Gamma Knife Surgery for AVM
Arteriovenous malformations (AVM) are an abnormal connection between arteries and veins lacking intervening capillary network characterized by a complex, tangled web of abnormal vessels and believed to be congenital although not hereditary. This abnormal tangled web of vessels is called nidus (Latin word for nest), which has abnormally high blood flow resulting from the absence of dampening effect of capillary and has high tendency to rupture resulting in stroke. Lack of capillary network also induces enlargement of especially venous side with subsequent “growth” of AVM resulting in mass effect to surrounding brain parenchyma. AVM also disrupts normal metabolism of surrounding brain tissue by depriving them of adequate circulation (steal phenomena) causing neurological symptoms depending on its location whether superficial in cortex or deep within thalamus, basal ganglia, corpus callosum, insula/sylvian fissure, brainstem, or cerebellum. These mechanisms stated above underlie pathogenesis of symptoms and signs including hemorrhage as the most common presenting symptom in 50–52% of cases, followed by seizure in 27–30%, headache in 5–14%, and progressive neurological deficit in less than 5% [1, 2]. Hemorrhage is associated with high morbidity and mortality as much as 35% and 29%, respectively, that it becomes a primary concern in AVM management. Overall annual risk of hemorrhage on untreated AVM ranges between 2 and 4%, and although multiple studies implicated factors such as nidus size, complexity of AVM morphology, deep brain location or draining vein, associated aneurysm, and younger age to be related to hemorrhagic events, multivariate analysis indicated that only hemorrhage in initial presentation, deep AVM location or deep draining vein, and large nidus size is to be consistently predictive of higher risk of subsequent rupture up to 15% of annual rates [1, 2]. Figure 32.1 shows presentation of AVM hemorrhage on brain computed tomography (CT) scan and angiogram.
32.1 Symptoms and Signs
AVM management includes treatment such as microsurgical resection, Gamma Knife surgery, and embolization either individually or in combination, or just simple clinical observation. The option between doing treatment and observation or among any treatment modality has to be carefully weighed according to risk-benefit ratio and must be balanced against the risk from natural course of the disease. This chapter focuses on Gamma Knife surgery as treatment modality for AVM, which has to be considered as treatment for selected patients.
32.2 Gamma Knife Surgery
Gamma Knife surgery is commonly used to treat AVM that carry high risk of morbidity for microsurgical resection such as Spetzler-Martin grade III or above because it has the advantage of accurately treating small and deep-seated lesions with minimal risk of injury to surrounding structures. It delivers multiple fine beams of gamma rays radiation in such a highly focused manner that it can induce changes in AVM structure over time resulting in cessation of blood flow through the abnormal vessels leading to occlusion. The pathological changes following Gamma Knife surgery for AVM includes endothelial cell damage, proliferation of smooth muscle cells, and elaboration of extracellular collagen that subsequently lead to luminal stenosis and obliteration of nidus. Once complete obliteration of nidus happens, there will be no more blood flow through abnormal vessels. Overall total obliteration rates of AVM following Gamma Knife surgery ranges between 72 and 85% in a typical single-session treatment, and this process usually takes 2–3 years to yield in result. High chance of cure together with low risk of morbidity makes this treatment modality an interesting option for AVM.
The aim of treatment is to accomplish total obliteration of AVM in which situation the risk of hemorrhage is alleviated and the patient is cured. In other situation where partial or subtotal obliteration occur, the risk of hemorrhage still persists and hence subsequent Gamma Knife treatment has to be considered. In predicting outcome after Gamma Knife surgery for AVM, several grading scales were developed such as Virginia Radiosurgery AVM Scale (VRAS) and radiosurgery-based grading AVM Score (RBAS), which includes into consideration history of hemorrhage and patient age, respectively, beside nidus volume and location. These grading scales are considered better in predicting favorable outcome after Gamma Knife surgery, compared to previously and more commonly used Spetzler-Martin Scale .
32.4 Preoperative Preparation
Terminology in Gamma Knife surgery
Volume of lesion that is defined as target. In AVM it represents nidus volume
Point where radiation beams are converged by using combination of different collimators. Isocenters are placed inside target volume
A line indicating distribution volume of same radiation dose. It is three-dimensional and is composed by combination of isocenters. For example, 12 gray (Gy) isodose line demonstrates nidus volume receiving 12 Gy
Isodose line of prescribed effective radiation dose that covers target volume. It is stated in margin or peripheral dose. For example, margin dose of 12 Gy in 50% isodose means that nidus margin is covered by 12 Gy, which is 50% of maximum dose
An index that compare isodose volume (prescription) inside target with total target volume. It demonstrates degree to which target is covered by prescription dose and greater index reflects more prescription dose inside the nidus
An index that compare isodose volume (prescription) inside target with total isodose volume. It demonstrates degree to which prescription dose is specific to target and do not spill to the adjacent neurovascular structure. Greater index reflects less prescription dose outside the nidus
An index comparing prescription isodose with half value of this isodose. It reflects the sharpness of radiation dose falloff around target margin, and smaller index demonstrates steeper dose falloff outside the nidus
32.6 Steps of the Surgery
When the frame is fixed, patients are ready for image acquisition (Fig. 32.5, right). Stereotactic DSA and MRI images are used for target localization. DSA is performed under local anesthesia; anteroposterior (AP) and lateral projections of feeding arteries, nidus, and draining veins are always sought for through both carotid and vertebrobasilar system bilaterally.
This way, every part of nidus can be visualized optimally and unlikely to be missed. It is essential to have optimal nidus visualization for maximum nidus coverage in treatment planning. The best images are then picked out for nidus delineation in order to define target volume, usually at the moment when the contrast agent starts leaving the nidus and filling up draining veins. In order not to miss this particular moment, DSA is set at minimum six frames per second (fps). Too late in taking images may result in overestimation of nidus size while too early may result otherwise. Since DSA images are made in two dimensions, they alone are not adequate to define stereotactic coordinates for accurate target localization in treatment planning that three-dimensional MRI images are needed. We use 1.5 Tesla MRI with thin slice of 1 mm-thickness without gap or overlap and usually without contrast injection. T2-weighted images together with T1 Fast Field Echo (FFE) technique are used to visualize the malformed vessels. Some patients cannot undergo MRI for many clinical reasons including severe claustrophobia and implantation of cardiac pacemakers. In these situations, three-dimensional CT angiogram is used in defining stereotactic coordinates for lesion.
As soon as treatment plan is finalized, the protocol is approved and exported from TPS into control system, which will control the whole operation of Gamma Knife unit during treatment delivery. Patient is then prepared for treatment.
When treatment is completed, patient is removed from treatment couch, and pins are unscrewed during frame removal. Some patients may experience temporary headache after frame removal that dexamethasone and painkiller are routinely given. Finally, patients are observed for 1–2 h before discharge from hospital.
32.7 Surgeon Plan to Handle the Complication
Many factors are related to these complications risk including larger nidus size, higher marginal dose, superficial and eloquent location, as well as less number of draining veins. Special care and considerations in treatment planning must be taken when factors related to increased risks such as larger nidus volume and eloquent locations are encountered. Lower marginal dose is typically given in these situations, and we would consider volume staging in large AVMs (nidus size above 4 cm or volume above 30 cm3).
Surprisingly, ARE is also considered a prognostic factor to complete obliteration. Those who have complete obliteration on long-term follow-up are more common to experience ARE starting earlier within 4–6 months, peaking within 7–12 months after Gamma Knife surgery, and to have more severe symptoms. Among this group of patients, ARE were evident in 74.4% and being significant in terms of symptom in 58.1% . Steroids such as dexamethasone is commonly used to treat this condition.
Gamma Knife surgery doesn’t increase risk of hemorrhage, compared to natural history of AVM. In fact, although debatable, it seems to reduce the risk especially when higher margin dose is administered [6, 7]. It is reasonable to have higher margin dose for small nidus, while for larger ones, dose should be lowered. Risk of cyst formation and neoplasia or malignancy on long-term follow-up is even far less. There are very few reports currently on neoplasia or malignancy induced by Gamma Knife surgery.
32.8 Things to Be Observed and Postoperative Care/Follow-Up
Rarely we find excessive bleeding from pin site that we need to put stitches on. Some patients experience temporary headache or even being nauseous right after frame removal that we recommend observation at least an hour or two before discharge.
Patients are followed up clinically within 2 weeks posttreatment and every 6-month intervals subsequently within 3-year period. MRI examinations were scheduled within 6–12-month interval and DSA within 1–2-year interval. MRI is performed regularly to follow-up change and potential complications, while DSA is to confirm obliteration of AVM. For those who develop clinically significant ARE, intravenous steroid starting 10 mg dexamethasone four times a day is initiated and soon is tapered off as clinical condition improves.
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