipolar coagulation is more precise as the current travels between 2 electrical contacts which also act as tissue grasping forceps.

The laser is a tool that was in vogue among neurosurgeons during the early 1980's. It was touted to be superior for many procedures but was bulky, expensive and cumbersome compared to more conventional techniques. The laser is continues to be used in more limited circumstances and is most helpful when debulking large tumors, particularly in the posterior fossa, where the tumor is firm or hard and tumor movement must be minimized because of the risk to adjacent tissues. The laser is also used for tissue fenestration during endoscopic procedures but may not provide any advantages over conventional cautery via the same approach.

Ultrasonic liquefaction is the decomposition of tumor by direct application of an ultrasonic probe. The ultrasonic energy breaks up the interstitial cellular matrix permitting suction of tumor tissue from the operative field. This instrument may be referred to as the CUSA or the ultrasonic surgical aspirator. Suction and irrigation, as well as a probe for transmission of ultrasonic energy, are usually included in the tip of this hand held instrument.

Conclusion

When presented with claims that a new technique is superior, I recommend asking the following question:

1. How long has the technique or tool been in use?

2. How would the surgery be performed if the tool was not available?

3. Are there published studies that directly compare the technique with older techniques?

I hope this has provided you with an overview of the most important technical aspects of neurosurgery.

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