Radiosurgery technology development and use - Old City Publishing

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Physics

Radiosurgery technology development and use Sanford L. Meeks, Ph.D.1,2, Jason Pukala, M.S.1, Naren Ramakrishna, M.D.1, Twyla R. Willoughby, M.S.1 and Francis J. Bova, Ph.D.2 1

Department of Radiation Oncology M. D. Anderson Cancer Center Orlando, Orlando, FL, USA Department of Neurosurgery, University of Florida, Gainesville, FL, USA

2

Correspondence to: Sanford L. Meeks, Ph.D. Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, 1400 S Orange Ave, Orlando, FL 32806, USA Phone: (321) 841-8809; E-mail: [email protected] (Received September 28, 2010; accepted October 25, 2010)

Radiosurgery first became a clinical option in the 1960’s because of the Gamma Knife, and the technology proliferated in the 1980’s due to the availability of linear accelerator radiosurgery. The technology has continued to develop with both Gamma Knife and linac radiosurgery due primarily to advances in computer technology and robotic automation. Many of these advances include planning systems that enhance the conformity of the dose distribution, and delivery systems that can more safely and efficiently delivery these more complex treatment plans. This manuscript details the evolution of technologies in stereotactic localization and delivery for intracranial radiosurgery. Keywords: Gamma Knife, Sterotactic radiosurgery

Early in the technology development, different groups developed methods to minimize the errors in treatment delivery using both the Gamma Knife and linear accelerators (linacs). Using either treatment method, the radiation beam can be directed with high isocentric accuracy (2, 3). This precise delivery of radiation has remained the hallmark of radiosurgery, and technology developments have focused primarily on improving dose conformity, patient comfort, and treatment efficiency. Radiosurgery technology developments can be categorized in three basic areas: treatment devices, dose planning, and patient localization. Because dose planning and patient localization are specific to the treatment device, advances in these areas will be discussed separately for each.

1. INTRODUCTION Since Lars Leksell’s conception of stereotactic radiosurgery (1), the technology has proliferated and radiosurgery has become a standard consideration in the treatment of many benign and malignant central nervous system pathologies. Radiosurgery relies on three-dimensional, or stereotactic, image localization, thereby enabling co-identification of a virtual target in the treatment-planning computer with the actual target position for treatment delivery. To use this paradigm optimally, all errors from image acquisition for treatment planning through mechanical aspects of treatment delivery must be systematically minimized.

2. GAMMA KNIFE 2.1 Treatment Delivery System The first “gamma-knife” containing 179 cobalt-60 sources became operational at the Sophiahemmet in Stockholm in 1968 (4, 5). In 1975, clinicians at the Karolinska Hospital began using a new Gamma Knife, redesigned using 201 60Co sources arranged in a hemispherical configuration to create a more spherical dose distribution. Treatment is performed by centering the patient’s tumor in the center of this focused radiation.

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The radiation in this case is fixed and the patient is moved to treat different areas of the brain. To facilitate reloading of the 60Co sources, the unit was redesigned so that the sources were arranged in a circular configuration (Model B). The user could select one of four different sized collimation systems (helmets) that would focus the radiation to 4, 8, 12, or 18 mm diameter sphere. These devices were very large and heavy, requiring a lift in order to attach them to the patient treatment couch. To accommodate non-spherical targets or larger targets, several combinations of these helmets, along with repositioning of the patient would be used to target different parts of the tumor until the entire tumor was covered. Each of the patient position and helmet is called a shot. In 1999, the Model C was introduced. The Model C treatment planning system more easily allowed the creation of complex dose distributions requiring multiple shots of radiation, and an automatic couch position system (APS) was included that facilitated the patient positioning for delivery of these complex plans. In 2002, a panel of clinical experts began work with Elekta engineers to design a new Leksell gamma knife. In addition to maintaining the robust nature of previous models and the ability to create the same dose distributions as previous models, the group agreed on several key features: improved patient safety and comfort, improved radiation protection for patients and staff, full system automation (including changes in sphere size or helmets), and increasing the treatable tissue volume within the brain and beyond. The result of this effort is the Gamma Knife Perfexion, which was released in 2006 (6). The Perfexion has a different beam geometry than the previous gamma knife models (3, 6). It has 192 60Co sources arranged in a cylindrical configuration in five rings. The primary and secondary collimators of previous models have been replaced by a single large 120 mm thick tungsten collimator array ring. Consequently, the Perfexion does not require collimator helmets. Only three collimator sizes, 4, 8 and 16 mm, are used in Perfexion, and they are integrated into the treatment machine itself so the user does not physically have to apply them. The 120 mm thick tungsten collimator ring is subdivided into eight identical regions, each region containing 72 collimators (24 collimators for 4 mm, 24 collimators for 8 mm, and 24 collimators for 16 mm). The beam size for each region is changed automatically by moving 24 sources over the selected collimator set. A sector containing 24 sources can be moved into one of the collimator positions or a position in which the 24 sources are completely blocked. The sector movement is performed by servo-controlled motors. The range of couch movement around the focal point of intersection of the radiation beams has been 22     Journal of Radiosurgery and SBRT 

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substantially increased in the Perfexion. The mechanical treatment range for the Perfexion has been increased by 60 mm in the X and Y directions, and by 55 mm in the Z direction. Because of the increased treatment range, the treatment of multiple brain metastases does not present the difficulty encountered in older Gamma Knife models. As mentioned previously, the stereotactic treatment coordinates were setup in the Gamma Knife C models using the APS, which moved only the patient’s head. This has been replaced by the patient positioning system (PPS), which moves the patient’s entire body to the desired stereotactic coordinates thereby enhancing patient comfort. The patient is docked into the PPS using a frame adapter that is attached to the Leksell head ring using three clips. The adapter is then directly docked to the PPS. The reproducibility of the stereotactic coordinates set up in the Perfexion system is better than 0.05 mm. This is an improvement over the 0.3 mm accuracy of the APS. The mechanical accuracy of the isocenter, guaranteed by the manufacturer, has been improved over previous gamma knife models, being better than 0.5 mm over its entire range. These tolerances have been tested up to a 210-kg load. Early users of the system have reported that in addition to the enhanced cranial reach, the Perfexion provides a significant improvement in treatment efficiency (3, 6-8). While the beam-on times are comparable between various Gamma Knife models, the set-up time is generally 20 minutes less with the Perfexion than the 4C model (6), and may be as much as 2 hours less(8). 2.2 Advances in Gamma Knife Treatment Planning As a result of the collimator and sector de­sign of the Perfexion, it is possible to develop treatment plans containing hybrid shots, that is, shots that have dif­ferent collimator values in different sectors (3, 7, 9). This composite shot feature allows each individual isocenter of a treatment plan to have its own optimized shape, thereby increasing the conformity of the overall treatment plan. In the treatment of multiple metastases in which a single isocenter may be used for each tumor, this individual isocenter can now be shaped to the target. Intermediate collimator sizes can be mimicked by alternating sectors with different collimator sizes. For example, a 6-mm collimator can be created by placing 4- and 8-mm collimators in opposing sectors. The ability to change collimators in less than 1 second removes the previous time penalty of approximately 8 minutes every time a collimator helmet needed to be changed. Extreme elongation of isodoses from a single isocenter can be achieved by selective blocking of sectors. This feature is similar to beam plugging used in Models B

Radiosurgery technology development and use

and C, but has more flexibility and can be performed automatically. Doses to organs at risk (OARs) can be limited by a process called dynamic shaping. The OAR is outlined and sectors that contain beams that pass through the OAR are blocked. The sectors are automatically blocked during treatment, resulting in no extra set up time, as would be required with older Gamma Knife models. Beam on times will increase, however, due to the lower dose rate from the partially blocked collimator array. While these treatment planning advances help improve the overall efficiency, early users have reported no significant improvement in dose conformity using the Perfexion (6, 8). 2.3 Frameless patient localization for Gamma Knife treatments To facilitate fractionated treatments, Elekta has designed a repositioning system under the commercial name Extend, which consists of a repositioning head frame and associated hardware and software(10). The main components are a carbon fiber frame, vacuum cushion headrest, mouthpiece, and carbon fiber front piece. Extend is basically the same as the HeadFix system (11, 12), only slightly modified to work with the Perfexion. To achieve high-precision positioning and reproducibility, the Extend system uses a mouthpiece with the patient’s upper palate impression and solid vacuum bonding with the hard palate. Before the first treatment fraction, a patient-specific maxillary mold is made using commercially available dental impression material. A small hole is left in the impression to allow a vacuum device to suction the impression to the patient’s upper palate. The mouthpiece is then attached to the front piece, the patient’s head is placed on the vacuum cushion. The front piece is then attached to the frame, and the sliding plate and mouthpiece are secured. The vacuum cushion headrest is then evacuated, creating an impression of the back of the skull with which to help guide the patient back upon repeat setups. For each subsequent patient setup, the reproducibility of the Extend system relies on bringing the patient’s head into the position in which the front piece rests flush and tensionfree against the frame, and the mouthpiece is suctioned to the upper palate. If done correctly, the patient’s head should be in the same position relative to the frame as during setup. After the front piece is clamped to the frame, the head cannot move without losing suction. While performance of the HeadFix system has been previously evaluated, the accuracy and reproducibility of the Extend system has been evaluated in one study for patients with intracranial tumors who received linac-

based stereotactic radiotherapy(10). These patients were setup and immobilized using Extend, and conebeam CT (CBCT) correlation with the planning CT was used as the gold-standard for comparison. Three hundred thirty-three fractions of radiation were delivered in 12 patients. The mean three-dimensional (3D) setup error was 0.8 mm with interpatient and interfraction variations of 0.1 and 0.4 mm, respectively.

3. LINEAR ACCELERATOR SYSTEMS 3.1 Conventional Linear Accelerators Recognizing the considerable expense, regulatory hurdles and source-replacement constraints of the Gamma Knife, in the 1980’s several different groups developed radiosurgery programs based on modifications of linacs used for conventional radiotherapy (2, 13-18). These efforts resulted in proliferation of radiosurgery facilities in the 1990s Recently, linac manufacturers have introduced linear accelerators specifically designed to deliver linac radiosurgery more accurately and efficiently (19-23). All of the linacs currently marketed for radiosurgery share common features: higher dose rates, integrated image guidance, integrated high resolution multileaf collimation, and improved mechanical accuracy. Varian was the first manufacturer to venture into the radiosurgery market with the introduction of the Clinac-600SR(24). The Clinac-600SR was a dedicated radiosurgery accelerator with a single 6MV that was delivered through a maximum 10x10 field size. This allowed the manufacturer to reduce the weight in the gantry and improve the rotational accuracy of the unit. Eventually, the Clinac-600SR evolved into the Novalis through Varian’s partnership with BrainLab (22, 25, 26). The linac comes equipped with the standard circular collimators used for conventional radiosurgery, and has a maximum dose rate of 800 MU/minute. For beam shaping or intensity modulated radiosurgery, the Novalis has a built-in integrated computer-controlled m3 micromultileaf collimator (mMLC) with leaves that project to 3 mm at the beam isocenter. Image guidance on the Novalis is performed using the fully integrated ExacTrac X-Ray 6D, which consists of two infrared (IR) cameras for patient positioning and tracking, two floormounted kilovolt x-ray tubes, and two ceiling-mounted amorphous silicon flat panel detectors for x-ray image guidance. The amorphous silicon detectors are mounted at a height of 2.1 m from the floor of the LINAC vault; because of their orientation, no collision with the gantry in any position is possible. Furthermore, any collision with the amorphous silicon detectors would be noticed

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by the autocalibration of the ExacTrac X-Ray 6D. The beam axes of both x-ray tubes cross the treatment beam axis at the LINAC isocenter. All components are fully integrated with the linac, and they allow computerassisted IR- and x-ray-based correction and verification of the patient’s position before and during treatment. Varian has since extended the radiosurgery system to reside on its high-energy linear accelerators, resulting in the Trilogy (21)and the Novalis TX(20). These have the high dose rate, mechanical accuracy, and integrated components of the Novalis. Additionally, these linacs have integrated cone beam CT on the linacs, allowing acquisition of soft-tissue imaging for image guidance. Similarly, both Elekta and Siemens have marketed radiosurgery linacs featuring higher dose rates, higher accuracy, high-resolution mMLCS, and CBCT for softtissue image guidance. 3.1.1 Treatment planning for conventional linac radiosurgery Linac radiosurgery treatment planning and delivery historically used noncoplanar arc therapy delivered through circular collimators. The intersection of multiple beams, each with unique entry and exit pathways, results in a peaked spherical dose at the isocenter and a very steep dose gradient outside of the target volume. The use of noncoplanar arc therapy results in a dose fall-off from the prescription isodose shell to half of the prescription dose in approximately 2-4 mm. As the target shape deviates from spherical geometry, the treatment planning parameters must be modified to generate a treatment plan that conforms to the shape of the target. Much like Gamma Knife planning, linac radiosurgery provides several treatment planning parameters to elongate the dose distribution and/ or avoid nearby critical structures(27). Arc weighting, collimator size, and arc length may all be altered to produce ellipsoidal dose distributions while maintaining a single isocenter plan. More irregular targets require multiple isocenter planning. Manipulating the arcs within a single isocenter can be used to conform the dose distribution to all spherical and most ellipsoidal target volumes. Most of the targets encountered in radiosurgery are neither spherical nor ellipsoidal, however, and multiple isocenters must be used to conform the dose distribution to any target that deviates from ellipsoidal geometry. Multiple isocenter planning requires packing small spherical dose distributions within the target volume. Any arbitrary shape may be achieved if these spherical dose distributions are placed correctly within the volume. Using a systematic approach to multiple isocenter planning, a conformal treatment plan can be generated for any target volume. 24     Journal of Radiosurgery and SBRT 

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A great deal of expertise and practice are required to generate conformal plans for highly complex target volumes. Therefore, automated algorithms have been developed for multiple isocenter planning (28). With the addition of mMLCs on linacs, conformal beam radiosurgery has become prevalent (29, 30). In conformal beam radiosurgery, the objective is to converge conformally shaped photon beams on a single isocenter while minimizing the overlapping volumes between beam entrance and exit paths (31-34). This objective is best achieved when beams are arranged with maximal spatial separation in three-dimensional (3D) space. Such arrangements have been approximated using manual beam placement (Bourland et al 1994, Marks et al 1995), starting with several equally spaced coplanar beams, and then rotating additional beams out of the beam plane between existing beams (31, 33). However, manually placing non-coplanar beams within the constraints of inter-beam geometry and patient-specific radiosensitive structures quickly becomes difficult, even when aided by modern 3D treatment planning systems, which is one of the reasons why early manually planning attempts were limited to at most eight beams. Partly because of this difficulty, automatic, iterative beam placement optimization algorithms have also been used for this purpose. These algorithms incorporate a measure of beam separation with beam intersection with non-target volumes as part of the objective function (32, 34). To improve the conformity of these plans, a logical evolution of beam bouquets is to use step and shoot intensity modulated radiation therapy (IMRT) for each of the noncoplanar fields (35-37). After geometrically optimizing the placement of the fields, inverse planning is used to determine the optimal fluence profile for each beam in order to maximize conformity and minimize dose to critical structures. Another logical extension of shaped beam radiosurgery is dynamic conformal radiosurgery(37-39). In dynamic conformal radiosurgery, the MLC leaves move continuously, conforming to the beam’s-eye-view projection of the target at every increment along the path of an arc. Recently, arc-based IMRT techniques have emerged as a promising progression from fixed-field techniques. Intensity modulated arc therapy (IMAT) was proposed in 1995(40) .The principle behind IMAT was the delivery of a form of IMRT from many gantry angles in a continuous arc. While traditional IMRT is delivered using static fields consisting of several subfields to modulate the radiation beam, IMAT used superimposing arcs to deliver additional subfields. Thus, under the IMAT philosophy, three subfields would require three superimposing arcs. Clinical implementation of IMAT encountered several practical challenges and never received widespread adoption, however.

Radiosurgery technology development and use

Improvements were made to the IMAT approach and a volumetric arc therapy (VMAT) algorithm that could perform IMRT in a single 360° gantry arc was later proposed(41). Varian Medical Systems adopted this technology for their commercial implementation of VMAT called RapidArc. RapidArc can simultaneously vary gantry speed, dose rate, and multileaf collimator (MLC) position to achieve highly conformal dose distributions. Recently, several investigators have explored the use of RapidArc for SRS (42-44). Results from comparisons of shaped beam and intensity modulated radiosurgery techniques with conventional circular collimator techniques have been mixed. In general, however, it has been shown that circular collimator planning techniques have the highest possible dose gradients, and have superior conformity for very small targets (i.e. ≤ 1 cm diameter). As the target size and complexity increase, however, mMLC planning techniques often have superior conformity. Also, because circular collimator techniques require multiple isocenters to cover complex lesions, the dose gradient worsens for circular collimator radiosurgery of complex lesions. Hence, the gradient is comparable among the various techniques when complex targets are planned. For complex target volumes, mMLC techniques always require less planning expertise than multi-isocenter planning techniques, and they always improve treatment efficiency. Hence, mMLC techniques have become increasingly popular. 3.1.2 Frameless patient localization with conventional linac To minimize errors, traditional radiosurgery uses an invasive headring to coordinate the virtual and real worlds, and minimize motion during image acquisition and treatment. The reliable immobilization and target localization accuracy of invasive frame-based radiosurgery have established the technique as a gold standard, but it is associated with significant disadvantages. Frame-placement involves risk of bleeding and infection, and requires pre-medication. Furthermore, the care of patients wearing head frames creates a clinical resource burden, requiring dedicated nursing and physician support. Frame-based treatment also requires treatment planning to be completed following frame placement on the day of treatment, making it difficult to use dose planning techniques such as IMRT. As an alternative to an invasive headring, several frameless radiosurgery systems have been developed, most of which rely on optical and/or image guidance. The first commercially available optical tracking system for radiation therapy was originally developed at the University of Florida (45-48) and is commer-

cially available as part of Varian’s optical guidance platform (OGP). This system uses the Polaris position sensor unit (Northern Digital, Inc., Waterloo, Ontario), which is an array of two 2D CCD cameras, to optically track the position of either active or passive infrared markers arranged in an array to form a rigid body. In the OGP, the Polaris is mounted in the ceiling above the linear accelerator. A calibration procedure is required to transform the coordinate system from the Polaris’s native coordinate system to the linear accelerator’s coordinate system. After this calibration, the position of any infrared marker in the room may be determined relative to the isocenter. Using this same calibration procedure, the origin and orthogonal reference system can be translated to any point. The system can therefore be adapted to any orthogonal reference system making it compatible with most imaging systems, treatment delivery systems, or stereotactic frame systems. Patient localization is accomplished through detection of an optical reference array containing four passive markers. This reference array is attached to a custom bite plate that links to the maxillary dentition of the patient to form a rigid system. The optically determined patient position is then compared to the desired patient position, as determined in the virtual simulation; the biteplate-reference array complex is kept in place during the virtual simulation CT scan, and the image coordinates of the reflective markers are determined as part of the virtual simulation. During treatment planning the desired target, or isocenter, coordinates are determined in CT space. The centers of the spherical fiducial markers in the optical reference array are also localized in CT space, thus determining their positions relative to the treatment isocenter and defining a stereotactic coordinate system. After selecting the fiducials from the scan, the code determines the best fit between the image-defined coordinates of the reference array and the known geometry of the reference array. During patient setup, the OGP is used to determine the patient’s position and report the displacement from isocenter in real time. The system reports the translational misalignments along three orthogonal axes and reports the rotational misalignment about each of these axes. In addition, the system reports overall vector misalignment, which is the root mean square of the three translational misalignments, and hence, the 3D displacement of the patient’s target from isocenter. The patient’s target is repositioned to the desired position in stereotactic space. The patient is monitored in real time during treatment delivery and patient motion is detected, the treatment can be interrupted until the patient’s position is corrected. This process is repeated for each treatment field.

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The OGP has been used primarily for fractionated stereotactic radiotherapy of intracranial and skull base tumors using circular cones, conformal static beams, and intensity modulation (46, 49, 50). However, it has been used by multiple groups for frameless radiosurgery (51-53).The reported clinical results are comparable with frame based radiosurgery series and suggest that frameless SRS is a viable option for a subset of patients (54, 55). The ExacTrac system (BrainLab, Munich, Germany) is operationally very similar to the OGP (48, 56, 57). ExacTrac employs two infrared cameras, which are mounted in the ceiling of the linear accelerator vault, to track passive optical markers that are attached to the patient. A calibration procedure is used to transform the coordinate system from the camera’s native coordinate system to the linear accelerator’s coordinate system. After this calibration, the position of any infrared marker in the room may be determined relative to isocenter. In the Novalis ExacTrac frameless radiosurgery system, a non-invasive thermoplastic mask system is used. A lightweight carbon fiber array equipped with six passive IR-reflective markers attaches to the base of the mask system. These markers are optically tracked to roughly position the patient relative to the linac isocenter. In addition, the ExacTrac utilizes dual floor-mounted kV X-ray tubes which project onto ceiling mounted-amorphous silicon detectors and generate stereoscopic oblique images through machine isocenter. The system creates an image fusion of the kV X-ray images with a digital reconstructed radiograph (DRR) library generated at the time of simulation and generates a predicted position shift to place the patient such that the target is coincident with the planning isocenter. The optical tracking system is then used to make relative shifts. Using a robotic couch, rotational misalignments can also be corrected. Phantom-based end-to-end tests have demonstrated overall system accuracy within tolerances appropriate for radiosurgery, and the system has been used clinically for frameless radiosurgery (56, 58, 59). 3.2 Other Linac Systems 3.2.1 CyberKnife The Accuray Cyberknife is a compact X-band linear accelerator mounted on a robot originally designed for assembly line fabrication (60-62). Because the X-band microwave frequency is 10 GHz instead of the 3 GHZ of conventional linacs, the waveguide is roughly 1/3 the length of a conventional linac. The overall weight of the linear accelerator with the microwave power supply is 180 kg. This enables the unit to be supported by a commercial 6-axis robotic arm.

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The multi-axis robotic arm allows positioning of the linear accelerator in any direction and at various surfaceto-axis distances. Since any of the individual beams is not constrained to be directed at a fixed isocenter, the delivery does not have to be along the path of an arc. The linear accelerator can treat from directions not possible with conventional units. For example, beams can be directed from the inferior-to-superior direction so that solid angles of delivery can be greater than 2 π. The distance from the collimator to the patient can be varied so that the collimator width can be changed from beam to beam. This flexibility adds considerable complexity to the planning process. In addition to choosing a set of directions defining the beams’ paths, the SSD and beam intensity for any of the sub-beams is also a free parameter. Automated procedures to choose optimal beam configurations and beam weights would have to search through a vast number of possible combinations. The most serious concern is collision avoidance. Additionally, two diagnostic X-ray sources are installed in the ceiling of the treatment room and digital image collectors placed orthogonally to the patient to provide real time image guidance. The robotic table can move around different axes and adjust the position of the patient according to the results of the image guidance. The diagnostic X-ray sources coupled to the X-ray image detectors determine the position of the linac without the need for a stereotactic frame. A series of internal radiographic reference points, provided by the bone structure or radio-opaque markers implanted close to the lesion, allow the localization of the target. The CyberKnife system comes with dynamic tracking software that identifies and measures the treatment volume, and communicates this information to the robotic arm that positions the linac. The newest version of the CyberKnife, known as the VSI System, was first installed in April 2010 (63). Many of the improvements in the technology relate to treatment efficiency. In the VSI, the robot now moves 20% faster than in previous units, and it now moves along an optimized path only between nodes at which one or more treatment beams are to be delivered rather than to many nodes where no radiation is delivered. Furthermore, the dose rate is increased to 1000 MU/ minute in comparison to the 300 MU/minute dose rate in older models. Additionally, the VSI has tighter accuracy specifications and more accurate dose planning. 3.2.2 TomoTherapy Helical tomotherapy is an innovative means of delivering IGRT and IMRT using a device called the HiART (TomoTherapy, Inc., Madison, WI) that combines

Radiosurgery technology development and use

features of a linear accelerator and a helical computed tomography (CT) scanner(64). Helical tomotherapy is analogous to helical CT imaging where the gantry and the couch are in simultaneous motion. Hence, beam delivery is continuous over all 360° in transverse planes about the patient. Temporal beam modulation is achieved by moving multi-leaf collimator vanes into and out of a fan beam. In addition to its ability to deliver IMRT, the HIART has the ability to obtain helical megavoltage CT (MVCT) images using the same megavoltage radiation beam that it uses for treatment. These MVCT images are used to image guide the radiation delivery, and offer verification of patient position prior to during radiation therapy. Similar to other image-guided radiotherapy (IGRT) systems, the Hi-ART II can increase the precision of radiation therapy by properly locating the planning target volume as well as normal structures relative to the treatment unit. TomoTherapy, Inc. has designed an intracranial stereotactic patient positioning system that consists of a sterereotactic head frame, head frame couch interface, megavoltage CT (MVCT), and optical tracking camera system (65). A system delivery verification test in phantom using film showed spatial agreement between planned and delivered dose distributions to within 1 mm(65). Different groups have compared radiosurgery plans generated for the Hi-ART with treatment plans for the Gamma Knife(66), circular collimator linac radiosurgery(67), and step and shoot IMRS(68). Because tomotherapy beams are all coplanar, there are concerns about its ability to match the dosimetry generated using noncoplanar delivery. Compared to conventional radiosurgery techniques, Hi-ART plans can be generated that have comparable or better conformity but generally have a worse dose gradient(66, 67). Compared to step and shoot IMRS, however, tomotherapy plans can achieve better dose conformity and dose gradient.

4. DISCUSSION Any radiosurgery system must be based upon stereotactic imaging and localization systems that provide sub millimeter accuracy in specifying the position of both target and non-target tissues, and these systems have existed and been in clinical use for more than 40 years. Due to advances in computer technology and robotic automation, many changes have occurred in the technology of radiosurgery. Because these highly precise systems have existed for so long, the primary advances in radiosurgery technology have revolved around planning systems that enhance the conformity of the dose

distribution, and delivery systems that can more safely and efficiently delivery these treatment plans.

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