Stereotactic Radiosurgery of Vascular Malformations

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Chapter 47 Stereotactic Radiosurgery of Vascular Malformations

Chun Po Yen, Chris Cifarelli, David Schlesinger, Jason P. Sheehan

Clinical Pearls

• Regardless of modality, radiosurgical devices achieve the desired characteristics of small fields, fast-dose falloff, and highly accurate targeting through the use of two basic principles: superposition of beams and stereotactic targeting.

• The goal of radiosurgery for arteriovenous malformations is to deliver a high absorbed radiation dose to the nidus, while largely sparing the surrounding normal brain tissue. Endothelial damage followed by subendothelial and intimal-medial proliferation of smooth muscle cells and subsequent cellular degeneration and hyaline transformation eventually obliterate the nidus.

• The reported obliteration rate following radiosurgery ranges between 60% and 90%. The outcomes of radiosurgery for arteriovenous malformations are dose and volume dependent. The role of preradiosurgical embolization remains to be fully defined.

• Complications associated with radiosurgery for arteriovenous malformations include symptomatic radiation-induced brain damage in 8.7% (permanent in 1.8%), late cyst formation in 1.6%, and radiation-induced tumor within 10 years following radiosurgery in 0.7%.

• Radiosurgery has a smaller role for the treatment of dural arteriovenous fistulas owing to its delayed effects. For patients with cavernous malformations, radiosurgery should be used primarily in compelling cases of repeated hemorrhages from a lesion located in an inoperable site.

Intracranial vascular malformations include arteriovenous malformations (AVMs), dural arteriovenous fistulas (dAVFs), cavernous malformations (CMs), venous malformations, and capillary telangiectasias. Venous malformations and capillary telangiectasias usually have a benign clinical course and rarely hemorrhage; thus, treatment is usually not required. In contrast, AVMs, dAVFs, and CMs might present with hemorrhage, seizure, headache, or neurological deficits that necessitate intervention. Stereotactic radiosurgery, microsurgery, and embolization are important tools in the neurosurgical armamentarium for treating patients with vascular malformations. We will focus on the current role of stereotactic radiosurgery in the management of patients with intracranial AVMs, dAVFs, and CMs.

Arteriovenous Malformations

Management

AVMs are congenital cerebrovascular anomalies consisting of poorly developed vascular channels forming the nidi and shunting arterial blood directly to the venous system. The high blood flow and malformed vascular wall pose the risk of rupture and intracerebral hemorrhage. Other clinical symptoms patients might experience include seizure, headache, and neurological deficits via the steal phenomenon or from mass effect.

Three management modalities are employed alone or in combination for the management of cerebral AVMs: microsurgery, embolization, and radiosurgery. Several factors should be weighed when choosing the most appropriate choice of treatment. Microsurgery should be used in superficially located nidi because it eliminates the risk of hemorrhage immediately upon a complete total extirpation. Embolization is most commonly used to reduce the size of large AVMs, making them amenable to subsequent microsurgery or radiosurgery. However, new liquid embolization materials (e.g., Onyx) have increased the chance of obliteration solely by embolization.1,2 Radiosurgery is usually reserved for small to moderate-sized AVMs located in deep or critical areas of the brain.

Role of Radiosurgery

History

The first case of AVM treated with radiotherapy was reported by Magnus.3 In a patient he operated on with an AVM at the motor cortex, he did not attempt surgical removal because of the high possibility of neurological deficits. After decompressive craniectomy, he treated the patient with radium therapy and reported that the patient was seizure-free 2 years after radiotherapy. No imaging or histological studies were available. Cushing and Bailey reported the first successful surgery on an AVM.4 Cushing explored a vascular tumor and felt that the lesion could not be attacked without fatal hemorrhage. The patient was treated with radiotherapy. He reexplored the lesion 3 years later and described that the tangle of pulsating vessels previously encountered was largely thrombosed and transformed into a multitude of small bloodless shreds which could be easily separated from the adjacent normal cortex.

There was intense interest in the use of radiation for AVMs following Cushing’s discovery, but the initial results were not encouraging. Although some studies5,6 with small numbers of cases provided evidence of the possible utility of radiation in the treatment of AVMs, most did not provide imaging or histological proof of the efficacy of radiotherapy. This led to an almost unanimous consensus in the assessment of radiation as being worthless in the management of AVMs.

With the introduction of Gamma Knife, the potential value of irradiation in vascular malformations was reassessed. Contributing factors included an increasing body of evidence that the cells constituting the vessel wall were responsive to ionizing radiation. Long-term angiographic follow-up of a small series of AVMs treated with fractionated conventional radiation by Johnson in the 1950s revealed that the AVMs were obliterated in 45% of cases.7 In 1970, the first radiosurgical treatment for an AVM was performed by Ladislau Steiner and associates at Karolinska Institute in Stockholm. The patient refused surgery, and given the patient’s renal insufficiency, the risk of surgery was considered too high. Although the intention was to deliver focused radiation to the nidus, because only small collimators were available, the feeding arteries were targeted and 25 Gy was given as the prescription dose to the 50% isodose line. On angiography 19 months after the treatment, the feeding vessels were obliterated, and the malformation no longer filled. Subsequently, larger collimators were available and could cover the AVM nidi, and more patients were treated successfully. Today more than 50,000 AVM patients have been treated with Gamma Knife radiosurgery, which has been proved to be a safe and effective treatment alternative for AVMs.

Modality

The goal of radiosurgery for AVMs is to deliver a high absorbed radiation dose to the AVM nidus, typically in a single session, while largely sparing the surrounding brain significant dose and thereby minimizing undesirable effects from the treatment. As the number of modalities for delivering radiosurgery have increased over time, so have the numbers that have been applied to the treatment of AVMs. Reports in the literature exist for AVM treatment with various radiosurgical modalities including the Gamma Knife, isocentrically mounted linear accelerators,8–11 and robotic linear accelerators (CyberKnife).12 Regardless of modality, radiosurgical devices achieve the desired characteristics of small fields, fast-dose falloff, and highly accurate targeting through the use of two basic principles: superposition of beams and stereotactic targeting.

Radiosurgery achieves highly conformal dose distributions by spreading the total energy delivered over a targeted volume, either through the superposition of many small beams on the target or through the use of noncoplanar arcs (where each arc can be thought of as a large number of small beams). The energy of any given single beam is too low to cause a significant biological effect. However, the superposition of all the beams at the target delivers a substantial amount of radiation, sufficient to cause biological changes resulting in the occlusion of the AVM nidus.

The details of beam superposition vary by modality. In the case of the Gamma Knife, 192 (or 201, depending on the model of the unit) individual beams are precisely aimed at a focal point, or isocenter, to achieve this effect. The beams are collimated through individual beam channels. In older units, this was achieved with a combination of internal primary and secondary collimation and external “helmet”-based final collimation. In the case of the newer Perfexion model Gamma Knife, the collimator assembly is housed entirely within the main body of the unit.13

For early linear accelerators adapted for radiosurgery, finely collimated beams were achieved through the use of circular collimator “cones” that could be attached to the accessory tray of the accelerator.14 Many currently available accelerators are equipped with micro-multileaf collimators that can achieve irregularly shaped beams that can more precisely conform to the target morphology.15 Both approaches are often used with a non-coplanar arc technique, which directs the fields at the target while spreading out the overall delivered energy.16

The ability to create a focal, high-dose distribution does little good in itself if there is no way to precisely and accurately aim at the target. This problem is elegantly solved using the principles of stereotaxy. Traditionally in intracranial radiosurgery a rigid frame is fixed to the skull. This frame defines a coordinate system by which any point within the brain can be localized. Fiducial markers, which are visible in stereotactic imaging studies as part of the procedure, are directly related to this coordinate system; thus, the target can be visualized and localized in “stereotactic space.” Accuracy and precision of treatment are thus guaranteed by the precision and accuracy with which the target can be localized and the assurance that this target will not move during a treatment owing to the rigid head frame. More recent innovations in radiosurgery have included frameless stereotaxy.17 In these systems image-guidance plays a greater role both before and during the procedure. Less invasive restraint systems such as thermoplastic masks are used in place of the rigid head frame, and periodic imaging is used to track and correct for patient motion.

Histopathology

Several histological studies have described the changes of irradiated vessels with progressive narrowing and obliteration of the lumen.18 The earliest changes are endothelial damage and endothelial-intimal separation. These are followed by subendothelial and intimal-medial proliferation of smooth muscle cells with elaboration of extracellular matrix components. Cellular degeneration and hyaline transformation of vessel walls follow, and finally the vessels obliterate completely. The above-mentioned histopathological changes are correlated with time after radiosurgery and tend to occur in smaller vessels.

Gamma Knife Radiosurgery

At the University of Virginia, we perform AVM radiosurgery with the Gamma Knife. Patients are evaluated at least 1 day before the Gamma Knife surgery (GKS). Anticonvulsion medication is used for patients already on these medications and for patients with supratentorial AVMs.

The technique of GKS begins with the placement of the Leksell G-Frame on the patient’s head by the neurosurgeon. The Leksell G-Frame consists of a rectangular aluminum base ring to which four aluminum posts are attached. The frame is affixed to the patient through the posts using titanium pins, which are screwed to the outer table of the skull. In our center, we place the frame in the operating room using controlled sedation, local anesthesia, and strict attention to aseptic conditions.

The accuracy of a GKS is ultimately dependent on the visualization of three-dimensional views of the intended target in the brain. Stereotactic biplane angiography is the gold standard for delineating the nidus. Additionally, stereotactic magnetic resonance imaging (MRI) and magnetic resonance angiography source images provide information, especially in the axial plane, which can enhance visualization of the nidus for the purpose of treatment planning.

Treament Planning

Treatment planning is the process of creating a dose distribution that conformally covers the intended target by defining one or more isocenters, or “shots,” which each contribute to the total dose distribution. It is an iterative technique requiring detailed knowledge of neuroanatomy, neuroradiology, the biological effect of single-fraction radiosurgery, and the compromises required to create a treatment that will be effective to obliterate the nidus and at the same time tolerable for the surrounding brain structures. Optimal prescription dose has been described by Steiner and associates within the range of 23 to 25 Gy.19 However, in cases with large nidi or nidi close to critical structures, the dose needed to be adjusted.

Follow-up

The need for adequate and thorough clinical and imaging follow-up cannot be overemphasized. Clinical and imaging responses as well as GKS-related complications and hemorrhage during the latency period should be evaluated following the GKS. We suggest that MRI be performed at 6- to 12-month intervals following radiosurgery until signs of AVM obliteration are evident. When MRI suggests AVM occlusion, follow-up angiography should be performed to confirm complete obliteration of the nidus. Thereafter, patients are advised to have further MRI every 3 to 5 years to rule out delayed adverse effects. To evaluate the outcome and causes of treatment failures, all the images should be analyzed both by neurosurgeons and neuroradiologists interested and experienced in the interpretation of MRI and angiograms.

Outcomes

Following radiosurgery, angiography reveals that hemodynamic changes occur before changes in the size and shape of an AVM.20 First, the flow rate decreases progressively. This may be related to the changes in the sizes of the feeding arteries and outflow veins. The outcome of an AVM following radiosurgery may be a total, subtotal, or partial obliteration of the nidus.

Total obliteration of the AVM after radiosurgery was defined as “complete absence of former nidus, normalization of afferent and efferent vessels, and a normal circulation time on high-quality rapid serial subtracted angiography.”20 Any remaining nidus, regardless of its size, is considered partial obliteration. Subtotal obliteration of an AVM means the angiographic persistence of an early filling draining veins without demonstrable nidus.21 The early filling venous drainage suggests that some shunting persists. Our studies have shown that these subtotally obliterated AVMs have very low risk of hemorrhage in spite of the fact that per definition the AVM nidus is still patent as indicated by the shunting. It should be noted that more than 70% of them went on to obliterate completely without further treatment.21

The reported obliteration rate following radiosurgery ranged between 30% and 92%.10,22–24 One should be cautious in terms of the interpretation of the results owing to the biases injected from different cutoff time and imaging modality used to conclude total obliteration. Studies excluding patients with short follow-up, reporting only patients undergoing angiography, or including MRI as an imaging study to conclude obliteration tend to overestimate the success rate of radiosurgery.23,25,26

Since 1989, a total of 1023 patients with AVMs treated with GKS at the University of Virginia with follow-up for at least 2 years were analyzed (82 patients completely lost to follow-up and 139 patients with follow-up less than 2 years were excluded; an additional 106 patients with large AVMs undergoing partial treatment will be discussed later). There were 523 males and 500 females with a mean age of 34.2 years (range 4 to 82 years). The presenting symptoms leading to the diagnosis of AVMs was hemorrhage in 529, seizure in 237, headache in 133, and neurological deficits in 94. In 30 patients, the AVMs were incidental findings. The locations of the AVMs were in the cerebral hemispheres in 630, basal ganglion in 96, thalamus in 82, corpus callosum in 38, brainstem in 84, cerebellum in 68, and insula in 25 patients. The Spetzler-Martin grading of the AVMs were grade I in 174 (17%) patients, grade II in 328 (32.1%) patients, grade III in 440 (438%) patients, grade IV in 78 (7.6%) patients, and grade V in 3 (0.3%) patients. One hundred twenty-two patients (11.9%) had previous partial resection of the nidi, and 244 patients (23.9%) underwent preradiosurgical embolization. The nidus volume ranged from 0.1 to 33 cm3 (mean 3.5 cm3). The mean prescription dose was 21.1 Gy (range 5-36 Gy), and the mean maximum dose was 39.0 Gy (range 10-60 Gy). The mean number of isocenters was 2.7 (range 1-22).

The mean follow-up after GKS was 80 months. GKS yielded a total angiographic obliteration in 552 (54%) and subtotal obliteration in 42 (4.1%) patients (Fig. 47.1). In 290 (28.3%) patients, the AVMs remained patent and in 139 patients (13.6%) no flow voids were observed on the MRI. The angiographic total obliteration was achieved in 65.2% of patients with nidus less than 3 cm3; 43.8% between 3 and 8 cm3, and 27.6% with nidus volume larger than 8 cm3. Small nidus volume, high prescription dose, and low number of isocenters are predictive of obliteration. Preradiosurgical embolization has a negative effect on obliteration.

image

FIGURE 47.1 A 24-year-old man diagnosed with a left-sided arteriovenous malformation (AVM) at left sensorimotor cortex following a hemorrhage. A and B, Lateral and frontal projections of angiograms. Patient underwent a partial embolization. The nidus obliterated completely 3 years following Gamma Knife surgery (C and D).

Complications

Early Adverse Effects

Patients might experience headache from frame placement and nausea or vomiting from pain or sedative medications. Few patients developed seizure in the immediate post-GKS period; most who did were those with a prior history of seizure.

Radiation-Induced Changes

Radiation-induced change is an increased T2 signal around the AVM seen on MRI (Fig. 47.2). Radiation damage of glial cells, endothelial cell damage followed by breakdown of blood-brain barrier, excessive generation of free radicals, or release of vascular endothelial growth factors have been proposed to explain this imaging finding. The severity of radiation-induced changes on images and associated neurological deficits varies, ranging from asymptomatic, being only a few millimeters of increased T2 signal surrounding the treated nidus to massive brain edema with symptoms and signs of increased intracranial pressure. From our 1500 Gamma Knife procedures performed for AVM patients with follow-up MRI available for analysis, 34.4% of patients developed radiation-induced changes. Among them, 60% had mild (a few millimeters of increased T2 signal surrounding the nidus), 33% had moderate (compression of ventricle and effacement of sulci), and 7% had severe (midline shift) radiation-induced changes. The mean time to the development of radiation-induced changes was 13 months after GKS, and the mean duration of the changes was 22 months. Larger nidus volumes, higher prescription doses, history with preradiosurgical embolization, and nidus without previous hemorrhage were associated with higher risk of radiation-induced changes.

image

FIGURE 47.2 Pre-GKS (Gamma Knife surgery) lateral (A) and frontal (B) projection angiograms of a 52-year-old male patient with a history of seizure. The nidus obliterated 6 years following GKS (C and D). The patient developed radiation-induced changes as shown on the T2-weighted magnetic resonance imaging (MRI) scan 8 months (E) and 2 years (F) after GKS. A cyst formation with increased T2 signal was seen 6 years after GKS (G).

One hundred twenty-two (8.7%) patients developed headache, worsening or new seizures, or neurological deficits associated with radiation-induced changes. Patients with severe radiation-induced changes and nidus at eloquent areas were more likely to develop symptoms. Twenty-six patients (1.8%) had permanent neurological deficits.

Cyst Formation

Cyst formation is a rare complication following GKS. Cysts that develop after resolution of previous hemorrhages or fluid cavities from encephalomalacia after surgeries should not be considered as complications related to GKS. Direct radiation injury to the perilesional brain tissue, increased permeability of the blood-brain barrier with accumulation of the exudative fluid, hemodynamic perturbations during gradual obliteration of the nidus with subsequent ischemic tissue damage, and tissue destruction due to subclinical perilesional hemorrhages have been proposed as the possible mechanisms of cyst formation. From our 1272 patients with follow-up MRI available, we found a total of 20 patients (1.6%) developing a cyst after a mean of 8.1 years after GKS. Four cysts were found in 710 patients with follow-up shorter than 5 years, eight cysts were found in 302 patients with follow-up between 5 and 10 years, and another eight cysts were found in 260 patients with follow-up between 10 and 20 years. Of the 20 patients, 18 had regular MRI follow-up and 14 (78%) of them had radiation-induced changes before the development of cysts. Six patients had large cysts and three of them were symptomatic, requiring surgery. Two patients underwent craniotomy and drainage of the cyst. The cyst wall showed no evidence of neoplasia.

Radiosurgery-Induced Neoplasia

We found two meningiomas from 1333 AVM patients treated with GKS; however, follow-up imaging was performed over a period of at least 10 years in 288 of these patients. If we conservatively estimate that radiosurgery-induced lesions would be evident within a 10-year time interval, then our incidence of radiosurgery-induced neoplasia is 2 in 2880 person-years or 69 in 100,000 person-years. Thus, there is a 0.7% chance that a radiation-induced tumor may develop within 10 years following GKS. This is less than the 1.9% risk detailed by Brada and colleagues,27 but our results encompass a follow-up period of only 10 years. The long latency and relative rarity of these lesions following radiosurgery may defy a conclusive determination of the true incidence.

Specific Applications of Radiosurgery

Incompletely Obliterated Arteriovenous Malformations

Most reported studies state that the risk of hemorrhage persists as long as the AVM nidus is still patent. These data provided the rationale for re-treatment of still-patent AVMs following the initial GKS.

In our experience, 74 males and 66 females with a mean age of 33 years underwent repeat GKS for still-patent AVM nidi following initial GKS from 1989 to 2007. Causes of initial treatment failure included inaccurate nidus definition in 14, failure to fill part of the nidus due to hemodynamic factors in 16, recanalization of embolized AVM compartments in 6, and suboptimal dose (less than 20 Gy) in 23 patients. Nineteen patients had repeat GKS for subtotal obliteration of AVMs. In 62 patients, the AVM failed to obliterate in spite of correct target definition and adequate dose. At the time of re-treatment, the nidus volume ranged from 0.1 to 6.9 cm3 (mean 1.4 cm3) and the mean prescription dose was 20.3 Gy. Clinical follow-up time ranged from 15 to 220 months with a mean of 84.2 months after repeat GKS.

Repeat GKS yielded a total angiographic obliteration in 77 (55%) and subtotal obliteration in 9 (6.4%) patients. In 38 (27.1%) patients, the AVMs remained patent. In 16 patients (11.4%) no flow voids were observed on the MRI. Higher prescription dose, smaller nidus volume, nidi with only superficial venous drainage, and a negative history of prior embolization were significantly associated with increased rate of AVM obliteration. Clinically, 126 patients improved or remained stable and 14 experienced deterioration (8 due to a rebleed, 2 caused by persistent arteriovenous shunting, and 4 related to radiation-induced changes).

In the early 1990s, angiography was the only imaging modality available for nidus definition and treatment planning during AVM radiosurgery. So far, our treatment planning still depends mainly on angiography and the nidus can be fully appreciated with multiple projections and real-time observation of the hemodynamic changes during the procedure of angiography. However, MRI does sometimes provide extra information for the outlining of the nidi and during the treatment planning less normal brain tissue would be included within the prescribed isodose.

We advise repeat GKS in cases with still-patent nidi 3 to 4 years after initial GKS when open surgery or endovascular procedures were expected to yield higher risk of complications than GKS. Using repeat GKS, we achieved a 55% angiographic cure rate. Although radiation-induced changes were slightly higher (39%) than those in patients undergoing only one GKS procedure, only 4 patients (3.6%) developed permanent neurological deficits. Our experience showed that when repeating GKS a dose of at least 20 Gy led to a higher chance of subsequent nidus obliteration (77% versus 47% with prescription dose less than 20 Gy).

Embolization of Arteriovenous Malformations

The effectiveness of partial embolization followed by GKS in the management of relatively large AVMs remains controversial. Small series of cases managed with this combined approach reported diverse results with obliteration rates ranging between 50% and 76%.10,28,29 When comparing the outcome in patients treated with Gamma Knife alone to those with combined embolization and Gamma Knife treatment, recent studies reported less favorable outcome in patients with preradiosurgical embolization.10,30

Between 1989 and 2007, a total number of 217 AVMs with prior partial embolization were treated with radiosurgery at the University of Virginia. There were 107 males (49%) and 110 females (51%). The mean age at the initial GK treatment was 32.8 years. The presenting symptoms were hemorrhages in 93 (42.9%) patients, seizures in 67 (30.9%) patients, headaches in 27 (12.4%) patients, and neurological deficits in 25 (11.5%) patients. In 5 patients the AVMs were incidental findings. Most of the AVMs were embolized with liquid embolics (59.8%) such as NBCA (N-butyl 2-cyanoacrylate) or ethanol. Other embolic materials used were coils (9.4%), silk (1.6%), Onyx (0.8%), or a combination of them. In 167 patients the nidus was compact after the embolization, whereas the angiogram of 50 patients revealed that the nidus was broken apart after the endovascular procedure. The mean volume of the nidus at the time of GKS was 5.1 cm3 (0.02-24.9 cm3). The mean maximum dose of the GKS was 37.2 Gy (range 20-50 Gy), and the mean prescription dose was 19.6 Gy (range 10-28 Gy).

After GKS an angiographically confirmed total obliteration of the AVMs was achieved in 71 patients (27.1%). A total obliteration on MRI confirmed by the absence of flow voids was observed in 26 patients (9.9%). In 157 patients (59.9%) only a partial obliteration could be obtained after a follow-up period of at least 2 years. Eight patients (3.1%) presented with a subtotal obliteration. Comparing the outcome after GKS between embolized and nonembolized AVM patients (obliteration rate 72%), the Kaplan-Meier curves revealed a significant lower obliteration rate (p < 0.001) in patients with pre-GKS embolization.

Twenty-six hemorrhages were recorded during the follow-up period yielding an annual hemorrhage rate of 2.1%. Radiation-induced changes detected on MRI were observed in 94 patients (46%), which were higher than those in patients undergoing GKS alone. Eleven patients developed neurological deficits.

Recanalization of previously embolized parts of the nidus,31 difficulty in nidus delineation following previous embolization,10 and attenuation of radiation dose by embolization materials32 have been proposed to explain the less favorable outcome in patients with pre-GKS embolization. Theoretically, volume reduction following embolization affords a lower chance of GKS-related adverse effect but this expectation was not shown based on our data. Additionally the complications from embolization are not negligible. Therefore, the use of pre-GKS embolization remains problematic and awaits further investigation.

Brainstem Arteriovenous Malformations

Studies have shown that AVMs located in the posterior fossa carry a higher risk of hemorrhage compared to AVMs in other locations.33,34 Furthermore, owing to the critical location in proximity to vital neuronal pathways and nuclei, there is a high risk of morbidity and mortality once the brainstem AVMs rupture. With the advance of microsurgical techniques, extirpation of the AVMs involving the brainstem is feasible but the associated risks are not negligible. Several surgical series have demonstrated a less favorable obliteration rate with a high risk of complications.35–37

Between 1989 and 2007, a total number of 96 patients with AVM nidi mainly located in the brainstem were treated with GKS at our institute. Thirteen cases with a follow-up period shorter than 2 years after GKS were excluded, leaving 83 patients for analysis. There were 53 males and 30 females with a mean age of 33 years (range 6-81 years). The presenting symptoms leading to the diagnosis of AVM were hemorrhage in 53 (63.9%) patients, seizure in 4 (4.8%), headache in 6 (7.2%), cranial nerve palsies in 12 (14.4%), long-tract signs in 3 (3.6%), and hydrocephalus in 2 (2.4%). Three patients were asymptomatic. Nine patients underwent embolization prior to GKS. Incomplete surgical resection was carried out in five patients. One patient had partial resection and embolization before undergoing GKS. The AVMs were located in the midbrain in 43 (51.8%) patients, pons in 29 (34.9%), and medulla oblongata in 11 (13.3%). The maximum diameters of the nidi ranged from 7 to 41 mm (mean 18.6 mm) and the volumes ranged from 0.1 to 8.9 cm3 (mean 1.9 cm3). All nidi had deep venous drainage. The mean prescription dose was 19.8 Gy (range 5-32 Gy) and the mean maximum dose 33.8 Gy (range 10-50 Gy).

Following a single GK procedure, 35 (42.2%) patients still had a residual nidus shown on MRI or angiography. In 7 (10.8%) patients, the last MRI revealed absence of flow voids. In 38 (45.8%) patients, total obliteration was confirmed on follow-up angiography. The interval between GKS and angiographic obliteration ranged from 6 to 148 months (mean 38.3 months). Three (3.6%) patients had a subtotal obliteration.

Eighteen patients had a second GKS for still-patent AVM residuals performed at a mean of 4.4 years (range 2.0-10.1 years) after the initial GK procedures. Two patients had the third GK procedures 7 and 16 years after a failed repeat GKS. Of 18 patients undergoing repeat GKS, 11 achieved a total obliteration based on angiography (including 2 patients who underwent a third GK procedure). Two patients obtained nidus obliteration based on MRI. In 5 patients, the nidus remained patent.

Following one or more GKS, angiography follow-up was available in 68 (82%) patients. A total obliteration was confirmed in 49 (59%) and subtotal obliteration in 3 (3.6%). Twenty-two (26.5%) patients still had patent residual nidus. In 9 (10.8%) patients, obliteration was confirmed on MRI only. Prescription dose greater than 20 Gy (p = 0.037) was significantly associated with increased rate of obliteration.

The clinical follow-up period ranged from 24 to 264 months (mean 100 months). Following GKS, three patients had two and seven patients had one bleeding. In total, 10 patients experienced 13 episodes of hemorrhage in 457 risk-years, yielding an annual hemorrhagic rate of 2.8%. Of these 10 patients with hemorrhage, six had a complete recovery and four had residual neurological deficits. Two patients with persistent AVMs deteriorated clinically presumably due to mass effect or steal phenomenon. In our series, one patient died owing to complications of hemorrhage. One patient whose AVM had obliterated died from disease unrelated to the brainstem AVM.

Radiation-induced changes were observed in 33 of 81 (40.7%) patients who had series MRI follow-up. Twenty-three (28.4%) patients were asymptomatic upon the imaging finding of radiation-induced changes, one (1.2%) presented with headache, and nine (11.1%) developed new or aggravated neurological deficits. Among the patients with neurological deficits, four (4.9%) had a full recovery but five (6.2%) patients still had residual neurological deficits at the last follow-up including one who developed a large cyst 6 years following GKS.

Brainstem AVM is a formidable challenge for neurosurgeons because of its high risk of rupture and significant morbidity and mortality rates associated with the hemorrhages. Adding to the conundrum is the fact that none of the treatment modalities available can eliminate the risk of hemorrhage without a significant risk of neurological deteriorations. GKS is a reasonable treatment option, especially if the nidus is located within the parenchyma of the brainstem. In our series, GKS achieves a 59% rate of complete obliteration. However, one should be cautious that the neurological deficits associated with radiation-induced changes are relatively high.

Arteriovenous Malformations in Pediatric Patients

Although AVMs only account for 1.4% of intracerebral hemorrhage in the adult population, they represent the underlying cause of 20% to 50% of cerebral hemorrhage in pediatric patients.38,39 Studies have shown that pediatric patients have a high cumulative lifetime risk of hemorrhages, and AVMs in the pediatric population also have a high propensity to rupture.39,40 Radiosurgery has been increasingly used for the management of pediatric AVMs following the success in adult patients.

Between 1989 and 2007, 200 AVM patients under 18 years of age were treated with GKS at the University of Virginia. Fourteen cases with follow-up times shorter than 2 years after GKS were excluded, leaving 186 patients for analysis. There were 98 males and 88 females with a mean age of 12.7 years (range 4-18 years). The presenting symptoms leading to the diagnosis AVMs were hemorrhage in 133 (71.5%) patients, seizure in 29 (15.6%) patients, headache in 11 (5.9%) patients, and neurological deficits in 8 (4.2%) patients. Five (2.7%) patients were asymptomatic and the AVMs were an incidental finding. Thirty-eight patients underwent embolization prior to GKS. Incomplete surgical resection was carried out in 24 patients. Five patients had partial resection and embolization before undergoing GKS. One patient had previous proton beam radiotherapy.

The locations of the AVMs were hemispheric in 101 (54.3%) patients, thalamus in 24 (12.9%), basal ganglia in 23 (12.4%), corpus callosum in 9 (4.8%), brainstem in 18 (9.7%), insula/sylvian fissure in 5 (2.7%), and cerebellum in 6 (3.2%). Five patients had coexistent intranidal aneurysms, and seven had perinidal aneurysms. Three patients had non-flow-directed aneurysms. The nidus volumes ranged from 0.1 to 24 cm3 (mean 3.2 cm3). Sixty-two nidi had only superficial venous drainage and 124 had deep venous drainage. The Spetzler-Martin grading at the time of initial GKS was grade I in 23 (12.4%) patients, grade II in 55 (29.6%) patients, grade III in 87 (46.8%) patients, grade IV in 20 (10.8%) patients, and grade V in 1 (0.5%) patient. Forty-one patients had a second GKS for still-patent AVM residuals performed at a mean of 2 to 5 years (range years) after the initial GK procedures. The volumes ranged from 0.2 to 15.9 cm3 (mean 2.3 cm3) at the time of repeat GKS.

The treatment parameters at the initial GKS were as follows: mean prescription dose 21.9 Gy (range 7.5-35 Gy); mean maximum dose 40.1 Gy (range 20-50 Gy). The treatment parameters of the second GKS were as follows: mean prescription dose 20.7 Gy (range 4-27.5 Gy); mean maximum dose 39.6 Gy (range 8-50 Gy).

Following a single GKS, 92 (37.1%) patients still had a residual nidus shown on MRI or angiography. In 15 (8.1%) patients, the last MRI revealed absence of flow voids but patients or parents refused to have an angiography to confirm the obliteration of nidus. In 92 (49.5%) patients, total obliteration was confirmed on follow-up angiography. Ten (5.4%) patients had a subtotal obliteration. Of 41 patients undergoing repeat GKS, 17 achieved a total obliteration based on angiography. Three patients obtained nidus obliteration based on MRI. In 16 patients, the nidus remained patent. Five patients had subtotal obliteration of AVMs.

Following one or more GKS, a total obliteration was confirmed in 109 (58.6%) and subtotal obliteration in 9 (4.8%). Forty-nine (26.3%) patients still had patent residual nidus. In 19 (10.2%) patients, obliteration was confirmed on MRI only. The actuarial angiographic obliteration rate was 34% at 2 years, 46% at 3 years, and 51% at 5 years. In general, the imaging outcome of pediatric patients is similar to that observed in the adult population. A negative history of pre-GKS embolization (p = 0.049) and a high prescription dose (p = 0.001) were significantly associated with increased rate of obliteration.

The clinical follow-up period ranged from 24 to 240 months (mean 98.4 months). Following GKS, seven patients had two bleeding episodes, and 10 patients had one bleeding episode. In total, 17 patients experienced 24 episodes of hemorrhage in 1013 risk-years (assuming patients with completely obliterated AVMs were no longer at risk for hemorrhage), yielding an annual hemorrhagic rate of 2.4%. We do not observe any hemorrhage after angiography concludes a total obliteration. If the 14 patients with follow-up less than 2 years were included, two more hemorrhages in 1016 risk-years yield a hemorrhage rate of 2.6%. The hemorrhage rate was 5.4% per year for the first 2 years and reduced to 0.8% per year from 2 to 5 years after GKS. There were no deaths in our series.

The follow-up MRI period ranged from 6 to 222 months with a mean of 80 months. Radiation-induced changes were observed in 68 of 180 (37.8%) patients who had series MRI follow-up. Fifty-five (30.6%) patients were asymptomatic upon the imaging finding of radiation-induced changes, seven (3.9%) presented with headache, and six (3.3%) developed new or aggravated neurological deficits. Among the patients with neurological deficits, four (2.2%) had a full recovery, but two (1.1%) patients still had residual neurological deficits at the last follow-up. Five patients developed a large cyst following GKS. A 7-year-old boy and a 12-year-old girl each developed a small asymptomatic meningioma 12 and 10 years after GKS.

Only a small series of children went through a systemic psychological test analyzing the cognitive faculties in a long-term follow-up after GKS. However, yearly follow-ups including questioning the parents, the patients, and the referring doctors about the intellectual development and possible cognitive or endocrinological deficits were conducted. According to this information, 95% of the children had normal intellectual development after radiosurgery with satisfactory or good school performance. As adults, they performed from average to excellent and were socially well adjusted. Riva and associates studied patients, ranging in age from 9 to 18 years, treated for AVMs using GKS to record potential effects of radiosurgery on cognitive and neuropsychological performance.41 Tests for general intelligence, nonverbal intelligence, memory and its components, and attention performance were administered to patients and compared with test results of age-matched siblings or first cousins. No statistically significant difference was found between the performance of patients and control subjects in any of the tests administered.

The reported obliteration rate for pediatric AVMs following radiosurgery ranged between 53% and 86%.25,42–45 Some studies had proposed that in pediatric patients the response to radiosurgery seems to be less favorable.25 Hypotheses such as the immature vessels in pediatric patients were more likely to recover from radiation-induced damage and neovascularization in response to radiation have been proposed. Our experience shows comparable result in children compared to adults. Additionally, we observe that radiation-induced damage seems to be more tolerable for kids, suggesting that radiosurgery has a favorable benefit-risk profile in the management of pediatric AVMs. However, the risk of hemorrhage remained in pediatric patients and the development of secondary tumor cannot be overlooked. For pediatric AVMs amenable to surgery, microsurgery should be considered as the first-line treatment.

Large Arteriovenous Malformations

Although satisfactory results in small and moderately sized AVMs following radiosurgery are well documented, reports on the imaging and clinical outcomes in large AVMs are sparse. Few neurosurgeons treated large enough series with appropriate follow-up periods. The main problem with large AVMs is due to the dependence of the obliteration response on dose and volume; this dependency requires a delicate balance in deciding the dose that will be efficient but low enough to avoid adverse neurological deficits.

The following strategies are currently available to treat large AVMs with radiosurgery. First, one can embolize a portion of the AVM and then perform radiosurgery if the nidus shrinks to a size manageable with radiosurgery. However, embolization should effectively shrink the nidus for radiosurgery to achieve good results. Also fragmentation of the nidus into a number of segments should be avoided because that will make the radiosurgical planning difficult and increase the probability of radiosurgery failure. Another strategy involves serial staged radiosurgery to selected volumes of the AVMs. Sirin and associates used staged volumetric radiosurgery in 28 large AVMs.45a Of the 21 patients, seven underwent repeat radiosurgery and were eliminated from outcome analysis. Of the remaining 14 patients, 3 had total obliteration on angiograms, and 4 had no flow voids on MRI but had no follow-up angiography. Four patients had hemorrhages after radiosurgery, resulting in two deaths. Worsened neurological deficits occurred in one patient. The third approach is to treat the whole nidus in one session with low-dose radiosurgery. Pan and associates reported an obliteration rate of 25% for AVMs with volume larger than 15 cm3. The obliteration rate increased to 50% at 50 months’ follow-up. The morbidity rate was 3.3%. Post-treatment hemorrhage occurred in 9.2%.

For the past 20 years, we evaluated a protocol using combined radiosurgery and microsurgery for the management of large AVMs. Radiosurgery was performed for the deep medullary portion of the AVM as a first step. The second step was planned as microsurgical extirpation of the superficial segment if the goal of the first step, obliteration of the deep segment of the AVM, was achieved. However, this goal was achieved in less than 5% of the patients.

The management of large AVMs demonstrates that every treatment has its limitations. In an effort to solve the problems of the management of large AVMs, a cautious approach is warranted pending the development of new techniques and agents for embolization. In very large AVMs perhaps “wait and see” may occasionally be the best management.

Dural Arteriovenous Fistulas

Intracranial dAVFs comprise a unique subset of vascular malformations from the perspectives of etiology and treatment paradigms. Although dAVFs make up approximately 15% of all intracranial vascular malformations, the precise mechanism of formation remains unknown, with leading theories including adjacent venous sinus stasis as well as alterations in local expression of vasogenic factors, such as vascular endothelial growth factor (VEGF) and fibroblastic growth factor (FGF).46,47 Unfortunately, the presence of venous occlusion proximal or distal to the dAVFs is not an absolute requirement, complicating the interpretation of its role. Similarly, changes in expression levels of VEGF and FGF have been found in animal models of dAVFs as well as samples of patients treated surgically, but no causative relationship has been shown to exist between these vasogenic substances and dAVF formation.48–51

From a treatment standpoint, the experience with dAVFs is distinct from that with AVMs. Whereas size and location of AVMs are correlated with the natural history and optimal treatment protocols, several studies have established that the flow dynamics of dAVFs are the most important indicator of the need to treat and modality of choice, be it endovascular, open microneurosurgical, or radiosurgery.52–54 Specifically, the current classification systems for dAVFs, excluding those involving the cavernous sinus, have been defined based on the direction of fistulous flow as well as the presence or absence of cortical venous reflux (Box 47.1). In an attempt to validate these classification systems based on a large single institution sample, Davies and co-workers55 retrospectively analyzed 102 dAVFs in 98 patients focusing on venous anatomy and direction of flow, confirming that the single best predictor of hemorrhage is retrograde leptomeningeal drainage. As the aggressive natural history of lesions with cortical venous reflux differs significantly from lesions without angiographic evidence of cortical venous reflux, early definitive therapy via endovascular procedures or open surgical resection appears to be preferable to radiosurgery as a first-line treatment. Once again, the ionizing radiation delivered via radiosurgery is thought to be an effective agent for decreasing neovascularization in dAVFs, but the time interval needed for the desired effect is too great to justify radiosurgery as a first-line therapy.48,56

BOX 47.1 Classification of Dural Arteriovenous Fistulas

Borden Scale

Type I – Drainage into dural venous sinus or meningeal vein only

Type II – Drainage into dural venous sinus or meningeal vein with cortical venous reflux (CVR)

Type III – CVR only

Cognard Scale

Type I – Drainage into dural venous sinus only with normal antegrade flow

Type IIa – Drainage into dural venous sinus only with retrograde flow

Type IIb – Drainage into dural venous sinus with antegrade flow and CVR

Type IIa + IIb – Drainage into dural venous sinus with retrograde flow and CVR

Type III – CVR only without venous ectasia

Type IV – CVR only with venous ectasia

Type V – Drainage into spinal perimedullary veins

Adapted from: Borden JA, Wu JK, Shucart WA. A proposed classification for spinal and cranial dural arteriovenous fistulous malformations and implications for treatment. J Neurosurg 1995;82:166-179; Cognard C, Gobin YP, Pierot L, et al. Cerebral dural arteriovenous fistulas: clinical and angiographic correlation with a revised classification of venous drainage. Radiology 1995;194:671-680.

Given the relatively low incidence of clinically identified lesions with respect to other types of vascular malformations and the percentage of those diagnosed dAVFs treated with radiosurgery, large clinical series are few in number and published studies report the findings of single institutions.57–60 The long-term analysis of radiosurgery for dAVFs over 25 years at the Karolinska University Hospital in Stockholm, Sweden, included 52 patients treated between 1978 and 2003. The obliteration rate reported in this study was 68% with 16 dAVFs presenting as less aggressive Borden I or Cognard I/IIa lesions. Moreover, 43 of 52 patients received GKS only, with the remaining 9 patients having undergone prior surgical or endovascular treatment.60 In a similar institutional experience at the University of Virginia between 1989 and 2005, 55 patients with dAVFs were treated with GKS, primarily as an adjunct to surgery or embolization. Obliteration rates measured by angiography at 3 years ranged from 54% to 65%, with the 16 patients classified as Borden I lesions (Fig. 47.3). Unlike the Karolinska study, the majority of patients treated at the University of Virginia received GKS as a secondary therapy, with 41 of the 54 patients receiving surgical or endovascular intervention prior to radiosurgery. Regardless of the difference in utilization of radiosurgery as a primary or secondary treatment modality, the results of these long-term studies indicate that GKS is an effective and safe treatment for intracranial dAVFs. From a purely logistic standpoint, the role of GKS may remain a second-tier therapy as the majority of lesions are still initially examined via angiography, facilitating treatment at the time of radiographic diagnosis. Of course, radiosurgery may serve as a first-line therapy for deep lesions that are either inaccessible to endovascular treatment given current microcatheter technology or in situations in which there is low risk of intracranial hemorrhage.

image

FIGURE 47.3 Lateral view angiogram of a 48-year-old man who presented with sudden onset of severe headache without evidence of intracranial hemorrhage. The patient underwent five neuroendovascular attempts at embolization over the 12 months prior to radiosurgery referral. A, Lateral view, right vertebral artery injection. The pretreatment angiogram demonstrated a multihole fistula with sinus drainage involving the torcula (arrow). B, Lateral view, right vertebral artery injection. The post-treatment angiogram performed at 25 months following radiosurgery documents obliteration of the dural arteriovenous fistula.

Cavernous Malformations

Cavernous malformations (CMs) are another type of intracranial vascular malformation. They can also be referred to as cavernous hemangiomas, angiographically occult vascular malformations, or cavernous angiomas. From a histological standpoint, they are discrete and lobulated lesions composed of dilated sinusoidal vascular channels formed by a single layer of endothelial lining and variable layers of fibrous adventitia. CMs do have a hereditary component and may occur as multiple lesions within the same patient. The proportion of CMs that are developmental anomalies as opposed to acquired malformations and the extent to which the etiology of a cavernous malformation alters its natural history remain the subject of much debate.61,62 Annual rates of hemorrhage have been reported to be less than 1% to as high as 7% for those with multiple recent hemorrhages.63,64 Acquired malformations are frequently associated with deep venous anomalies and may point to an underlying pathophysiology associated with venous hypertension.65,66 Radiosurgery and radiation therapy have even been linked to the formation of CMs.67,68

CMs have been associated with headaches, seizures, focal neurological deficits, and intracerebral hemorrhages. In some patients who have symptomatic yet microsurgically inaccessible lesions, radiosurgery has been employed. However, unlike the consensus regarding radiosurgical indications and outcomes for AVMs and dAVFs, opinions vary greatly as to the risk-to-benefit profile for radiosurgical treatment of CMs. Moreover, few proponents of radiosurgery for CM patients advocate treatment unless there has been at least one if not several prior hemorrhages. In addition, as compared to AVMs or dAVFs, histopathological studies of CMs resected after prior radiosurgery show little in the way of protective changes associated with the treatment.69

Proponents of radiosurgery for CMs maintain a reduction in the risk of hemorrhage within 2 years after radiosurgery.70,71 Less favorable assessments of the benefits of radiosurgery for altering the natural history of hemorrhage associated with CMs have been put forth by others.24,72,73 It does seem clear that radiosurgery of CMs is associated with a higher rate of complications than for AVMs or dAVFs. This may be related to radiosensitization of surrounding brain parenchyma by iron deposition from repeated clinical and subclinical hemorrhages associated with CMs.74 If radiosurgery is attempted for a young patient with a surgically inaccessible CM that has repeatedly hemorrhaged (e.g., a deep-seated thalamic lesion or one intrinsic to the brainstem), a lower radiosurgical dose (15 Gy or less) compared to AVMs seems warranted.24,75,76

The results in terms of reduction of CM-associated epilepsy following radiosurgery seem a bit more promising.77 In a randomized, multicenter study of 49 cases, Bartolomei and associates showed that 53% of patients achieved a seizure-free status (Engel’s class I) using a mean marginal dose of 19.2 Gy. Two patients in this cohort experienced severe complications.77 Hsu and colleagues showed Engel’s class I seizure control in 64.3% of patients treated with linear accelerator (linac)–based radiosurgery.78 The improvement in seizures need not be accompanied by a protection from hemorrhage associated with these vascular lesions. Further investigation of the role of radiosurgery for symptomatic CMs in inaccessible or eloquent brain tissue is required. Until then, radiosurgery for CMs must be employed only after careful scrutiny of the risks and benefits likely afforded the patient.

Conclusions

Radiosurgery for AVMs is associated with a high rate of obliteration and acceptable risk of complications. The treatment outcome is dose and volume dependent; small volume and high prescription dose are associated with high chance of obliteration and low incidence of adverse effects. Unless surgery is medically contraindicated, patients with surgically accessible nidus should be managed with surgery because a complete extirpation of the nidus eliminated the risk of hemorrhage immediately. The management of large AVMs remains a great challenge for neurosurgeons. Although embolization has been used for decades to reduce the size of the nidus and make subsequent radiosurgical procedure feasible, the literature has proved that this approach does not improve the obliteration rate of AVMs treated with radiosurgery.

Radiosurgery has a smaller role for the treatment of dAVFs. For patients with CMs, radiosurgery should be used sparingly and primarily in compelling cases of repeated hemorrhages from a CM located in an inoperable site. A multidisciplinary approach and well-defined treatment plan should be set up before the initiation of each patient’s treatment.

Selected Key References

Adler J.R.Jr., Chang S.D., Murphy M.J., et al. The Cyberknife: a frameless robotic system for radiosurgery. Stereotact Funct Neurosurg. 1997;69:124-128.

Hsu P.W., Chang C.N., Tseng C.K., et al. Treatment of epileptogenic cavernomas: surgery versus radiosurgery. Cerebrovasc Dis. 2007;24:116-120.

Karlsson B., Lindquist C., Steiner L. Prediction of obliteration after Gamma Knife surgery for cerebral arteriovenous malformations. Neurosurgery. 1997;40:425-430.

Kondziolka D., Lunsford L.D., Kestle J.R. The natural history of cerebral cavernous malformations. J Neurosurg. 1995;83:820-824.

Pan D.H., Kuo Y.H., Guo W.Y., et al. Gamma Knife surgery for cerebral arteriovenous malformations in children: a 13-year experience. J Neurosurg Pediatr. 2008;1:296-304.

Soderman M., Edner G., Ericson K., et al. Gamma Knife surgery for dural arteriovenous shunts: 25 years of experience. J Neurosurg. 2006;104:867-875.

Please go to expertconsult.com to view the complete list of references.

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