Hi, my name is Kaisan Chachana, and I'm a professor of neurosurgery at the Mayo Clinic in Jacksonville, Florida, and today I'll be talking about minimally invasive approaches for deep-seated brain tumors, namely high-grade gliomas, using exoscopes and tubular retractors. My relevant disclosures are that I'm a course lecturer for Nico, who has designed the BrainPath device. In this presentation, I will be mainly talking about accessing and resecting tumors within the deep subcortical space. The subcortical space is the vast area of brain below the cortical surface and includes structures such as the central semiovalley, intraventricular spaces such as the lateral and third ventricle, basal ganglia, thalamus, deep nuclei, as well as white matter tracts including the SLF, arcuate fasciculus, IFOF, ILF, and frontal asthma tract, among others. Access to these deep lesions often requires transgressing uninvolved cortical and white matter structures in lesions that do not present themselves to the cortical surface. Traditional techniques used for accessing these deep-seated lesions can cause significant morbidity. However, advances have allowed us to access and visualize these deep-seated brain tumors to not only access but resect these tumors. The traditional approaches to subcortical brain tumors involve large craniotomies, extensive white matter dissection, and the use of fixed retractor systems to keep the brain parted to allow light to access these deep-seated regions for access and visualization. However, this creates several potential sources of injury which include increased exposed brain from the large craniotomies, the use of retractor blades which can cause tissue ischemia to the underlying cortex and white matter tracts, shear forces from the forces applied during the retractor blades, white matter dissection as the white matter is dissected to enter the tumor, repetitive entry into the resection site, especially when there's tissue creep into the corridor, and all of this is used just to access the lesion before the lesion is even resected. The use of a tubular retractor is not a new concept but has existed at least since the 1980s. Patrick Kelly from Mayo Clinic described the use of these metal tubes that came in different sizes based on the diameter of the tumor. These were very similar in structure and composition to the Metrex tubes that are used for spine surgery today. He described using these for deep-seated brain tumors. When you're accessing these deep-seated brain tumors, it is important to know what cortical and subcortical regions you're encountering. There are several critical cortical regions, not only the language areas, the so-called broken Wernicke's areas, but also the primary motor cortex, the primary sensory cortex, primary visual cortex, the basal ganglia, and thalamus, among others. In addition to the cortical areas, what is becoming more evident is the importance of white matter tracts. White matter tracts can be divided into three categories and include projection, commisural, and association fibers. Projection fibers are those fibers that connect higher brain areas with lower brain areas, the most important being cortical spinal tracts and optic radiations. Commisural fibers are fibers that connect the two hemispheres and include the corpus callosum as well as the anterior and posterior commissure. Association fibers are those fibers that connect adjacent gyri and adjacent lobes within the same hemisphere. You have long association and short association fibers. The short association fibers are your U fibers that connect adjacent gyri, while the long association fibers connect adjacent lobes within the same hemisphere. The most important association fibers are the SLF or superior longitudinal fasciculus and the arcuate fasciculus, the inferior longitudinal fasciculus, the inferior frontal occipital fasciculus, the uncinnate fasciculus, and the frontal aslan tract. Each of these tracts have a different function and have a different function on each hemisphere. Therefore it's important to understand these white matter tracts, where they originate, where they terminate, and what structures they go through. The concept of accessing deep-seated brain tumors is termed minimally invasive parafascicular surgery. The minimally invasive component of this wording is really minimally disruptive. You can still have small skin incisions, small bone openings, small dural openings, but what is mainly important is minimally disrupting the overlying cortex and surrounding white matter tracts when accessing and resecting these deep-seated brain tumors. There are different approaches to accessing the tumors and they include transsocal and transgyral approaches. A transsocal approach means to go through the sulcus, while a transgyral approach means going through the gyrus. There are advantages and disadvantages to each of these. The advantages of the transsocal approach are that you minimize the amount of brain tissue that you have to traverse in order to access the lesion. So in the bottom left image, if you use the sulcus, you decrease the working corridor or the amount of brain tissue you have to go through to access the lesion, as opposed to the dotted blue line, which is the transgyral approach. Also the advantage of the transsocal approach is that you engage subcortical U-fibers, which are typically less important than your long-association fibers. When you're going transgyral, not only do you damage the surrounding cortex, but you can damage the white matter tracts that are below these areas. A transgyral approach, though, has some advantages over the transsocal approach in that you do not engage the sulcal vessels and therefore minimize potential collateral damage to surrounding gyri. Periphysicular means to go parallel with the white matter tracts or fascicles. When you're going through a transsocal approach, you're going perpendicular to the U-fibers, but parallel, typically, with the rejection fibers. When you're going with the transgyral approach, you're going with the white matter tracts, but you're engaging more of these tracts. The goal in these approaches is to minimize tension and shear forces on both the cortex and white matter tract that are uninvolved when you're accessing a deep-seated brain tumor. So in the bottom right image, when you put the tubular retractor and you're engaging the sulcus, you'll engage the U-fibers, which are the yellow lines, as opposed to the association fibers, which are the blue lines. There are a variety of tubular retractors that exist, the most common being the peel-away catheter that come in different sizes, as you can see in the top, which come from 12 French to 17 French. They're called peel-away catheters because you insert them into a cavity and you peel the catheter away to get the determined distance. There's also the vicor of u-site, which is in the bottom left, as well as the necobrain path, which is in the bottom right. These are the most common tubular retractors used for intracranial surgery, but other ones exist. Tubular retractors allow you to access these deep-seated spaces. However, arguably what's more important than accessing these deep-seated spaces is visualizing within these deep-seated corridors. There are a variety of tools that are used to provide light and visualize within these deep-seated spaces and include endoscopes, microscopes, and exoscopes. Endoscopes are cameras or scopes that you insert into body cavities, as you can see in the leftmost pictures. Microscopes in the middle column hover over the surgical field and allow magnification as well as illumination, and then exoscopes are on the far right. The exoscope is similar to the endoscope in that it is a camera, but instead of being held inside the body cavity, it's held outside of the body cavity, hence the term exo. In the top leftmost image, you see an exoscope where the camera is being held by a metoc arm outside of the body cavity. The main advantage of the exoscope is that it is ergonomic. So the tenet of microscopic surgery is that you typically want to be perpendicular to your surgical field. So you're looking straight ahead into the eye pieces and then you're operating straight down. Most of the time this is possible, however, when you're accessing lesions such as a deep-seated brain tumor within the posterior thalamus and trying to access that through a superior parietal lobule approach, it can be quite challenging using the microscope because you either have to extend your back to look upwards or you have to flex the bed upwards and stand significantly high off the ground in order to look down in a perpendicular field. The exoscope can take on these unergonomic positions and maintain your ergonomics. So in the top right image, as you see here, me operating straight ahead using a superior parietal lobule approach where the patient is supine with their head minimally flexed. Also an advantage of the exoscope is that magnification is based on the screen size. So the bigger the screen size you have, the bigger the magnification. When you're using a microscope, as you go to maximum magnification, that is your limit. However, with the exoscope, the bigger the screen, the bigger the magnification. So if you're operating on, let's say, a trans-Sylvian dissection, the Sylvian vessels can be the size of your arm with a 70-inch screen. It also has wider focal points. So when you're operating with a microscope, I typically use the mouthpiece and the foot pedal because you're constantly moving in and out of focus. With the exoscope, there's a wider range, so you have less need for adjusting your focus. The disadvantages are that most exoscopes are two-dimension, so it's similar to endoscopic surgeries that you're operating in 2D rather than three dimensions. There are nowadays three-dimensional exoscopes that make this easier. There's also an indirect surgical field. What that means is you're not actually looking at the exact area you're operating on. You're looking at a screen straight ahead, just like with an endoscope. It's also costly in that a lot of surgeons don't use the exoscope, so it would be something that you have to purchase in addition to the microscope. There are several surgical adjuncts that are critical for operating in these deep-seated spaces. You need high-quality MRI, especially 2-2 sequences, intraoperative navigation, DTI or diffusion tractography imaging, so you know where the white matter tracts are to avoid these white matter tracts if possible. Ultrasound monitoring to help guide extent of resection, ultrasound for not only accessing but resecting these tumors and looking for complications such as a hematoma, fluorescence for high-grade tumors, as well as brain mapping. Peel-away catheters are the least invasive and the least disruptive of the tubular retractors. The advantages of these peel-away catheters are that they have no brain retraction, they have minimal collateral damage because they are so small, and they can be used through either trans-socal or trans-gyral approach. The disadvantages, however, is that in order to visualize down these peel-away catheters, it requires an endoscope. It doesn't work with a microscope or an exoscope because the corridor is so small. Because you're working with an endoscope through a peel-away catheter, it's typically limited to cavity-based or fluid-filled surgical cavities such as intramentricular surgery. There's also a limited degree of freedom, there's a limited number of instruments that can work down these channels, there's a lack of bimanual techniques where it's one instrument at a time rather than bimanual or two instruments at a time, and it can be challenging to achieve hemostasis. An example of using the peel-away catheter is this colloid cyst case here. This is a 22-year-old female with known colloid cysts who presented with increasing headaches. She initially underwent imaging that revealed a colloid cyst that was 3 millimeters but increased its size with 8 millimeters with increasing headaches over a 2-year time span. So in this case, we're using surgical navigation, and we make an incision over the cochris point on the right side, drill a burr hole, open the dura, and insert the peel-away catheter using navigation guidance into the lateral ventricle on the right side. Once we enter the ventricle, we make sure we're on the right side by identifying the relevant anatomy, including the choroid plexus, septal vein, and the thalamus striae vein. We then cauterize the choroid plexus to minimize potential sources of bleeding. Then we use a nicomeriad to open a hole in the colloid cyst to bulk its internal contents to decompress the cyst and open up the foramen munro. The advantage of this nicomeriad is that it has an aperture that's open on one side where the remaining 270 degrees is protected so that it's good to work adjacent to the fornix. Once we enter into the third ventricle, we visualize for any tumor remnants. As you can see in here, the foramen munro is open. You can visualize the mammillary bodies and the aqueduct. And in order to minimize any potential hydrocephalus from obstruction from this ipsilateral foramen munro, we do a septum pellucidotomy by using the Bugbee or monopolar caudary within the septum to identify the contralateral lateral ventricle and expand the opening with the nicomyriad. And here we're using the biting device to open up the septum to increase the aperture between the two ventricles. And then we use the endoscope to enter into the contralateral lateral ventricle and visualize the contralateral foramen munro. In this patient, she was discharged to home on post-operative day two, and the final pathology was a colloid cyst and she's recurrence-free after 36 months. Another type of tubular retractor is the bicore of u-site tubular retractor that you can see in the top right. It's oval shaped and it comes in a variety of sizes, both length as well as width. It provides quote-unquote circumferential retraction. Why I say quote-unquote is that it's really oval shaped, therefore it has a longer working axis in one dimension while the other dimension is narrower. It causes minimal to moderate collateral damage. It's improved degree of freedom, especially along the long axis. So when you're inserting this tubular retractor, you wanna have the long axis parallel with your left and right hand or parallel with your line of view. It can be used with an endoscope or a microscope as well as an exoscope. And there's only slight limitation usable instruments where typically you can use most instruments that you would use for a microscopic surgery. The disadvantages of using this tubular retractor is that it's limited to primarily trans-gyro approaches. You can use this device through a sulcus. However, because of it's larger in size than other tubular retractors, as well as the oval shape and the blunt shape, it makes it very difficult to access sulci unless they're very big sulcus, sulci such as the salivary fissure. It also requires relatively wide cortisectomies. It moderately dilates the brain. I'll show you a video of that and can sever white matter tracks and its difficulty with navigation guidance. A typical way around that is to apply bone wax to the back of the tubular retractor and allow the navigation probe to be locked in place. Here's an example of a case where we use the VICOR ViewSight tubular retractor to access the deep seated brain tumor. So this is a 54 year old male who presented with progressive left-sided arm and leg weakness and imaging showed a very large enhancing right dylamic brain tumor that displaced the internal capsule anteriorly. So here we're accessing the lesion through a superior parietal lobule approach using the tubular retractor or the VICOR ViewSight with microscopic visualization. We do a typically a larger craniotomy or a larger in skin incision in case there's recurrence. The dura is open in cruciate fashion then elevated. Because of its blunt tip, it's very difficult to access the sulci in this region. Therefore we go transgyral. It's held in place with a Greenberg retractor. And as you can see, it's oval shaped where it has one long working access and a shorter working access. As you can see here, we're accessing the lesion, performing white matter dissection and then removing the tumor. We're using the standard sucker as well as standard tumor forceps and standard bipolar. Here we're drawing the tubular retractor. As you can see, the brain is moderately dilated. Besides the VICOR ViewSight tubular retractor, another commonly used tubular retractor is the NECO BrainPath. NECO BrainPath is in top right where it is a circular shaped retractor rather than oval shaped. It has a obturator as well as an inner cannula as opposed to being blunt tip, it's shard tip. And it can be held in place with the shepherd's hook. It provides circumferential retraction because of its circular shape rather than ovular shape. It causes minimal collateral damage. It can be used either transsocial or transgyral. And visualization can be done with either a microscope or an exoscope as well as an endoscope. The disadvantages of this tubular retractor are the limited corridor being only 13.5 millimeters in diameter. Because of this narrow diameter, there's a limitation available instruments that can be used, typically pituitary type instruments, as well as hemostasis. An example of a case where we use this BrainPath tubular retractor is in this 54 year old male who presented with progressive right-sided arm and leg weakness. Here we're accessing also through a superior parietal lobular approach. We're working through the exoscope here as opposed to the microscope. So everything is in two dimensions. We provide cotinoids to protect the brain surfaces and provide countertension. Access this sulcus. As you can see, this is being done with a large screen to allow magnification of these sulcal vessels to appear much larger than they actually are. Once we have freed up the sulcal vessels, then we insert the tube with the sharp tip going through the sulcus into the lesion, provide a working corridor to resect the lesion. And it's important when you're working at these depths that you minimize collateral damage and therefore minimize bipolar cautery to only areas that are bleeding. And as you can see, once we remove the tube, the brain surfaces go back together rather than being dilated. This patient did well after surgery. They were discharged to home on post-operative day three with improved right-sided strength. Final pathology was glioblastoma. They had recurrence at 11 months and then was treated with Bevacizumab and currently remain alive at 14 months. We have applied this approach to deep-seated excisional biopsies where our goal is to not achieve meaningful resection, but to provide tissue for diagnosis. The advantage of this approach over standard needle biopsies is that you provide more tissue for more accurate diagnoses and to minimize the chance of sampling error. And another advantage is that in certain cases, you can debulk enough tumor to provide symptomatic relief or relief from mass effects, such as this bottom right image to relieve hydrocephalus. We have also applied this approach to deep-seated metastatic brain tumors. This allows us to access these deep-seated brain tumors that cause significant mass effect. And despite accessing these deep-seated tumors, we're still able to achieve meaningful resection where the majority of these patients underwent gross total resection. Also important is that it allows them to undergo adjuvant radiation therapy in a timely fashion. This approach is also ideal for deep-seated high-grade gliomas that were typically only allowed needle biopsies in the past. The advantage of using this approach with deep-seated brain tumors is that you can provide more tissue and you can debulk these tumors, which can change the natural history of these lesions. So there are a variety of types of lesions that are ideal for this approach. The first tenant is that you want to be able to readily differentiate tumor from non-tumor. So the lesions that are very obvious from the surrounding parenchyma and white matter are metastatic brain tumors, intracranial hemorrhages, colloid cysts, plus or minus glioblastoma, as well as abscesses. Non-ideal tumors are low-grade gliomas and meningiomas. Low-grade gliomas because you can't differentiate sometimes normal tissue from tumor tissue, and meningiomas because those typically present to the cortical surface. So here are examples. The top left is a metastatic non-small cell lung cancer. Top right is an intracranial hemorrhage. The middle is a colloid cyst. The middle right is a glioblastoma within the visual tracts, and an intracranial abscess within the basal ganglia. Location is also important. So these are ideal for deep-seated locations. Typical locations are those that involve the white matter association tracts, as well as the basal ganglia, thalamus, cerebellar hemisphere, and vermis. Non-ideal tumors are those that are cortically based. So here are examples of a metastatic renal cell carcinoma within the cortical spinal tract. In the top left, a basal ganglia gliosarcoma, a thalamic anaplastic astrocytoma, a cerebellar hemisphere and metastatic non-small cell lung cancer, and a cerebellar vermion intracranial hemorrhage. These tubular retractors are typically reserved for lesions that are below the sulcal boundary. So when you're looking at the coronal MRI, if the tumor is not below the sulcal boundary or the medial edge of the sulcus, then a tubular retractor is not advantageous over conventional approaches. If the lesion is superficial to the sulcal boundary, a trans-gyral approach would just be sufficient to access the lesions. Whereas using a trans-sulcal approach below the sulcal boundary is advantageous for the tubular retractor approach. So ideal lesions are below the sulcus around three centimeters. Those located in deep seated areas such as the basal ganglia, the thalamus, the middle cerebellar peduncles, and obviously non-ideal tumors are superficial lesions. So here are all examples of deep cannulation. So here's a three centimeter right below the sulcal boundary involving the motor cortex. An intracranial abscess that required a five centimeter trajectory. Middle cerebellar peduncle cavernoma that was seven centimeters. And a basal ganglia intracranial hemorrhage that was 8.5 centimeters at its most posterior aspect. The size abnormality is typically limited to around 50 millimeters. This number comes from the fact that most tubular retractors are around 13.5 millimeters and you can toggle this tube in one diameter in each direction. And therefore you multiply the two by three, you get around 50 millimeters. However, this is relative because if there's a deeper lesion, the arc or the degree of movement of the tubular retractor is larger and therefore the larger lesion can be resected that's deeper as opposed to a more superficial lesion. In addition, softer lesions can be larger because they're easier to resect than harder lesions. So here are all examples of different size tumors that were resected using a tubular retractor. Here's a 1.1 centimeter glioblastoma in the top right is a 3.7 low grade astrocytoma, a four centimeter glioblastoma in the middle left, a 4.6 centimeter colloid cysts in the middle right, a 6.0 centimeter in the bottom left that was resected through two tubular retractors, both from a right frontal and a left frontal approach. Also experience is very important when using these tubular retractors. The most challenging lesions are those that are within the ventricle. The reason why it's difficult to achieve hemostasis during this, when you're working in a parenchyma, the parenchyma limits where the bleeding can occur, which is up the tube. However, when you're in the ventricle, it can escape the tube and go within the ventricular system. So the ventricle is probably the most challenging place to use these tubular retractors. Also, obviously hemorrhagic lesions, such as your arteriovenous malformations and your hemangioblastomas, as well as thalamic lesions because of the lack of forgiveness in this location. So here are some of my most challenging cases that I've had difficulty with. Here's an interventricular renal cell carcinoma that we achieved a subtotal resection, but had some thalamic injury just because of the bleeding. It was very difficult to control. A cerebellar hemangioblastoma that went well, but it's difficult to get around this lesion to stay outside of the lesion within the parenchyma. And then a thalamic anaplastic astrocytoma that crossed to the other side. In conclusion, oncofunctional balance is critical. What that means is to achieve extensive resection, but maintaining function. It is important to preserve as much cortical and subcortical structures as possible, even outside of the known eloquent structures. Tubular retractors can allow safe access and resection corridors to deep-seated brain tumors. And visualization is just as critical as accessing these lesions because you have to visualize in order to resect safely. And exoscopes may provide some advantages over traditional surgical microscopes. Thank you for this opportunity.

minimally invasive approaches

deep-seated brain tumors

high-grade gliomas

tubular retractors

exoscopes

subcortical space