Report of AAPM TG 135: Quality assurance for robotic radiosurgery
2011; Wiley; Volume: 38; Issue: 6Part1 Linguagem: Inglês
10.1118/1.3579139
ISSN2473-4209
AutoresSonja Dieterich, Carlo Cavedon, Cynthia Chuang, Alan Cohen, Jeffrey A. Garrett, Charles L. Lee, Jessica Lowenstein, M DˈSouza, David D. Taylor, Xiaodong Wu, Cheng Yu,
Tópico(s)Advanced X-ray and CT Imaging
ResumoMedical PhysicsVolume 38, Issue 6Part1 p. 2914-2936 Task group reportFree Access Report of AAPM TG 135: Quality assurance for robotic radiosurgery Correction(s) for this article Erratum: “Report of AAPM TG 135: Quality assurance for robotic radiosurgery” Sonja Dieterich, Carlo Cavedon, Cynthia F. Chuang, Alan B. Cohen, Jeffrey A. Garrett, Charles L. Lee, Jessica R. Lowenstein, Maximian F. D'Souza, David D. Taylor, Xiaodong Wu, Cheng Yu, Volume 38Issue 9Medical Physics pages: 5264-5264 First Published online: August 30, 2011 Sonja Dieterich, Sonja Dieterich Stanford University Cancer Center, Stanford, California94305Search for more papers by this authorCarlo Cavedon, Carlo Cavedon Azienda Ospedaliera Universitaria Integrata di Verona, U.O.di Fisica Sanitaria, Verona, 37126 ItalySearch for more papers by this authorCynthia F. Chuang, Cynthia F. Chuang University of California San Francisco, Department of Radiation Oncology, San Francisco, California 94143-0226Search for more papers by this authorAlan B. Cohen, Alan B. Cohen Accuray Inc, Sunnyvale, California 94089Search for more papers by this authorJeffrey A. Garrett, Jeffrey A. Garrett Mississippi Baptist Medical Center, Jackson, Mississippi 39202Search for more papers by this authorCharles L. Lee, Charles L. Lee CK Solutions, Inc., Edmond, Oklahoma 73034Search for more papers by this authorJessica R. Lowenstein, Jessica R. Lowenstein UT MD Anderson Cancer Center, Houston, Texas 77030Search for more papers by this authorMaximian F. d'souza, Maximian F. d'souza St Anthony Hospital, Oklahoma City, Oklahoma 73101Search for more papers by this authorDavid D. Taylor Jr., David D. Taylor Jr. US Radiosurgery, Nashville, Tennessee 80304Search for more papers by this authorXiaodong Wu, Xiaodong Wu University of Miami, Department of Radiation Oncology, Miami, Florida 33101Search for more papers by this authorCheng Yu, Cheng Yu USC Keck School of Medicine, Los Angeles, California 90033Search for more papers by this author Sonja Dieterich, Sonja Dieterich Stanford University Cancer Center, Stanford, California94305Search for more papers by this authorCarlo Cavedon, Carlo Cavedon Azienda Ospedaliera Universitaria Integrata di Verona, U.O.di Fisica Sanitaria, Verona, 37126 ItalySearch for more papers by this authorCynthia F. Chuang, Cynthia F. Chuang University of California San Francisco, Department of Radiation Oncology, San Francisco, California 94143-0226Search for more papers by this authorAlan B. Cohen, Alan B. Cohen Accuray Inc, Sunnyvale, California 94089Search for more papers by this authorJeffrey A. Garrett, Jeffrey A. Garrett Mississippi Baptist Medical Center, Jackson, Mississippi 39202Search for more papers by this authorCharles L. Lee, Charles L. Lee CK Solutions, Inc., Edmond, Oklahoma 73034Search for more papers by this authorJessica R. Lowenstein, Jessica R. Lowenstein UT MD Anderson Cancer Center, Houston, Texas 77030Search for more papers by this authorMaximian F. d'souza, Maximian F. d'souza St Anthony Hospital, Oklahoma City, Oklahoma 73101Search for more papers by this authorDavid D. Taylor Jr., David D. Taylor Jr. US Radiosurgery, Nashville, Tennessee 80304Search for more papers by this authorXiaodong Wu, Xiaodong Wu University of Miami, Department of Radiation Oncology, Miami, Florida 33101Search for more papers by this authorCheng Yu, Cheng Yu USC Keck School of Medicine, Los Angeles, California 90033Search for more papers by this author First published: 25 May 2011 https://doi.org/10.1118/1.3579139Citations: 159AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Abstract The task group (TG) for quality assurance for robotic radiosurgery was formed by the American Association of Physicists in Medicine's Science Council under the direction of the Radiation Therapy Committee and the Quality Assurance (QA) Subcommittee. The task group (TG-135) had three main charges: (1) To make recommendations on a code of practice for Robotic Radiosurgery QA; (2) To make recommendations on quality assurance and dosimetric verification techniques, especially in regard to real-time respiratory motion tracking software; (3) To make recommendations on issues which require further research and development. This report provides a general functional overview of the only clinically implemented robotic radiosurgery device, the CyberKnife®. This report includes sections on device components and their individual component QA recommendations, followed by a section on the QA requirements for integrated systems. Examples of checklists for daily, monthly, annual, and upgrade QA are given as guidance for medical physicists. Areas in which QA procedures are still under development are discussed. I. INTRODUCTION Fundamental to stereotactic radiosurgery (SRS) is the accurate placement of the intended radiation dose. Small errors in the placement of radiation dose from individual beams or beamlets can result in inaccurate estimates of accumulated dose as well as inaccurate estimates of the steepness and location of the high dose gradient regions that may be designed to protect adjacent critical structures and organs at risk. The Accuray CyberKnife® Robotic Radiosurgery system1–3 is at the time of publication the only robotic radiosurgery device in clinical use. It consists of a compact x-band linear accelerator mounted on an industrial robotic manipulator arm. The manipulator arm is configured to direct the radiation beams to the region of beam intersection of two orthogonal x-ray imaging systems integrated to provide image guidance for the treatment process. The patient under treatment is positioned on an automated robotic couch such that the target to be treated is located within this radiation beam accessible region. The movements of the robotic manipulator arm and the robotic patient support assembly are under the direct control of a computer system that is in turn controlled by the radiation therapist (during patient treatments) or the medical physicist (for quality assurance measurement purposes). The treatment planning system for the CyberKnife® is device-specific. It is an inverse planning system which uses linear optimization to optimize the beam angle and beam monitor units (MU). The user selects the preconfigured treatment path, collimator size, dose calculation algorithm (ray-tracing or Monte Carlo), and sets the dose constraints. While most CyberKnife® treatments are nonisocentric, there is a reference point in the room which serves as the origin for several coordinate systems used within the CyberKnife® application, and to which the robot and imaging calibration is defined. This point in space is defined by an “isocrystal” which is mechanically mounted on the “isopost.” In this report, this point in space is defined as the “geometric isocenter.” It must not be confused with the “treatment isocenter,” which refers to an isocentric treatment to a target which may be located at a distance from the geometric isocenter. While a small fraction of CyberKnife® treatments are either isocentric or an overlay of isocentric shots of different collimator sizes, the majority of treatments are “nonisocentric.” This means that beams are pointing away from the geometric isocenter to create highly irregular target shapes that can contain surface concavities. This document will cover the aspects of the CyberKnife® system that were well established at the time this report went to review, and therefore excludes devices or software which had a very limited user base (e.g., IRIS™ collimator, Monte-Carlo dose calculation, InTempo©, and external physician workstations). This report aims to define standards for an institutional quality assurance (QA) protocol for robotic radiosurgery. Efficacy and efficiency are key considerations in our process of developing the QA methodology. This report intends to give guidelines on setting up a comprehensive quality assurance (QA) program for robotic radiosurgery systems to complement the vendor guidelines. Acceptance testing and commissioning are outside the scope of this report; this report focuses on routine QA after commissioning and serves as a supplement to TG 142.4 Each institution should develop a comprehensive QA program for their robotic radiosurgery program that is customized to the unique nature of this treatment delivery system. It is incumbent upon the physicist to develop and implement such a program, based on how the equipment is to be used. In this task, he/she should refer to professional guidelines such as this document, manufacturer's recommendations, and the experience of other users. Any program must minimally meet state and federal regulatory requirements. In the following sections of this report, the words shall andmust are italicized to emphasize that they are being used in the special sense conveyed by the definition given below. • “Shall” and “must” are used when the activity is required by various regulatory agencies, or may be essential to meet currently accepted standards. • “Recommend” and “should” are used when the task group expects that the procedure should normally be followed as described. However, equivalent processes, criteria or methodologies may exist which can produce the same result. I.A. Structure of report This report is structured in five parts: an introduction, two major parts discussing QA, a summary section including QA checklists, and references. Section II is titled “QA for Individual System Components.” Each of the subsystems (robot and room, accelerator, imaging subsystem, and software) will be described and QA recommendations developed. Section III is titled “QA for Integrated Systems.” In this section we will discuss how the individual components are linked and describe the QA to check the various links between subsystems, leading to overall system QA. Section IV contains tabulated checklists for daily, monthly, and annual QA, as well as recommendations for special situations. I.B. Record-keeping In the current environment, technology is rapidly evolving. Hence, thorough quality assurance (QA) and quality control (QC) become an essential component in treating patients safely. With the arrival of new treatment techniques and modalities it is very important that the new procedures for QA tests and QC are well documented. Good record-keeping5 can increase work efficiency and reduce the risk of making errors for newly implemented QA tests. It will also make it easier to compare the test results to previous test results and ensures easy repeatability by multiple individuals, thus limiting the potential for errors. For every QA test, there should be a written guideline which clearly defines the objective, lists the action levels for the test, and corrective action(s) to be taken when these levels are exceeded. The QA guideline should include all tests necessary to evaluate equipment safety, patient safety, and overall treatment accuracy. In addition, the guideline must also meet state, federal, and/or any other regulatory agency requirements. It is essential to keep either a handwritten record or electronic record in a well-organized file. This file will provide documentation for a site visit or a department audit, as well as educate new personnel to the status and service history of the equipment. A good record allows another physicist to come into a clinic and completely understand what has been done previously and to recreate the tests performed.6,7 There should be a clear and concise description of each test. The results should be legible (if one is keeping paper copies) and should be compared to data which is clinically relevant. The comparison should clearly state if the result is or is not within the required criteria level. If it is outside the criteria level then it should clearly state what corrective action was taken, when, and by whom. Also, if the procedure has several different action levels (i.e., morning checks) it should clearly define each step and who should be notified at each of the different action levels. All documents should be dated and have a legible (if applicable, digital) signature of the person who completed the test. If a second check is made by another physicist then it should be clearly signed and dated by that physicist. Glossary AQA “Auto QA,” a Robot pointing test: The centering of a radiographic shadow of a 2 cm diameter tungsten ball hidden in a cubic phantom is measured on a pair of orthogonal films. CNR Contrast-to-noise ratio. Code of Practice: A systematic collection of rules, standards, and other information relating to the practices and procedures followed in an area. DQA Delivery Quality Assurance: The DQA plan is an overlay of a patient plan on a phantom. The plan is delivered and the measured dose in the phantom can be compared with the calculated dose for quality assurance, typically by using a gamma-index pass/fail criteria. The DQA assesses both spatial and dosimetric accuracy of delivery, and is the most comprehensive, overall assessment of the system. DRR Digitally reconstructed radiograph. E2E End-to-End test. A phantom containing a hidden target and orthogonal films is taken from simulation through treatment delivery. The spatial distribution of delivered dose is compared to the plan dose for the 70% isodose line. The E2E test is performed using an isocentric treatment plan. Its purpose is to be a more sophisticated Winston–Lutz test,8 checking spatial delivery accuracy together with tracking modality accuracy. Unlike the DQA test, the E2E does not have a patient-specific dosimetry component. EMO Emergency Motion Off. EPO Emergency Power Off. IGRT Image-Guided Radiation Therapy. Geometric Isocenter A point in space defined by the position of the isocrystal. Treatment Isocenter The common crossing point of the CyberKnife® beams in an isocentric (single center) treatment plan. This point is not required to be coincident with the Geometric Isocenter. Isocrystal A light-sensitive detector of about 1.5 mm diameter mounted at the tip of a rigid post whose position of peak internal sensitivity marks the alignment center for the ideal pointing direction of the center of all CyberKnife® radiation beams as defined by the position of the centerline laser. MC Monte Carlo. MTF Modulation transfer function. MU Monitor unit. OCR Off-center ratio. PDD Percent depth dose. QA Quality assurance. QC Quality control. SAD Source-to-axis distance. SNR Signal-to-noise ratio. SRS Stereotactic radiosurgery (including stereotactic radiotherapy, SRT). TG Task group. TPR Tissue-phantom ratio. TPS Treatment planning system. II. QA FOR INDIVIDUAL SYSTEM COMPONENTS II.A. Robot and room safety Any robotic system that causes the motion of either the patient couch or treatment apparatus in the immediate vicinity of a patient must have collision safeguards to prevent a potential collision with the patient. The details of how collision safeguards are implemented vary with the component and the overall system configuration. In general, collision safety precautions are dealt with in three stages in the use of a robotic radiosurgery system: (1) Design specification: Adequate space for all system components such that clearance issues for both the equipment and patient are verified prior to and during facility design and construction. (2) System installation, acceptance, commissioning, and upgrades: Items that are fixed by system design are verified as functional and adequate. In this category are elements of electrical safety (emergency offs, system motion disable, etc.), patient and robot movement restrictions, patient safety zones where robotic motion is excluded for patient safety, etc. (3) On-going system accuracy and safety testing: The periodic testing of safety systems to document the on-going function of system components. II.A.1. Mechanical safety and collision avoidance The CyberKnife® uses a minimally modified industrial robot to support and position a linear accelerator weighing approximately 160 kg. In the clinical implementation, the robot range of motion is restricted to a hemisphere around the patient. There are no inherent mechanical restrictions placed on the robot's movement, with the exception of the collimator assembly collision detector. We recommend checking the collimator assembly collision detector as part of the daily QA. The definition of any motion-restricted space is completely executed in the controlling computer software. It is very important to note that robot–patient collision control software is only functional while the system control software is running. If the robot is operated under manual control, software defined safety zones are not functional and cannot stop a violation of the robot exclusion zone and a subsequent collision. The CyberKnife® maintains separate zones of motion restriction. One zone is fixed with respect to the robot and includes system components that do not move, such as imaging system components, floor, walls, and ceiling. The second zone, the patient safety zone, is defined relative to the patient couch, and thus must be tested at various couch locations within the range of couch motions. Both fixed and patient safety zones shall be tested prior to the first clinical use of the system, and after any major software upgrade. A testing procedure is provided by the manufacturer during installation, but requires the assistance of a field service engineer. If an unusual patient position is required to access a particular treatment location such that a portion of the patient may extend beyond the patient safety zone, there will be no collision protection for this part of the patient. In this case, the setup should be evaluated for potential collisions by running the patient plan in simulation/demonstration mode with the couch and a phantom positioned similar to the realistic patient setup. The “simulation/demonstration mode” provides a mock treatment with the robot moving, but the accelerator switched off so the motion can be studied with observers in the treatment room. Alternatively, the patient position might be modified with the robot exclusion zone in mind to make better use of the patient safety zone. For instance, for a mid-pelvis treatment, a patient might be positioned feet first supine on the treatment table in order to have the feet extend out of the robot exclusion zone instead of the head. II.A.2. Ancillary safety systems All safety systems incorporated into the facility design must be verified initially and periodically as part of daily and monthly QA. These systems include emergency interruption for robot movement, emergency power off, audio and visual monitors, and door interlocks. In addition to the routine checks outlined below, these systems must be checked at installation and each time they may have been disabled or disconnected during maintenance work. Interlocks must occur immediately upon activation and remain engaged until the generating condition is reversed and acknowledged by the operator. Emergency power off (EPO) and emergency motion off (EMO) switches are required on robotic systems with components which could collide with a patient. The EPO will shut off power to the complete system, while the EMO only engages the robot mechanical brakes while leaving the accelerator and robot powered up. If a collision occurs and the EPO button is pressed instead of an EMO button, responders could lose precious minutes waiting for the robot system to be powered on before the robot could be moved away from the collision site. In addition, the EPO could potentially cause loss of robot mastering (see Sec. ???) due to the unclean shutdown of the robot controller PC. Therefore, the EMO button should be pressed in an emergency situation unless the electrical power is the cause of the unsafe condition, in which case the EPO should be used. All EMO and EPO wall switchesshall be tested annually. The EMO switch on the console should be tested on a daily basis, because it is the switch most likely to be used should an emergency situation arise during treatment. Audio and visual patient monitoring: As with all radiation therapy installations, state regulations requiring the presence of audio and visual patient monitoring also apply to a robotic system. Because the linear accelerator of a robotic treatment system is so flexible in its ability to be positioned around the patient, the likelihood of the robot and/or linac obscuring the view of the patient is high if there are only one or two observation sources. It is therefore recommended that at least three (preferably four) closed circuit television cameras (CCTV) be positioned in the treatment room such that any possible patient contact points can be seen by at least two of the monitoring CCTV cameras. Equally important as the presence of adequate CCTV cameras is the staffing requirement that at least one person in charge of treatment delivery must watch the video monitors during robot movement. II.A.3. Room shielding and radiation safety An example of room shielding design is given in NCRP Report No. 151,9 including a thorough treatment of the special assumptions and calculations required to execute an adequate shielding specification for this type of therapy machine. II.B. Accelerator QA Radiation for robotic radiosurgery devices is produced by compact linear accelerators that differ in some aspects from their isocentric gantry-mounted counterparts. The robotic nature of treatment delivery necessitates smaller weight and dimensions than conventional radiotherapy accelerators. The CyberKnife® beam source is a 9.5 GHz X-band accelerator producing 6 MV X-rays using a fixed tungsten alloy target with primary and removable secondary collimators. The secondary collimators have circular apertures with diameters ranging from 5 to 60 mm [defined at a source-to-axis distance (SAD) of 800 mm]. In addition, there is an in-line dual ion chamber for dose monitoring. Other collimator configurations with moving leaves similar to a camera aperture have become available (IRIS™) and will require additions to the QA procedures described in this report. Despite the differences between a robotic radiosurgery linear accelerator and the S-band accelerators used in conventional radiotherapy applications, most QA concerns and questions remain the same for both types of devices. With this approach in mind, it is straightforward to develop a quality assurance schedule for a robotic radiosurgery accelerator based on existing AAPM Reports.4,7,10–14 II.B.1. Daily accelerator QA It is important that the linear accelerator is sufficiently warmed up prior to obtaining any quality assurance measurements. It is recommended that each site establish a fixed number of monitor units (MU) for warm-up consistency. The number of MUs needed may depend on accelerator generation and chamber type (open vs closed). Older CyberKnife® accelerators have monitor ion chambers that are open to ambient temperature and pressure changes, while newer systems have “closed” chambers. Figure 1 shows the output of a closed and an open ion chamber as a function of warm-up MU. Running a warm-up should be considered after a machine downtime of more than 4 h. For accelerators with closed chambers, a warm-up of 2000 MU is sufficient. Figure 1Open in figure viewerPowerPoint Output of a closed (sealed) vs. open (vented) chamber as a function of warm-up MU. Data courtesy of Accuray, Inc. An open chamber will continue to warm up and cool down during a normal treatment day. A warm-up of about 6000 MU will put the chamber at a temperature which reflects the average chamber temperature status during a typical treatment. The actual fluctuation of the chamber during a treatment day is smaller than the full range of 2.5% graphed in the plot. The output of the linear accelerator in general should be measured once per treatment day, e.g., using a Farmer chamber with buildup cap. More frequent measurements for open-chamber systems may be justified if significant changes in temperature or atmospheric pressure occur within the course of a treatment day. In order to minimize the possibility of manual entry errors leading to incorrect output, it is strongly recommended that each CyberKnife® site determine an acceptable tolerance level, e.g., 2%, within which no adjustment to the calibration factor is made. This daily variation is less than the 3% recommended in TG-40 (Ref. 7) and TG-142,4 but the large fractional doses delivered in radiosurgery and hypo-fractionated radiotherapy justify a more stringent guideline. It is also strongly recommended that if the variation exceeds 2%, a Qualified Medical Physicist corrects the calibration. On a daily basis, we also recommend inserting an incorrect secondary collimator in treatment mode to verify the collimator interlock. Similarly, the interlock for a missing collimator should be checked daily. II.B.2. Monthly accelerator QA The dose output, energy constancy, and the consistency of the beam shape and beam symmetry should be checked monthly and compared to values obtained during commissioning. Typically, the largest collimator (60 mm) is used for this check. Symmetry measurements are similar to those performed on radiotherapy linear accelerators.10 Film irradiation and analysis may use point or area methods to evaluate beam symmetry, but following TG 45 and TG 142 (Ref. 4) are encouraged. Symmetry should be measured at a depth of 50 mm in two orthogonal planes (nominal in-plane and cross-plane). The measurements should pass the criterion established at the institution, which should be the same or more stringent than the acceptance testing criteria. Because the CyberKnife® linear accelerator does not have a flattening filter, beam profiles are curved in the central portion of the beam. Therefore, the concept of “flatness” normally measured for radiotherapy beams is not applicable. While any number of point or area measurements for the beam profile may be used to establish constancy, it is recommended to use at least three radial locations within the central portion of the beam. The relative values should not differ from beam data in the treatment planning system by more than 1%. For example, irradiate radiochromic film using the 60 mm collimator and compare the ratios of intensity values at 10, 15, and 25 mm radii to the treatment planning system (TPS) beam data. II.B.3. Annual accelerator QA Though recommendations on commissioning15,16 are beyond the scope of this report, it is recognized that commissioning is a critical aspect from the point of view of patient safety. In small beam dosimetry, the choice of an inadequate detector can result in severe dosimetric errors. AAPM TG 106 (Ref. 14) on “Accelerator Beam Data Commissioning and Equipment” contains guidance on appropriate equipment for use in the commissioning and annual QA process, including guidance on which detectors may or may not be appropriate for measuring data for small beam sizes. TG 51 (Ref. 13) or IAEA TRS-398 (Ref. 11) will be the assumed method for performing annual dose calibrations until new standards for small beam dosimetry are developed. The key difficulty with employing either method for CyberKnife® calibration is the assumption of a 10 cm × 10 cm radiation field for determining the value for kQ.13,17 Instead, a machine-specific reference field,17 i.e., the 60 mm collimator, is used for CyberKnife®. Equivalent field size corrections can be estimated for either %dd(10)x or TPR(20/10) using, for example, the BJR Supplement 25 tables.18 Only a 0.3% error is made if the kQ from a 6MV linac with TPR(20/10) of 0.68 is used.19 For consistency, the PDD at SSD = 100 cm for the 60 mm collimator should be measured with the same (small) chamber that is used for the TG-51 calibration. Converting the round field size of the 60 mm collimator and adjusting the collimator size for the extended SSD, an equivalent square field size of 6.75 mm × 6.75 mm results. An interpolation leads to the PDD at 10 cm depth. The PDD at 10 cm depth can be compared with a standard reference such as the British Journal of Radiology (BJR) Supplement 25 (Ref. 18 for the 6.75 cm square field size. From this value, the equivalent associated PDD value for a 10 cm × 10 cm field can be inferred. The active length of the detector used for absolute dose calibration has been shown to systematically change the calibration results.19 Detectors for absolute dose calibration of the CyberKnife® should not have an active length of more than 25 mm, and ideally have an active length of no longer than 10 mm. As with any clinical accelerator, the calibrationshall be traceable to NIST. The recommendation is to perform an independent verification as well, e.g., by participating in a TLD program through an accredited dosimetry calibration lab (ADCL). A secondary check using independent equipment by another qualified physicist similar to the annual peer review as recommended in Ref. 6 is also an option. The annual QA of the accelerator should repeat selected water phantom measurements performed during commissioning. It is important to verify that the accelerator central axis laser and radia
Referência(s)