Gamma Knife Radiosurgery

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CHAPTER 256 Gamma Knife Radiosurgery

Ajay Niranjan, Josef Novotny, Jr., Hideyuki Kano, Douglas Kondziolka, L. Dade Lunsford, John C. Flickinger

Professor Lars Leksell first coupled an orthovoltage x-ray tube with his first-generation guiding device to focus radiation on the gasserian ganglion for the treatment of facial pain. He subsequently investigated cross-fired protons, as well as x-rays, from an early-generation linear accelerator for radiosurgery. In the 1960s, he became dissatisfied with the cumbersome nature of cross-fired proton beams and the poor reliability and wobble of then existing linear accelerators. Leksell and Larsson finally selected cobalt 60 as the ideal photon radiation source and developed the Gamma Knife.1 They placed 179 60Co sources in a hemispherical array so that all gamma rays (radiation from the decay of 60Co) focused to a single point, thereby producing cumulative radiation isocenters of variable volume depending on the beam’s diameter. The first Gamma Knife created a discoid-shaped lesion suitable for neurosurgical treatment of movement disorders and intractable pain.

Clinical work with the Gamma Knife began in 1967.2 In the early 1970s, a series of surgical pioneers at the Karolinska Hospital, Stockholm, began to use a re-engineered Gamma Knife (which created an oblate spheroidal isocenter) for the treatment of intracranial tumors and vascular malformations.3–6 Units 3 and 4 were placed in Buenos Aires and Sheffield, England, in the early 1980s. Lunsford and colleagues introduced the first clinical 201-source Gamma Knife unit to North America (the fifth gamma unit worldwide). Lunsford and associates first performed Gamma Knife radiosurgery in August 1987 at University of Pittsburgh Medical Center.7 In the United States, based on the available published literature, arteriovenous malformations (AVMs) and skull base tumors that failed other treatments were considered the initial indications for radiosurgery. A cautious approach was adopted while waiting for increased scientific documentation. The encouraging results of radiosurgery for benign tumors8 and vascular malformations9 led to an exponential rise in radiosurgery cases and sales of radiosurgical units (Tables 256-1 and 256-2). In recent years, metastatic brain tumors have become the most common indication for radiosurgery. Brain metastases now constitute 30% to 50% of radiosurgery cases at busy centers.10

TABLE 256-1 Numbers of Active Gamma Knife Units Worldwide by December 2006

CONTINENT COUNTRY ACTIVE UNITS
Asia China 17
Hong Kong 1
India 5
Japan 51
Korea 11
Philippines 1
Singapore 1
Taiwan 6
Thailand 1
Vietnam 1
North America Canada 4
Mexico 2
United States 115
Europe and Middle East Austria 2
Belgium 1
Croatia 1
Czech Republic 1
Egypt 2
France 3
Germany 5
Greece 1
Iran 1
Italy 5
Jordan 1
The Netherlands 1
Norway 1
Romania 1
Spain 1
Sweden 2
Switzerland 1
Turkey 3
United Kingdom 3
Latin America Argentina 1
Brazil 1
Total units   257

TABLE 256-2 Brain Disorders Treated Worldwide With Gamma Knife Radiosurgery by December 2006

BRAIN DISORDER INDICATIONS NUMBER OF PATIENTS TREATED
Vascular disorders Arteriovenous malformation 48,407
Aneurysm 270
Cavernous malformation 1,887
Other vascular disorders 3,777
Benign tumors Vestibular schwannoma 36,843
Trigeminal schwannoma 2,312
Other schwannomas 1,005
Meningioma 49,558
Pituitary adenoma 31,901
Pineal region tumor 3,150
Craniopharyngioma 3,397
Hemangioblastoma 1,656
Hemangiopericytoma 946
Chordoma 1,619
Glomus tumor 1,107
Other benign tumors 3,490
Malignant tumors Glial tumors (grades I-II) 2,169
Glial tumors (grades III-IV) 23,610
Metastatic tumor 141,210
Chondrosarcoma 520
Nasopharyngeal carcinoma 1,277
Other malignant tumors 5,609
Functional targets Trigeminal neuralgia 25,198
Parkinson’s disease 1,309
Pain 566
Epilepsy 2,243
Obsessive-compulsive disorder 140
Other functional targets 867
Ocular disorders Uveal melanoma 1,354
Glaucoma 210
Other ocular disorders 65
Total indications   397,672

Evolution of the Gamma Knife

Models A, B, and C

The Gamma Knife has evolved steadily since 1967.11–13 In the first models (Model U or A), 201 cobalt sources were arranged in hemispherical array. However, these units present challenging cobalt 60 loading and reloading issues. To eliminate this problem, the unit was redesigned so that sources were arranged in a circular (O-ring) configuration (Models B, C, and 4C). Gamma Knife radiosurgery usually involves multiple isocenters with different beam diameters to achieve a treatment plan that conforms to the irregular three-dimensional (3D) volumes of most lesions. The total number of isocenters may vary depending on the size, shape, and location of the target. Each isocenter has a set of three x, y, and z stereotactic coordinates corresponding to its location in 3D space as defined with a rigidly fixed skull stereotactic frame. In terms of actual dose delivery, this means several changes in the patient’s head position within the helmet. In 1999 the Model C Gamma Knife was introduced. The first Model C in the United States was installed at the University of Pittsburgh Medical Center in March 2000. This technology combined advances in dose planning with robotic engineering and used a submillimeter-accuracy automated positioning system (APS). It obviated the need to manually adjust each set of coordinates in a multiple-isocenter plan. The robotic positioning system moved the patient’s head to the target coordinates defined in the treatment plan. The robot eliminated the time spent removing the patient from the helmet, setting the new coordinates for each isocenter, and repositioning the patient in the helmet. This advance significantly reduced the total time spent to complete the treatment and also increased accuracy and safety. Because treatment time was shortened, a precise 3D plan could be generated by using multiple smaller beams to enhance volumetric conformality. Such an approach results in a steeper dose falloff extending beyond the target (higher selectivity). Other features of the Model C unit included an integral helmet changer, dedicated helmet installation trolleys, and color-coded collimators.14–23 In 2005, the fourth-generation Leksell Gamma Knife (LGK) Model 4C was introduced. The first unit was installed at the University of Pittsburgh in January 2005. Model 4C was equipped with enhancements designed to improve workflow, increase accuracy, and provide integrated imaging capabilities. The integrated imaging, powered by Leksell GammaPlan (LGP), offered the ability to fuse images from multiple sources. These images can also be exported to a CD-ROM, so the referring physician could receive preoperative or postoperative images for reference and follow-up. The planning information could be viewed on both sides of the treatment couch. The helmet changer and robotic APS were faster and reduced total treatment time.

Leksell Gamma Knife PERFEXION

In 2002, Elekta Instrument AB, Stockholm, assembled a group of experts, including neurosurgeons, radiation oncologists, medical physicists, and engineers, who were assigned the task of defining specifications for the new LGK system. The group agreed on five critical features for the future system: (1) best dosimetry performance, (2) unlimited cranial reach, (3) best radiation protection for patient and staff, (4) full automation of the treatment process, and (5) patient and staff comfort. As a result of this development, a new system eventually called the LGK PERFEXION was officially introduced in 2006 (Fig. 256-1).24 The radiation unit was redesigned with beam geometry entirely different from that of the previous Gamma Knife Models U, B, C, and 4C. A total of 192 60Co sources were arranged in a cylindrical configuration in five concentric rings. This differed substantially from the previous hemispherical arrangements and resulted in a different source-to-focus distance for each ring that varied from 374 to 433 mm. The primary and secondary collimators were replaced by a single large 120-mm-thick tungsten collimator array ring (Fig. 256-2). Consequently, no external collimator helmets were needed for the PERFEXION system.

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FIGURE 256-1 Leksell Gamma Knife PERFEXION.

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FIGURE 256-2 Leksell Gamma Knife PERFEXION radiation unit and collimator system. A, Cross section of the Leksell Gamma Knife PERFEXION radiation unit. B, Detailed view of sectors. Each sector holds 24 60Co sources and can be moved independently of other sectors in the desired position to define collimator size or block beams. C, Sector position in a 4-mm collimator. D, Sector position in an 8-mm collimator. E, Sector position in a 16-mm collimator.

The range of collimators (beam size) was also changed from previous Gamma Knife models. Only three collimators are available for the PERFEXION system. The 4- and 8-mm collimators remain, but the 14- and 18-mm collimators were replaced with a 16-mm collimator. The tungsten collimator array is subdivided into eight identical but independent sectors, each containing 72 collimators (24 collimators for 4 mm, 24 collimators for 8 mm, and 24 collimators for 16 mm). Collimator size for each sector is changed automatically by moving 24 sources over the selected collimator set. A sector with 24 sources can be moved independently of other sectors to obtain five different positions: (1) home position when the system is standby, (2) 4-mm collimator, (3) 8-mm collimator, (4) 16-mm collimator, and (5) sector-off position, defined as a position between the 4- and 8-mm collimators that provides blocking of all 24 beams for the sector (see Fig. 256-2). Sector movement is performed by servo-controlled motors with linear scales located at the rear of the radiation unit.

Treatment volume has been increased by more than 300% in comparison to previous models. However, because of a better collimator system (120-mm tungsten ring), the average distance from the cobalt source to the focus is very close to that of previous models, which results in similar output between the 18-mm collimator of previous models and the 16-mm internal collimator of the PERFEXION unit. The enhanced treatment volume (more than 3 times) allows greater mechanical treatment range in the x/y/z dimensions. The mechanical range in x/y/z dimension is 160/180/220 for the PERFEXION system as opposed to 100/120/165 for other Gamma Knife models. As a result, PERFEXION has unlimited cranial reach and is ideal for the treatment of multiple brain metastases because multiple tumors at different locations can be treated without any difficulty in terms of collisions with the collimator system.

The APS responsible for setting the stereotactic coordinates was replaced by the bed patient positioning system (PPS). Instead of patient head movement in the Model C and 4C units, the whole patient couch now moves into preselected stereotactic coordinates in the PERFEXION model. This provides better patient comfort and facilitates completion of treatment in just a single run. Docking of the patient into the PPS is done by means of an adaptor that attaches to the standard stereotactic Leksell G frame with three clips. The adapter is then directly docked to the PPS (Fig. 256-3). The patient can be locked into three different neck positions, gamma angles of 70 (extension), 90 (neutral), or 110 (flexion) degrees. The gamma angle is the only treatment parameter that requires manual setup. The PPS replaces the APS and manual trunnion stereotactic coordinate setup and has repeatable accuracy in excess of 0.05 mm.

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FIGURE 256-3 The Leksell stereotactic frame is docked by means of a frame adapter in the Leksell Gamma Knife PERFEXION. A, Frame adapter attached to the Leksell G frame. B, The adapter is then directly docked to the PERFEXION patient positioning system.

The entirely redesigned hardware of PERFEXION also has a significant impact on the treatment planning performed by the LGP PFX, which is a new version of the LGP running on a personal computer platform with a Linux operating system. In principle, there are three possible approaches for treatment planning: (1) the classic approach using a combination of 4-, 8-, and 16-mm isocenters (shots), as has been used for other models; (2) the use of composite shots containing a combination of 4-, 8-, and 16-mm or blocked sectors; and (3) the use of dynamic shaping, which automatically blocks selected sectors to protect volumes defined as critical structures (Fig. 256-4). The most revolutionary change in treatment planning is the ability to generate isocenters composed of different beam diameters. Such composite shots can optimize dose distribution shapes for each individual shot (see Fig. 256-4). The setup for any sectors, combination of different collimators, or blocking is free of time penalty because all sector position changes are done automatically and take less than 5 seconds.

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FIGURE 256-4 Example of the treatment plan with composite shots for the Leksell Gamma Knife PERFEXION. A, Dynamic shaping technique to protect critical structures (brainstem in this example). B, Use of composite shots to shape the dose distribution.

The new PERFEXION system provides better patient and staff shielding. Sectors are always in the off position (blocked) during patient transportation in the treatment position, transition into new stereotactic coordinates, pause, or emergency interrupt. This results in significantly (about 5 to 10 times) reduced extracranial radiation delivered to the patient in comparison to Models B and C. Because the PERFEXION system is fully automatic, it provides high efficiency during treatment, especially for multiple brain metastases. Our preliminary comparison study showed that when compared with other systems, an average 1.5 to 2.0 hours is saved in treating a patient with 10 brain lesions. The new LGK PERFEXION provides excellent dosimetry performance, unlimited cranial reach, enhanced radiation protection for the patient and staff, full automation of the treatment process, and better patient and staff comfort in comparison to previous models (Table 256-3). The PERFEXION system certainly has the potential to increase the spectrum of treatable indications, including multiple brain metastases, upper cervical spine lesions, and pathologies of the cranial base or head and neck. However, applications such as the upper cervical spine will require the development of a special fixation device and new dose-planning algorithm.

TABLE 256-3 Comparison of the Leksell Gamma Knife PERFEXION with Other Gamma Knife Models

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The Radiosurgical Procedure

The following are the basic steps for Gamma Knife radiosurgery with the LGK 4C and PERFEXION:

1 Daily quality assurance of the radiosurgical system
2 Application of the stereotactic guiding device to the patient’s head
3 Frame adaptor and frame cap fitting check
4 Stereotactic brain imaging with magnetic resonance imaging (MRI), computed tomography (CT), angiography, or any combination of these modalities
5 Quality assurance of images
6 Determination of the target volume or volumes
7 Conformal radiosurgical dose planning by the radiosurgery team
8 Stereotactic delivery of radiation to the target volume by positioning the patient’s head inside a collimator system
9 Removal of the stereotactic guiding device

Daily Quality Assurance

Leksell Gamma Knife Model 4C

Gamma Knife quality assurance testing is performed by an authorized medical physicist every morning. The purpose of daily quality assurance is to ensure proper functioning of the system in standard treatment conditions and verification of all safety and emergency functions. The medical physicist ensures that all system tests required by Nuclear Regulatory Commission regulations are performed and the systems are functioning properly. These tests include the permanently mounted radiation monitor inside the treatment room and its remote indicator, handheld radiation monitor, patient viewing and communication systems, door interlock, timer termination of exposure, treatment pause and emergency stops, test of all required beam status indicators and alarm indicators, availability of release rods for emergency removal of a patient, and function of the helmet hoist used for collimator helmet exchange. Dummy runs simulating the standard treatment procedure are performed along with a test of APS accuracy. Apart from daily quality assurance tests, monthly (including mainly output measurement, timer accuracy, and linearity and constancy checks) and annual quality assurance (including mainly a focus precision check) are also performed in addition to preventive maintenance of the LGK.

Leksell Gamma Knife PERFEXION

Daily quality assurance for the PERFEXION system is very similar to that for the 4C system as described earlier. These tests include the permanently mounted radiation monitor inside the treatment room and its remote indicator, handheld radiation monitor, patient viewing and communication systems, door interlock, timer termination of exposure, treatment pause and emergency stops, and a test of all required beam status indicators and alarm indicators. Dummy runs simulating the standard treatment procedure are performed. These runs include a sector-positioning check to verify proper functioning of the automatic collimator setup. Patient docking device function is tested together with overall system accuracy (including the PPS and sector positioning) with the diode test tool and a focus precision check. Monthly and annual quality assurance is similar to that for the 4C system.

Application of the Stereotactic Guiding Device

For Gamma Knife radiosurgery, appropriate stereotactic frame placement is the initial critical aspect of the procedure. Before frame placement, the radiosurgery team should review the preoperative images and discuss the optimal frame placement strategy. The preoperative images should be kept in plain view while applying the head frame. Effort should be made to keep the lesion as close to the center of the frame as possible. The possibility of collision by the frame base ring, the post/pin assembly, or the patient’s head with the collimator helmet during treatment should also be considered before frame application. Steps to avoid possible collision should be taken during frame placement. The risk of collision is minimal with the LGK PERFEXION, and head frame placement is generally central because shifting of the frame is not needed.

Strategies for Optimal Frame Placement for the Leksell Gamma Knife Model 4C

To target lower lesions (skull base tumors, acoustic tumors, cerebellar metastatic lesions in general), the frame is positioned lower by placing the ear bars in the top holes of the earpieces on the Leksell G frame. For higher lesions (sagittal sinus meningioma, metastases high in the frontal or parietal lobe), the frame is positioned higher by placing the ear bars in the bottom holes of the earpieces. For anterior targets (anterior frontal lobe tumors, cavernous sinus tumors, sellar lesions and lesions anterior to the sella, anterior temporal lesions), the frame is shifted forward by placing the earpieces posteriorly on the base ring of the frame. The posterior edge of the earpiece is kept at 75 to 90 mm on the y-dimension of the head frame (instead of 95 to 100), depending on the shift that is needed to bring the lesion closest to the center of the frame. For anterior targets, short posterior posts are preferred to avoid collision of the posterior post/pin assembly with the collimator helmet. To target posterior lesions (occipital lobe tumors, transverse sinus tumors, cerebellar lesions), the frame is shifted backward by positioning the earpieces forward. The posterior edge of the earpieces is kept at 110 to 125 mm instead of 95 to 100 mm. The anterior posts are positioned as low as possible on the supraorbital region to avoid collision of the frontal post/pin assembly with the collimator helmet. For radiosurgical planning, a higher gamma angle (120 to 140 degrees) is used if a collision is detected at the default angle of 90 degrees. To reach lateral targets (lateral metastases, convexity tumors, far lateral tumors), the frame is shifted laterally (right or left) toward the lesion. While shifting the frame laterally, it is important to ensure that enough space is available on the contralateral side to allow positioning of the fiducial box on the base ring of the frame. The MRI or CT fiducial should be tried on the frame before sending the patient to the MRI unit. If the fiducial box does not fit on the frame because of excessive shifting of the frame, the frame will have to be repositioned.

Frame Adaptor and Frame Cap Fitting Check for PERFEXION Radiosurgery

Checking that the frame adaptor fits on the patient’s head frame is an important step. If the frame is shifted too anteriorly and the back of the head ring is too close to the neck, the adaptor may not fit and consequently treatment cannot be carried out. Tight fitting of the adaptor may cause neck discomfort, especially during a long treatment.

The frame cap check provides information about the geometry of all stereotactic frame parts, including posts and screws, as well as information about patient head geometry in relation to the treatment planning system. This information is needed for prediction of potential collisions or close contact with the Gamma Knife collimator system. If the frame cap does not fit, exact post and screw geometry measurements are required to be entered into the treatment planning system to avoid the possibility of potential collision.

Techniques of Stereotactic Imaging

Aside from frame application, the next most important aspect of radiosurgery is accurate imaging of the target. MRI is the preferred imaging modality, but CT is used when MRI is not possible. Angiography is used in conjunction with MRI for AVM radiosurgery.

Stereotactic Magnetic Resonance Imaging

The highlights of stereotactic imaging include optimal contrast between normal and abnormal tissues, in addition to high spatial resolution, short scan time, and thin slices so that accurate target localization can be achieved. The use of MRI in stereotactic planning has enhanced accurate targeting of lesions that are not usually adequately defined by any other imaging modalities. Some physicians prefer fusion of MRI and CT for stereotactic guidance because they believe that with certain type of scanners, geometric distortion may affect the accuracy of target localization in MRI. For the initial 2 years, we used both MRI and CT for stereotactic planning. Significant target coordinate differences were not observed with the Leksell stereotactic system. Since 1993, 1.5-T MRI has been used for stereotactic radiosurgical planning in almost all eligible patients. In addition, AVMs are imaged by biplane angiography.

At our institution, a high-resolution, gadolinium-enhanced, 3D localizer (T2* images) image sequence is used first to localize the tumor in axial, sagittal, and coronal images. This sequence (3-mm-thick slices 2 mm apart) takes just 45 seconds for 11 axial, 11 sagittal, and 11 coronal slices. With axial images, the fiducials can be measured and compared with the opposite side to exclude the possibility of MRI artifacts and confirm that there is no angulation or head tilt. Alternatively, T1-weighted sagittal scout images (3-mm-thick slices 1 mm apart) using a spin echo pulse sequence can be obtained for localization of lesions. The average time for this sequence is approximately 1.5 minutes. For stereotactic imaging of most lesions, 3D volume acquisition using a fast spoiled gradient recalled acquisition in steady state (GRASS) sequence with a 512 × 256 matrix and 2 NEX (number of excitations) covering the entire lesion and surrounding critical structures is preferred. To define the radiosurgical target, this volume is displayed as 1- or 1.5-mm-thick axial slices. The field of view is kept at 25 × 25 cm to visualize all fiducials. The approximate imaging time for this sequence is 6 to 8 minutes.

We generally prefer a 3D spoiled GRASS sequence for most lesions. Extra sequences are performed when more information is needed. An additional 3D T2 sequence can be used for vestibular schwannomas to visualize the cochlea. Pituitary lesions are particularly difficult to image, especially in patients who have previously undergone surgery. A half dose of paramagnetic contrast material is usually given to image pituitary adenomas. For residual pituitary tumors after transsphenoidal resection, a fat suppression spoiled gradient (SPGR) sequence is recommended to differentiate tumor from the fat packed in the resection cavity. For cavernous malformations, additional variable echo multiplanar (VEMP) imaging is obtained to define the hemosiderin rim. For thalamotomy planning, an extra fast inversion recovery sequence is performed to differentiate basal ganglia from white matter tracts. Patients with brain metastases receive a double dose of contrast agent, and the entire brain is imaged in 2-mm slices to identify all the lesions. Before removing patients from the MRI scanner, the images must be checked for accuracy.

Stereotactic Computed Tomographic Imaging

When using CT instead of MRI, it is advisable to use short posterior posts to avoid artifacts from the posts and pins. Care should also be taken in deciding the optimal place for the pins because they cause artifacts on CT. Effort should be made to keep the lesion away from the pin artifacts. With modern CT scanners, 1- or 2-mm-thick slices (depending on the size of the lesion) without any gap can be obtained in just minutes. Before removing the patient from the CT table, accuracy checks are performed to make sure that the images would be accepted by the planning system and the lesions have coordinates that are achievable in the Gamma Knife unit.

Stereotactic Angiography

Angiography is the “gold standard” for planning AVM radiosurgery. It should be used in conjunction with MRI or CT to provide the third imaging dimension. The technique of angiography differs slightly from conventional digital angiography in that the stereotactic angiographic images are used not only for definition of the AVM nidus but also to guide radiation to the target. Orthogonal images (instead of oblique or rotated) are preferred but are not necessary. For AVM nidi that are not properly visualized in orthogonal planes, rotation of up to 10 degrees in two dimensions or aspects can be performed without compromising the accuracy of radiation delivery. Before removing the angiography catheter, the images should be reviewed to make sure that all the fiducials are seen on the images. Digital subtraction techniques, despite the potential for radial distortion error, provide satisfactory spatial accuracy.

Quality Assurance of Images

Regular quality control checks of the MRI unit are performed to maintain accuracy of images. With a properly shimmed magnet, regular servicing, and strict quality assurance of the unit and images, MRI provides high-resolution images with accurate target localization. A special frame holder is used to avoid head movement during MRI. Image accuracy is checked for each image sequence by comparing the known frame measurement with image measurements, in addition to the distance from the posterior fiducials to the middle fiducials. The images are exported from the imaging suite via the Ethernet. Images are defined with the LGP software after being transferred to the LGP computer. The measurements are again checked and compared with the known frame measurement and also the distance to the middle fiducial to confirm the absence of any distortion during image transfer.

Determination of Target Volume or Volumes

Target determination is an important step in making a conformal plan. Target volume can be outlined with the LGP software (manual or semiautomatic mode), but experienced surgeons can create conformal dose plans without outlining the target. However, for new centers, especially those in which physicists assume the initial responsibility for planning, target definition becomes an important first step. The surgeon must define the radiosurgical targets for patients with AVMs, tumors, and functional lesions. By defining target volume and volumes of critical structures, better evaluation and quantification of the treatment plan can be achieved. Various parameters such as dose-volume histograms for the target volume and critical structures and conformity indices can be obtained.

Techniques of Conformal Dose Planning

Leksell Gamma Knife Model 4C

In the process of treatment planning, several strategies can be used. Model C allows treatment in a robotic APS mode, manual positioning (trunnion mode), or mixed treatment (some isocenters in the APS mode and some in the trunnion mode). A different approach would be used if only a trunnion treatment plan was possible versus an APS treatment plan. Universally, with the LGP software one can start planning from the middle of the target and then move to the top and bottom. Another approach is to start at the bottom or top and build from the starting point. Beginners can also use the inverse dose-planning algorithm (Wizard) to create a plan and then optimize it manually. When planning a treatment with the use of trunnions only, one might tend to use larger collimators (especially for larger lesions) to reduce the time and maximize coverage of the target. For example, for a medium-sized acoustic tumor, in the trunnion mode one might use a few 14-mm collimators for the majority of the tumor and a few 4-mm collimators for the intracanalicular portion of the tumor. In APS mode, however, one would most likely use multiple 8-mm isocenters for the majority of the tumor and 4-mm isocenters for the intracanalicular portion because the total time spent would be less. There would be no need to go into the treatment room to set each isocenter. As long as the isocenters are in close proximity to one another, the software would automatically put them into the same treatment run and the patient would move from one set of coordinates to the next until all isocenters of one collimator size were treated. Conformal dose planning is enhanced by the use of multiple small collimators.

Other techniques can be used in planning, such as using a steep (125 degree) gamma angle for posterior lesions (cerebellar or occipital) to avoid frame collisions. Another technique available for single-isocenter lesions is to match the gamma angle to the angle of the target. In APS mode, during the setup phase of planning, efficiency can be improved by grouping as many isocenters in the same run as possible, even if it means changing all the isocenters to high docking or a different gamma angle than the default of 90 degrees. If a different gamma angle is used, the plan must be rechecked for accuracy and adjusted if necessary. In the current version of the LGP, multiple targets (multiple tumors) can be treated with different isodose prescriptions and different central doses with the use of multiple matrices.

Leksell Gamma Knife PERFEXION

Three different approaches to treatment planning can be applied when using the LGK PERFEXION. The first is to use the same strategy as described for the 4C system. Because just 4-, 8-, and 16-mm collimators are available, only a combination of these three different collimators can be used to cover the entire target volume. The second approach is to use dynamic shaping, a new feature in treatment planning introduced for the PERFEXION system. This automatic procedure will provide solutions to block selected sectors for protection of volumes defined as critical structures (see Fig. 256-4). Different levels to reduce the dose delivered to critical structures can be selected. The treatment planning system then automatically calculates which sectors should be blocked for each individual shot. One should be aware that each blocking will significantly increase total exposure time. The third approach is to use isocenters composed of different beam diameters, sectors, and blocked sectors. Any pattern of sectors, including 4-, 8-, and 16-mm collimators and blocks, can be generated. This can help significantly in shaping the dose distribution, especially for irregular volumes. For example, it is very easy to design an elongated dose distribution by using a combination of small and large collimators. Composite shots can also reduce the dose delivered to critical structures in close proximity to the treated target volume. Use of composite shots consisting of any sectors, a combination of different collimators, or blocking is free of time penalty because all sector position changes are done automatically and take less than 5 seconds.

Techniques of Stereotactic Radiation Delivery with the Gamma Knife

Leksell Gamma Knife Model 4C

The Model 4C Gamma Knife allows delivery of radiation in the trunnion mode (manual patient positioning) or APS mode (robotic positioning) or a combination of the two (mixed treatment). In trunnion treatment, the x, y, and z coordinates of each isocenter are set manually and double-checked to avoid error. The APS plan is transferred directly from the planning computer to the control computer. The operator selects the run (a combination of isocenters of the same beam diameter) that matches the collimator helmet on the Gamma Knife unit. The APS is moved to the dock position and the patient’s head frame is fixed into the APS. The accuracy of the docking position is checked. The system prompts the user to perform clearance checks first for all planned isocenters in which the pins, posts, frame, or patient’s head would be less than 12 mm away from the inner surface of the collimator helmet (even though they may not match the collimator size that is being used for the first run). The clearance check is performed by moving the patient to these positions under APS or manual control and by visual check of collision with the collimator helmet. After the clearance check, the system prompts the surgeon to carry out position checks. In position checks, all the isocenters using the same helmet are checked, one by one, by moving the patient’s head to these positions under APS or manual control to make sure that patient will handle all changes in head position with sufficient comfort.

The team moves out of the treatment room after the position checks are completed, and the radiosurgical dose is administered. The APS moves the patient to all the planned positions, one by one, until the isocenters using that size collimator helmet are completed. The team monitors the patient and coordinates of the different isocenters on the control computer. If other runs with a different gamma angle but the same helmet are planned, the patient is taken out, the next run is selected, the APS is moved to the dock position, and patient’s head is again fixed in the APS at the planned angle (72, 90, 110, or 125 degrees). Position checks are performed, and the procedure commences. Similarly, if additional runs using different helmets are planned, the helmet is changed, the patient’s head is positioned in the APS, and position checks for all the isocenters for that helmet and gamma angle are performed.

Leksell Gamma Knife PERFEXION

The treatment procedure for the LGK PERFEXION is a fully automated process in all aspects of the entire treatment, including setup of the stereotactic coordinates, setup of different sector positions defining collimator size or blocked beams, and setup of exposure times. All treatment data are exported to the operating console, which is used to control and monitor patient treatment. The only manual part of the treatment procedure is positioning the patient’s head in the docking device in a selected angle and adjusting treatment bed height for the best patient comfort. Patient identity must be confirmed by operating personnel before treatment can start. Most PERFEXION treatments are administered in a single run. In a small number of patients (according to our experience, about 5%), the treatment is delivered in two separate runs with different angles defining different patient head angulations in the sagittal position. A clearance check for shots that involve close contact with the collimator system needs to be performed in a minority of patients. To perform a clearance check, a special test tool simulating the shape and dimensions of the inner collimator is attached and rotated around the patient’s head for the tested positions. This situation usually occurs in patients with multiple metastases.

After setup and confirmation of the patient’s identity are completed, the radiosurgery team moves out of the treatment room and treatment is initiated. The PPS moves the patient’s entire body to reach the planned stereotactic coordinates. This provides better patient comfort than the APS used in Model 4C does, which involves head movement only. The team monitors the patient and the setup for coordinates, exposure times, and sectors for different isocenters on the control computer of the operating console. The system allows audiovisual communication with the patient during treatment, and the process can be interrupted at any time if required by clinical conditions.

Conclusion

In the past 20 years we have witnessed dramatic improvements in stereotactic radiosurgical technologies. Gamma Knife radiosurgery now offers better image-handling features, including image fusion; faster, more compact platforms that make calculations nearly real time; automated robotic patient positioning, which reduces the potential for human error; and inverse treatment planning. In the future, more accurate imaging techniques, improved software to handle these images, and advanced inverse planning software will provide better treatment and result in better patient outcomes.

Suggested Readings

Bradford CD, Morabito B, Shearer DR, et al. Radiation-induced epilation due to couch transit dose for the Leksell gamma knife model C. Int J Radiat Oncol Biol Phys. 2002;54:1134-1139.

Chiou TS. Patients treated by Model-C Gamma Knife with APS are less exposed to non-therapeutic irradiation. Minim Invasive Neurosurg. 2008;51:47-50.

Goetsch SJ. Risk analysis of Leksell Gamma Knife Model C with automatic positioning system. Int J Radiat Oncol Biol Phys. 2002;52:869-877.

Horstmann GA, Schopgens H, van Eck AT, et al. First clinical experience with the automatic positioning system and Leksell gamma knife Model C. Technical note. J Neurosurg. 2000;93(suppl 3):193-197.

Horstmann GA, Van Eck AT. Gamma knife model C with the automatic positioning system and its impact on the treatment of vestibular schwannomas. J Neurosurg. 2002;97(suppl):450-455.

Kondziolka D, Lunsford LD, Flickinger JC, et al. Emerging indications in stereotactic radiosurgery. Clin Neurosurg. 2005;52:229-233.

Kondziolka D, Maitz AH, Niranjan A, et al. An evaluation of the Model C gamma knife with automatic patient positioning. Neurosurgery. 2002;50:429-431.

Kuo JS, Yu C, Giannotta SL, et al. The Leksell gamma knife Model U versus Model C: a quantitative comparison of radiosurgical treatment parameters. Neurosurgery. 2004;55:168-172.

Leksell L. A note on the treatment of acoustic tumours. Acta Chir Scand. 1971;137:763-765.

Leksell L. Sterotaxic radiosurgery in trigeminal neuralgia. Acta Chir Scand. 1971;137:311-314.

Leksell L. Cerebral radiosurgery. I. Gammathalamotomy in two cases of intractable pain. Acta Chir Scand. 1968;134:585-595.

Leksell L. The stereotaxic method and radiosurgery of the brain. Acta Chir Scand. 1951;102:316-319.

Lindquist C, Paddick I. The Leksell Gamma Knife Perfexion and comparisons with its predecessors. Neurosurgery. 2007;61(3 suppl):130-140.

Lunsford LD, Flickinger J, Lindner G, et al. Stereotactic radiosurgery of the brain using the first United States 201 cobalt-60 source gamma knife. Neurosurgery. 1989;24:151-159.

Lunsford LD, Kondziolka D, Flickinger JC. Radiosurgery as an alternative to microsurgery of acoustic tumors. Clin Neurosurg. 1992;38:619-634.

Lunsford LD, Kondziolka D, Flickinger JC, et al. Stereotactic radiosurgery for arteriovenous malformations of the brain. J Neurosurg. 1991;75:512-524.

Niranjan A, Gobbel GT, Kondziolka D, et al. Heritage of radiosurgical research, current trends and future perspective. Prog Neurol Surg. 2007;20:359-374.

Niranjan A, Lunsford LD, Gobbel GT, et al. Brain tumor radiosurgery: current status and strategies to enhance the effect of radiosurgery. Brain Tumor Pathol. 2000;17:89-96.

Niranjan A, Maitz AH, Lunsford A, et al. Radiosurgery techniques and current devices. Prog Neurol Surg. 2007;20:50-67.

Regis J, Hayashi M, Porcheron D, et al. Impact of the model C and Automatic Positioning System on gamma knife radiosurgery: an evaluation in vestibular schwannomas. J Neurosurg. 2002;97(suppl):588-591.

Steiner L, Leksell L, Forster DM, et al. Stereotactic radiosurgery in intracranial arterio-venous malformations. Acta Neurochir (Wien). suppl 21. 1974:195-209.

Steiner L, Leksell L, Greitz T, et al. Stereotaxic radiosurgery for cerebral arteriovenous malformations. Report of a case. Acta Chir Scand. 1972;138:459-464.

Tlachacova D, Schmitt M, Novotny JJr, et al. A comparison of the gamma knife model C and the automatic positioning system with Leksell model B. J Neurosurg. 102 suppl. 2005:25-28.

Yu C, Jozsef G, Apuzzo ML, et al. Fetal radiation doses for model C gamma knife radiosurgery. Neurosurgery. 2003;52:687-693.

References

1 Leksell L. The stereotaxic method and radiosurgery of the brain. Acta Chir Scand. 1951;102:316-319.

2 Leksell L. Cerebral radiosurgery. I. Gammathalamotomy in two cases of intractable pain. Acta Chir Scand. 1968;134:585-595.

3 Steiner L, Leksell L, Forster DM, et al. Stereotactic radiosurgery in intracranial arterio-venous malformations. Acta Neurochir (Wien). suppl 21. 1974:195-209.

4 Steiner L, Leksell L, Greitz T, et al. Stereotaxic radiosurgery for cerebral arteriovenous malformations. Report of a case. Acta Chir Scand. 1972;138:459-464.

5 Leksell L. A note on the treatment of acoustic tumours. Acta Chir Scand. 1971;137:763-765.

6 Leksell L. Sterotaxic radiosurgery in trigeminal neuralgia. Acta Chir Scand. 1971;137:311-314.

7 Lunsford LD, Flickinger J, Lindner G, et al. Stereotactic radiosurgery of the brain using the first United States 201 cobalt-60 source gamma knife. Neurosurgery. 1989;24:151-159.

8 Lunsford LD, Kondziolka D, Flickinger JC. Radiosurgery as an alternative to microsurgery of acoustic tumors. Clin Neurosurg. 1992;38:619-634.

9 Lunsford LD, Kondziolka D, Flickinger JC, et al. Stereotactic radiosurgery for arteriovenous malformations of the brain. J Neurosurg. 1991;75:512-524.

10 Kondziolka D, Lunsford LD, Flickinger JC, et al. Emerging indications in stereotactic radiosurgery. Clin Neurosurg. 2005;52:229-233.

11 Niranjan A, Gobbel GT, Kondziolka D, et al. Heritage of radiosurgical research, current trends and future perspective. Prog Neurol Surg. 2007;20:359-374.

12 Niranjan A, Lunsford LD, Gobbel GT, et al. Brain tumor radiosurgery: current status and strategies to enhance the effect of radiosurgery. Brain Tumor Pathol. 2000;17:89-96.

13 Niranjan A, Maitz AH, Lunsford A, et al. Radiosurgery techniques and current devices. Prog Neurol Surg. 2007;20:50-67.

14 Bradford CD, Morabito B, Shearer DR, et al. Radiation-induced epilation due to couch transit dose for the Leksell gamma knife model C. Int J Radiat Oncol Biol Phys. 2002;54:1134-1139.

15 Chiou TS. Patients treated by Model-C Gamma Knife with APS are less exposed to non-therapeutic irradiation. Minim Invasive Neurosurg. 2008;51:47-50.

16 Goetsch SJ. Risk analysis of Leksell Gamma Knife Model C with automatic positioning system. Int J Radiat Oncol Biol Phys. 2002;52:869-877.

17 Horstmann GA, Schopgens H, van Eck AT, et al. First clinical experience with the automatic positioning system and Leksell gamma knife Model C. Technical note. J Neurosurg. 2000;93(suppl 3):193-197.

18 Horstmann GA, Van Eck AT. Gamma knife model C with the automatic positioning system and its impact on the treatment of vestibular schwannomas. J Neurosurg. 2002;97(suppl):450-455.

19 Kondziolka D, Maitz AH, Niranjan A, et al. An evaluation of the Model C gamma knife with automatic patient positioning. Neurosurgery. 2002;50:429-431.

20 Kuo JS, Yu C, Giannotta SL, et al. The Leksell gamma knife Model U versus Model C: a quantitative comparison of radiosurgical treatment parameters. Neurosurgery. 2004;55:168-172.

21 Regis J, Hayashi M, Porcheron D, et al. Impact of the model C and Automatic Positioning System on gamma knife radiosurgery: an evaluation in vestibular schwannomas. J Neurosurg. 2002;97(suppl):588-591.

22 Tlachacova D, Schmitt M, Novotny JJr, et al. A comparison of the gamma knife model C and the automatic positioning system with Leksell model B. J Neurosurg. 2005;102(suppl):25-28.

23 Yu C, Jozsef G, Apuzzo ML, et al. Fetal radiation doses for model C gamma knife radiosurgery. Neurosurgery. 2003;52:687-693.

24 Lindquist C, Paddick I. The Leksell Gamma Knife Perfexion and comparisons with its predecessors. Neurosurgery. 2007;61(3 suppl):130-140.

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