AAPM TG 179

Quality assurance for image-guided radiation therapy utilizing CT-based technologies: A report of the AAPM TG-179Jean-Pierre Bissonnettea) Task Group 179, Department of Radiation Physics, Princess Margaret Hospital, University of Toronto, Toronto, Ontario, Canada, M5G 2M9 Peter A. Balter and Lei Dong Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center, Houston, Texas 77030 Katja M. Langen Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, Florida 32806 D. Michael Lovelock Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, New York, New York 10021 Moyed Miften Department of Radiation Oncology, University of Colorado School of Medicine, Aurora, Colorado 80045 Douglas J. Moseley Department of Radiation Physics, Princess Margaret Hospital, University of Toronto, Toronto, Ontario, Canada, M5G 2M9 Jean Pouliot Department of Radiation Oncology, UCSF Comprehensive Cancer Center, 1600 Divisadero St., Suite H 1031, San Francisco, California 94143–1708 Jan-Jakob Sonke Department of Radiation Oncology, The Netherlands Cancer Institute—Antoni van Leeuwenhoek Hospital, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands Sua Yoo Department of Radiation Oncology, Duke University, Durham, North Carolina 27710 (Received 11 August 2011; revised 19 January 2012; accepted for publication 10 February 2012; published 20 March 2012) Purpose: Commercial CT-based image-guided radiotherapy (IGRT) systems allow widespread management of geometric variations in patient setup and internal organ motion. This document provides consensus recommendations for quality assurance protocols that ensure patient safety and patient treatment fidelity for such systems. Methods: The AAPM TG-179 reviews clinical implementation and quality assurance aspects for commercially available CT-based IGRT, each with their unique capabilities and underlying physics. The systems described are kilovolt and megavolt cone-beam CT, fan-beam MVCT, and CT-on-rails. A summary of the literature describing current clinical usage is also provided. Results: This report proposes a generic quality assurance program for CT-based IGRT systems in an effort to provide a vendor-independent program for clinical users. Published data from long-term, repeated quality control tests form the basis of the proposed test frequencies and tolerances. Conclusion: A program for quality control of CT-based image-guidance systems has been produced, with focus on geometry, image quality, image dose, system operation, and safety. Agreement and clarification with respect to reports from the AAPM TG-101, TG-104, TG-142, and TG-148 has been addressed. C V 2012 American Association of Physicists in Medicine. [http://dx.doi.org/10.1118/1.3690466] Key words: quality assurance, cone-beam CT, fan-beam MVCT, CT-on-rails I. INTRODUCTION The goal of radiation therapy (RT) is to deliver accurately a curative or palliative dose distribution to a well-defined target volume. Unlike dose calculation and measurement accuracy, the geometric accuracy of RT has been a challenge that could only recently have been quantitatively and pragmatically ascertained.1 Lately, medical linear accelerator (linac) 1946 Med. Phys. 39 (4), April 2012 manufacturers and third-party vendors have developed integrated imaging systems to improve and facilitate internal patient anatomy visualization, enabling efficient positioning of these anatomical structures relative to the treatment room. These systems often use the accelerator isocenter as a reference point. The initial use of daily computed tomography (CT) has been for assessing internal organ position and defining the subsequent isocenter shifts to be performed at C V 2012 Am. Assoc. Phys. Med. 0094-2405/2012/39(4)/1946/18/$30.00 1946 and is paving the way toward advanced and adaptive RT. usually by 180 .9 vendor literature.5 while kilovoltage CBCT (kV-CBCT) provides high contrast volumetric datasets using imaging components mounted orthogonally with respect to the treatment beam. by assessing the long-term performance of this novel equipment by analyzing data from quality control (QC) tests. 4. By improving the geometric accuracy of RT. II.4 Automatic couch motion None Submillimeter 3–10 1. and individualized PTV margins can be achieved.6 Clinical implementation of both kV-CBCT and MV-CBCT systems necessitates conducting calibration procedures that correct for accelerator and imaging component sags and flexes and to properly register to the treatment beam isocentre. incremental improvements in tumor control probability. kV-CBCT has become an important tool for localization and patient monitoring in .7–3.05–1 3 s per sec Translation Rotation Geometric accuracy Dose (cGy) Image acquisition and reconstruction time the treatment unit. such as the AAPM TG-40 report. No.10 geometric accuracy. reduction in toxicity (thereby allowing dose escalation). In this report.1–3.5 2 min Varian On-Board Imager kV-CBCT 45 Â 45 Â 17 Automatic couch motion None Submillimeter 0.7 The clinical introduction of these guidance systems (Table I) has allowed the assessment and correction of patient positioning uncertainties.18–20 and imaging dose. and. we present a succinct review of commercially available CT-based IGRT systems and present the general QA principles for these devices. clinics need to employ robust quality assurance (QA) programs that ensure that the system performance meets high expectations consistent with patient care requirements.2–2. clear and concise recommendations regarding clinical commissioning and QA of these technologies and their related clinical processes are desired by the community. conformal avoidance by intensity-modulated radiation therapy.15. While diagnostic CT effective doses are in the range of 2 to 10 mSv. This report concludes with a brief discussion of the safe and efficient implementation of these technologies. evaluating their resources to determine if additional resources are required. the couch is aligned with the CT gantry motion path. CT-on-rails Integrating a diagnostic CT scanner into a RT treatment room was perhaps the earliest implementation of volumetric image-guided radiotherapy. As of this writing. Make and model Imaging configuration Field of view Correction method Elekta XVI kV-CBCT 50 Â 50 Â 25.28 This is because the image quality from low-dose CT imaging is sufficient for image alignment.5 min TomoTherapy MVCT 40 cm Automatic in 2 directions Optional Submillimeter 0. Early adopters of this technology have relied on the spirit of established standards. the couch rotates back to the linac side to proceed with treatment. which acquires a patient’s CT images while the patient remains in the immobilized position. which is mounted on rails (henceforth referred to as “CT-on-rails”) so that it can move across the patient instead of the couch moving the patient through the scanner as in conventional CT scanner design. The first integrated clinical system combining a linac and an in-treatment-room CT unit was developed by Uematsu et al.7. in Japan.5 min Siemens Artiste MV-CBCT 40 Â 40 Â 27.2 Fan-beam MVCT has been introduced clinically by integrating it with helical tomotherapy based with IMRT-based dose delivery. II.27 imaging doses typically can be reduced further by a factor of 2–4 when used for daily targeting. undesirable events in RT have resulted from human error rather than equipment failure. The distinguishing feature of the integrated CT-linac system is the moving gantry CT scanner. EXISTING TECHNOLOGIES II.20–24 Professional bodies interested in establishing QA and QC guidelines are beginning to expose device-specific QC to formal analysis of the frequency and severity of the risks or perceived failure modes involved with novel technologies. This is Medical Physics. By rotating the treatment couch. 39.6 Automatic couch motion Optional Submillimeter 0.0 1. Kilovoltage cone-beam CT In the past several years.25 Users designing their own IGRT QA program should first identify clearly the clinical aims and align the QA needs to these aims.1947 Bissonnette et al. revealed internal organ motion and deformation.8 Due to the rapid deployment and adoption of image-guided radiation therapy (IGRT).11–17 image quality. Physicists who are involved in starting CT-based IGRT technologies should study and understand not only the clinical potential of IGRT but also the intricacies of process design and development. if not most. To fully exploit the information provided by these systems.B.3 Megavoltage cone-beam CT (MV-CBCT) uses the accelerator’s treatment beam and its portal imaging system to provide volumetric datasets with sufficient contrast for image-guidance4. fractionated stereotactic treatments of brain and lung cancers. Subsequently. workflow improvement.: QA for image-guided radiation therapy utilizing CT-based technologies 1947 TABLE I. and change implementation within a busy clinic. Early publications on quality assurance of CT-based IGRT systems have evaluated safety. April 2012 because many. Vol. Commercially available CT-based IGRT systems. frameless. there exist no consensus guidelines for a comprehensive quality assurance of CT-based image-guidance systems. less frequently. experience acquired at the time of acceptance testing.0 5 s per slice Siemens Primatom kVCT-on rails 50 cm Manual couch motion Optional Submillimeter 0.26 The original system was primarily designed for noninvasive.A. 5–2 mm were reported.55 Using megavoltage x-rays for imaging also eliminates artifacts normally caused when high-Z materials are imaged with kilovoltage x-ray beams. Vol.54 In comparison with kV-CBCT (Sec. CA) consists of an aSi flat panel adapted for MV imaging attached to a linear accelerator and an integrated workflow application to generate a three-dimensional representation of the patient in treatment position.4 cm.24 Fan-beam MVCT image quality in terms of noise. 2. which translates into slice thicknesses of 2.4 cm. uniformity. CT-on-rails The CT-on-rails system produces diagnostic quality images. with the flat panel positioned at 145 cm source-to-imager distance (SID).19 Woodford et al. the imaging beam is in the megavoltage range.61 with the lower end used when daily acquisitions are performed on a patient.2 to 2 cGy per acquisition have been reported in the literature.45–51 II. respectively. The system performs the acquisition of projection images. The longitudinal extent of the scan is variable and is selected by the user. simplifying quality assurance of the system. the commercially available MV-CBCT system (ArtisteTM. For head and thorax phantoms.61.20.62 One benefit of the MV-CBCT system is its simplicity. thus rendering the images immune to typical high-Z artifacts. the kV-CBCTs can be reconstructed with submillimeter isotropic voxels. To acquire the kVCBCT projection data.C.40 Because of the high spatial resolution of the imaging panels.52. Siemens. No. and the scanned anatomical region. These projections are used to reconstruct the CBCT volumetric images. The imaging dose can be straightforwardly accounted for in treatment planning. The field of view is 40 cm in diameter.58–60 and sufficient soft-tissue contrast to visualize structures such as the prostate.21. inherently causes poor subject contrast. while 6 to 10 cGy are used for tumor monitoring studies or for treatment planning purposes. The MV-CBCT system demonstrates submillimeter localization precision23.4.57 This provides a 3D patient anatomy volume in the actual treatment position that can be aligned to the planning CT moments before the dose delivery. have tested the registration accuracy and precision of the fan-beam MVCT system using a set of anthropomorphic phantoms.5 MeV for fan-beam MVCT. with a slice thickness ranging from 0.: QA for image-guided radiation therapy utilizing CT-based technologies 1948 traditionally fractionated and hypofractionated RT. Concord. The use of the same imaging modality as that employed for treatment planning not only facilitates image registration to align the gross tumor volume (GTV) directly . and image artifacts.5 to 10 mm. The registration error was dominated by the error in the superior–inferior direction. The current generation of MV CBCT systems offers a half-beam acquisition mode. CBCT image reconstruction. II A). II. kV-CBCT uses a cone-shaped x-ray beam and acquires an entire volume (14–26 cm in length) in a single.14 They have shown that the Medical Physics.13. the EPID. soft tissue. Research is ongoing to alleviate these factors and likely to improve rapidly with advancements in acquisition techniques42–44 and reconstruction algorithms. or 3. 39. is generally adequate for imaging bone and.A.41 kV-CBCT image quality is limited compared to traditional CT for a number of reasons including motion blur due to the long acquisition time. increasing the reconstruction size in the axial plane of up to 40 cm. In radiotherapy applications. April 2012 As of this writing. The dose used for MV-CBCT depends on the clinical application but typically ranges from 3 to 10 cGy.53 The imaging beam is produced by the same accelerator that generates the treatment beam. Fan-beam MVCT registration accuracy depends on the imaging slice thickness. The MV-CBCT beam geometry is fixed by the manufacturer.D. contrast linearity. contrast. pitch value. and the field length is adjustable to a maximum of 27. relatively slow gantry rotation. Megavoltage cone-beam CT The helical tomotherapy delivery system can be used to obtain fan-beam MVCT images of the patient in the treatment position. typically resulting in 2 projections per degree over 195–360 arcs. flat-panel detectors are used in fluoroscopy mode.23. a configuration that is commercially available. the kV-CBCT tube and detector are mounted on the same gantry as the linac treatment head. however. Doses ranging from 0. the superior-inferior scan length. The field width is set by the manufacturer at 27. kV-CBCT imaging dose varies widely with the acquisition technique. obtaining multiple projections per second. The MVCBCT system can reconstruct a field of view of up to 27 cm. The fan-beam MVCT imaging dose is typically in the range of 1–3 cGy per scan. scattered radiation due to the volumetric image acquisition. TYPICAL CLINICAL APPLICATIONS III.06–2. though not of diagnostic quality.1948 Bissonnette et al.5 mm and 0. of 6 mm.29–39 Traditional CT uses a fan shaped x-ray beam to acquire one or more thin slices (0.29. 4. The megavoltage beam. and remote couch position adjustment. so. In contrast. in some anatomic sites. respectively.22. published studies have used doses as high as 6–10 cGy per MV-CBCT scan. 4. Similar to fan-beam MVCT. the beam is collimated to 4 mm at the isocenter and images are typically acquired with a pitch value of 1. those technologies using megavoltage beams for imaging suffer from fewer scatter and beam hardening artifacts.58 III. registration accuracies in the ranges of 0. which projects to the detector’s 40 cm active region. and spatial resolution has been reported by Meeks et al.4 cm in length) per tube=detector rotation. automatic CT to CBCT volumetric image registration.56 During image acquisition.5–1.19 Fan-beam MVCT scans are noisier than kVCT scans but the resulting low contrast resolution remains sufficient to identify some soft tissues. This geometry provides easier access to the patient by the therapists.40 kV-CBCT produces a full CT data set that. The image is directly referenced to the beam.20. but with the nominal electron beam energy reduced to 3. enabling the IGRT process. There is only one x-ray source and one detector. bony anatomy.109.133 III.D.127 The radiation response of an esophageal patient has been documented by Chen et al. 39.105–108 Furthermore.31. or implanted markers. permitting immediate correction of initial and intrafraction geometric discrepancies.104 Perhaps.111 or as a dataset for assessing dose-related anatomical changes. it may be impossible to correct for radiation delivery errors by modifying subsequent fractions.C. the relatively high dose per fraction increases the potential for normal tissue damage or serious target underdosing.125.23. 4.122–124 Deformation of the pelvic anatomy was reported for prostate patients.146 Target or normal structure positioning relies on the ancillary imaging equipment in the treatment room.B.129 Lastly. kV-CBCT offers a distinct advantage over projection imaging in that some soft tissue structures can be directly imaged and thus targeted.136 pelvic structures in the presence of hip replacement prosthetics. Stereotactic body radiotherapy requirements Daily fan-beam MVCT-based alignments are performed typically for all patients who are treated with helical tomotherapy based IMRT.36.100 esophagus.118.58.137 and.110 kVCBCT also makes adaptive planning possible.74–76 and stereotactic hypofractionated lung and paraspinal cancer treatments. head and neck. Alignments are based on soft tissue targets.32. Although the initial approach taken by SBRT developers was stereotactic in that the treatments were setup using body-frame coordinates. image quality. No.62 Other applications also include the monitoring of tumor growth and shrinkage23.120 and gynecological tumors. stereotactic body radiotherapy (SBRT).E. and spatial resolution need to be evaluated as part of a regularly scheduled QA program designed and managed by . depending on the visibility of the target in fan-based MVCT images. The use of fan-beam MVCT for alignments has been reported for prostate.130. Vol. uterus.138 An emerging use of MV-CBCT images includes dose verification using dose recalculation139–141 and dose-guided radiation therapy (DGRT). if even a single treatment is incorrectly delivered. finally.128 Li et al. kV-CBCT dose calculations. and lung alignments. The AAPM Task Group 101 recommends the use of image guidance for all SBRT treatments to eliminate the risk of a geometric miss. MV-CBCT In clinical applications. Fan-beam MVCT The MV-CBCT imaging procedure has been well integrated in the clinical workflow for the patient alignment and IGRT processes.1949 Bissonnette et al.121 In addition to patient alignment. such as paraspinal tumors in proximity of orthopedic hardware.115. April 2012 SBRT is characterized by the accurate delivery of high doses of radiation in five or fewer fractions. necessitating the development of a rigorous QA program. report anatomical variations for kidneys. allowing for either margin reduction104. geometric accuracy (including linearity and alignment between the imaging system and the radiation isocenter). When compared against conventional fractionation. an adaptive strategy where treatment modifications are based on comparisons of the dose-of-theday with the planned dose distribution.4 many papers have reported on the clinical applications of MV-CBCT.: QA for image-guided radiation therapy utilizing CT-based technologies 1949 but can also use the entire CT image set for adaptive replanning to account for interfractional anatomy changes.142–145 III. These applications include prostate. pancreas.131 Daily fan-beam MVCT scans can be used for Medical Physics.12. fanbeam MVCT imaging has been used to document anatomical variation for various anatomical sites.114 III. the high accuracy of kV-CBCT based target positioning has eliminated the need for body frames equipped with stereotactic coordinate systems.116 lung.117 head and neck. geometric changes in the parotid glands were reported for head and neck patients using fan-beam MVCT imaging. Lung cancer tumor regression measurements based on fan-beam MVCT imaging have been reported. and in many cases.55 brachytherapy applicators and catheter visualization.97–99 breast.135 MV-CBCT has been used clinically to improve the delineation of structures in CT images that suffer from metal artifacts. kVCBCT guidance is also utilized extensively in other treatment sites like head and neck.77–84 The potential use of repeat in-room CT for online or offline adaptive radiotherapy has been studied by various researchers. Patient safety. and sarcomas.102 and bladder.134 and more advanced IGRT strategies where a multiple adaptive plan IMRT procedure accounts for the independent movement of the prostate and pelvic lymph nodes.126.130. the relatively low doses delivered by this modality permit more frequent patient position monitoring during those long sessions.112. The geometric accuracy achieved by such machines has been deemed sufficient to permit bony or soft tissue localization or target-surrogates.113 Also.63–73 anatomy changes and their dosimetric impact in head and neck cancers.126 Movement of the mesorectal space was evaluated on fan-beam MVCT images by Tournel et al.132. normal tissues just prior to treatment.101 liver. Two clinical sites that directly benefit by this are the prostate95 and the lung.103. These spatial positioning issues have been addressed in large part by the widespread adoption of treatment machines with volumetric kilovolt or megavolt imaging capabilities. reducing the effect of intrafraction position uncertainties. the most important application of CBCT has been the simplification of hypofractionated. CT-on-rails systems have been used to study organ motion and soft-tissue localization for prostate cancer.96 the former cannot be directly targeted with projection imaging without implanted fiducial markers.85–94 III. Since the first MV-CBCT image of a patient was acquired in 2003. target position uncertainties due to organ motion and setup errors remained and were similar to those encountered with conventional radiotherapy. While robust patient immobilization for the long treatment times typical of this technique is still required. and the variations of target and organ at risk doses have been reported.119 breast. Furthermore. the intracranial stereotactic radiotherapy workflow has been adapted to benefit from the volumetric information obtained from CBCT. the measurement of small lesions near metallic implants. MVCBCT. manpower.148. No. to be well within 1 mm over extended periods of time. geometric accuracy. users can modify test frequency and tolerance according to clinical usage and machine capability. Therefore. and craniocaudal orientations are maintained (upon upgrade from CT to IGRT system) Long and short term planning of resources (disk space. Importantly. as specified elsewhere159–161 and in Sec. It is recommended that the geometric calibration be tested daily (see Sec. IV. must be considered carefully. The geometric calibration is typically expressed as a function of gantry angle since the TABLE II. is noted for daily QC checks of CT-based IGRT technologies for SBRT as described in the TG-142 report.11. Medical Physics. image quality test frequency could be aligned with those of conventional CT scanners after sufficient experience with the image-guided systems. Where noted. 39. a well-designed QA program will satisfy simultaneously the requirements of conventional and SBRT radiotherapy. daily quality assurance checks of geometric accuracy are recommended. henceforth termed geometric calibration..102. ms accuracy. While patients and tumors can be placed within the intended position immediately prior to treatment. noisea High contrast spatial resolutiona Low contrast detectabilitya CT number accuracy and stabilitya Imaging dose X-ray generator performance (kV systems only): tube potential. Geometric accuracy IV. or imaging dose necessitate full recommissioning of the IGRT system.15. as recommended by the manufacturer. or length of time spent on the accelerator couch. Frequency Daily Quality metric Safety System operation and accuracy Monthly or upon upgrade Geometric Quality check Collision and other interlocks Warning lights Laser=image=treatment isocentre coincidence OR Phantom localization and repositioning with couch shift Geometric calibration mapsa OR kV=MV=laser alignment Couch shifts: accuracy of motions Scale. 6–12 months after commissioning.17. II B of the AAPM TG-142 report. the relationship between the two isocenters. These checks can be easily made by imaging a phantom that has been positioned independently. These are generic test frequency and tolerance The value of CT-based image-guidance systems lies in their three-dimensional description of internal patient anatomy and its spatial relationship to the linac radiation isocenter.e. with the room lasers. distance.149 Because of the critical importance of the imaging system in SBRT patient positioning.A. image quality. and TG-148 reports. etc. for example. TG-179 is of the opinion that any major software or hardware upgrades that impact geometric accuracy. Fortunately. IV G).109 recommendations.18. and verifying that the setup correction is within tolerance. and orientation accuracya Uniformity. April 2012 . Likewise. a discrepancy.1950 Bissonnette et al. QUALITY ASSURANCE ISSUES Recommendations from this report are summarized in Table II. experience and performance. 4. and geometric fidelity have been demonstrated.7. in a number of publications.105. for technologies where the imaging and linac radiation isocenters are mismatched (i.151 there is mounting evidence that internal and external patient motion displaces the target away from the intended position. Vol.) Tolerance Functional Functional 62 mm 62 mm Replace=refresh 61 mm 61 mm Baseline Baseline 2 mm (or 5 lp=cm) Baseline Baseline Baseline Baseline Image quality If used for dose calculation Annual Image quality Dose Imaging system performance Geometric Accurate System operation Support clinical use and current imaging policies and procedures a These tests can be performed on a semiannual basis after stability has been demonstrated. and the periodic testing of this calibration.58 Such geometric accuracy is considered sufficient for both SBRT and conventional radiotherapy treatments.35. except for MV-CBCT where the localization accuracy is within 2 mm.18.12. performance status.: QA for image-guided radiation therapy utilizing CT-based technologies 1950 medical physicists.152–158 and patient position reassessment may be required throughout SBRT delivery depending on the chosen immobilization scheme. mA. discussed in Sec. localization. Because the geometric accuracy of CT-based imaging systems for image-guidance is inherently high. mediolateral. and linearity Anteroposterior. While this report is in full agreement with the AAPM TG-101 and TG-104.147 The resolving power of CTbased IGRT systems can also be on the order of 1 mm under favorable scatter conditions. representing the minimum imaging and registration performance needed for conducting IGRT with the technologies described in this report. service repairs and interventions should be followed by relevant and appropriate QC tests or baselines refreshing. Tolerances may change according to expectations.160 Of note. kV-CBCT.35.150. IV G. Summary of QC tests recommended for CT-based IGRT systems.1. and CT-on-rails). clinical and physiological process issues may ultimately affect the geometric accuracy of SBRT treatment delivery. after system upgrades or after service that could potentially invalidate it. To eliminate imprecision in the jaw position. the synchronization of the imaging and couch motions is explicitly tested. The accuracy of this method is generally limited to the alignment of the room . Under controlled conditions using rigid phantoms. such as BBs. the image acquisition.166 The residual error is well below 1 mm after correction irrespective of the correction type for image-guided systems based on CBCT or for the CT-on-rails systems. however. the ball bearing images are obtained using the image-guidance system. However.163 consists of placing a metal ball bearing (BB) near the radiation isocenter and using portal images acquired at the four cardinal angles to compare the ball bearing image centroid to the field edges. The couch coordinates of this location are recorded. the image registration is tested for consistency. The first approach is to digitally shift the projection images according to the measured flexmap prior to reconstruction of the volumetric datasets. as is done during simulation.11. on a daily basis. This is a phantom based end-to-end test intended to check the image registration and treatment delivery chain. For fan-beam MVCT units. The Winston–Lutz test is a comprehensive test to identify misalignment without explicitly identifying or correcting the cause of such misalignment. the imaging beam is generated by the same source that generates the treatment beam and the two beams share a common geometry. Fiducial markers. and registration process uses hardware and software components that have the potential to introduce geometric errors in the fan-beam MVCT IGRT process. 11 and 15)]. April 2012 performed on the IGRT components or after major software or hardware upgrades. The alignment between the CT and the linear accelerator isocenter. enabling the BB images to act as registration markers. This method relies on the room lasers acting as accurate surrogates of the accelerator isocenter. irrespective of conventional or hypofractionated radiotherapy regimes. Commercial vendors have proposed two automatic approaches to correct for system flex.161 and the interested reader is referred to that task group for a detailed discussion. and the subsequent alignment process is tested for accuracy. Shifts that will align the patient’s anatomy with the planned isocenter can then be calculated and reported as updated couch coordinates. The shifts identified in the measured flexmap are performed automatically by the image-guidance software. Similar to fan-beam MVCT. The second approach consists of moving the image-guidance system xray detector according to the flexmap to ensure that the detector is always coincident with the radiation beam central axis. While similarities exist between the kV-CBCT geometric calibration procedure and the Winston–Lutz test162 used to verify the alignment of stereotactic radiosurgery frames and attachment equipment. derived from the Winston–Lutz procedure162 and closely following the procedure described in Appendix G of the AAPM TG-66. The ball bearing can then be moved iteratively toward the accelerator radiation isocenter until it indicates accurately the location of the isocentre. and lateral and vertical shifts may be necessary to fit the couch through the CT bore. 60. Another approach to ensure high geometric accuracy is to image external fiducial markers.16 demonstrating that CT-based IGRT systems are capable of high geometric accuracy [typically.165 This first approach can take up to 2 h to perform.16. which couples patient location with couch coordinates. to transfer the radiation isocentre location into CT space. can be placed on the couch. reconstruction. or the patient’s surface at the laser intersection. The flexmap is typically measured at commissioning time. Users are cautioned. Finally. Spatial accuracy and geometry tests for fan-beam MVCT are described in TG-148.164 Once positioned.12 The clinical accuracy is expected to be worse because the process of rotating the patient couch into the CT imaging position introduces additional errors due to couch bearings eccentricity. The spatial and geometric accuracy of the fan-beam MVCT based IGRT system therefore needs to be tested routinely and consistently. Vol. immobilization device. patient motion. and the limited couch readout and control precision. Once this relationship is known. verified on a monthly basis. This leads to an inherent robustness of the MVCT imaging system geometry. the CT-on-rails solutions offer high CT gantry motion rigidity and reproducibility. No.3 mm (Refs. rendering it unsuitable for daily testing. A test of the image orientation and spatial integrity accuracy is recommended with a monthly frequency. An annual test of the imaging=treatment=laser coordinate coincidence is recommended. Briefly. A plot of the distance between the measured isocenter pixel and the pixel that would nominally intersect isocenter is termed a flexmap. to perform the manufacturer recommended calibration procedures whenever service that could modify geometric calibration is Medical Physics. A simultaneous test of the laser and imaging system coincidence with the imaging system enables the use of the laser system as a surrogate for the isocenter for daily and monthly consistency tests. The test frequencies and tolerance values that are recommended in TG-148 are consistent with TG-142.1951 Bissonnette et al.: QA for image-guided radiation therapy utilizing CT-based technologies 1951 imaging and therapy system components flex during gantry rotation. 39. 4. Analyzing the apparent travel of the ball bearing on the projection images used for reconstruction of volumetric datasets provides a measurement of the components’ flexing as a function of gantry angle. the pixel coincident with the isocenter can be determined and the projection image pixel locations referenced to the isocenter pixel. A convenient method for performing kV-CBCT system geometric calibration. images are acquired with the collimator rotated by 180 .7. The treatment couch is rotated prior to align the patient with the CT scanner. CT isocenter to treatment isocenter alignment accuracy of better than 61 mm has been demonstrated. The geometric calibration procedure described above expects flexes and other misalignments to occur and actively corrects them whereas the Winston–Lutz formalism assumes rigidity of all system components. the two should not be confused. Not only does the flexmap correction remove the blur due to the imaging component flexes but also aligns the resulting image with the accelerator isocenter. however. does not have the same rigidity as does other CT-based image-guidance systems since the imaging and treatment equipment do not share a gantry. and comparing the position of the projection of the crossing of the wires with the position of the central pixel corrected for the residual misalignments. patient too large for conventional CT bore or adaptive radiotherapy programs).1952 Bissonnette et al. so test tolerances and frequencies have not yet been determined. this procedure can repeated five times. Norfolk. NY) or the AAPM CT performance phantom (CIRS..58 The position accuracy is checked by placing a reference point at the center of each bead in the CB image of the phantom and ensuring that the recorded position of that reference point is within 1 mm of the physical bead position in the phantom.172 This section describes the general principles of these tests. x-ray scatter reduces soft tissue contrast. to the high quality detectors used in multislice CT scanners. a vertical position error may result in some image distortion. The phantom is a cylinder with four tungsten beads placed 90 apart on the surface.23. the large cone angle used by these technologies allow x-ray scatter to contribute undesirable signals to the reconstructed images. VA). However. differences that impact image quality. which is recommended to be checked daily. since the projection matrices derived during the geometry calibration procedure are only strictly valid at a SID of 145 cm. usually flat panel imagers (FPI) that are inferior. A specialized phantom can be used to test MV-CBCT geometric accuracy and registration. Therefore. No.58 EPID positioning errors with respect to the machine isocenter should not exceed 1 mm in the horizontal plane.40. Based on the available evidence and given the technological . It is recommended that the image quality tests be performed during system acceptance to obtain a system performance baseline that can thereafter be compared to quality assurance results acquired under identical conditions.20. the 3D CT geometric accuracy should be checked annually and the alignment between the CT and the radiation isocenter or the lasers and the radiation isocenters should be checked daily. the plane perpendicular to the beam central axis) is recommended to be checked daily by acquiring two portal images for a reticule with two orthogonal tungsten wires.18–20 Evidence-based guidelines have not yet been established because few authors have repeated these tests over extended periods of time. a longitudinal (lateral) EPID positioning error will result in an error in the longitudinal (radial) position of the image isocenter with respect to the machine isocenter. The design of other CT-based IGRT systems.18 Finally. and verified with megavoltage portal images. 4. Examples include the CatPhan 500 phantom (The Phantom Laboratory. introduces cupping and capping artifacts in 3D reconstructions. IGRT systems are usually used to localize targets and organs at risk and drive correction strategies to minimize geometric uncertainties.e. In either case. using a ruler. Most image quality control tests can be performed using commercially available phantoms that contain multiple inserts tailored to test various aspects of image quality. Moreover. CT number linearity and accuracy become important only if the CT scans are also used for dose calculation (e.g. with and without gantry sag effects.15. Similar phantoms are being adapted for megavoltage imaging purposes (Siemens Medical Solutions. the AAPM TG-179 recommends that image quality tests be performed initially on a monthly basis.18 Measurements acquired over extended periods of time have shown that most image quality parameters do not vary much over time. the methodologies described in the AAPM report 74 have been adopted for CT-based technologies. This can be done by imaging a device placed at radiation isocenter at a known offset. IV. CA). differs from conventional CT scanner designs.7% magnification of the image. the vertical position of the EPID is recommended to be checked monthly. Concord. with findings summarized below. The QA program of a CT-based IGRT system should be tailored to its utilization. Technologies based on cone-beam geometry (i.171 and reduces the reconstructed CT number accuracy. For example. after parameter stability has been demonstrated by the users. shifting based on an acquired CT. The residual misalignments are recommended to be recalibrated and checked every 6 months. A monthly check is recommended with all beads required to be within 61 mm in the three principal directions. blurring from internal structure motion affects the image quality since image acquisition takes up to 2 min.169 As a result. one at a gantry angle of 0 and the other at 90 .170.18 At this stage.168 Improvements in imaging physics and correction algorithms are required before image quality levels approach those achieved with fan-beam CT scanners.160 Recommendations of this report are directly applicable to the CT on Medical Physics. This tolerance was selected because an error of 1 cm in the SID results in a 0.: QA for image-guided radiation therapy utilizing CT-based technologies 1952 lasers and to the couch readout accuracy and tends to be on the order of 1 mm. on five consecutive images of the phantom in the same position.159. and ultimately on a semiannual basis. increases image noise. the positional accuracy of the EPID in the horizontal plane (i.e. in terms of dynamic range and detector quantum efficiency. The beads are positioned within a common transverse plane. as the machine isocenter still projects to the center of the EPID. Unlike the horizontal position. Salem. April 2012 rails systems operating in a fan-beam mode. Soft-tissue detectability is thus an important aspect of CT-based IMRT quality.e. 39. summarized in Sec II A.B. changes in the scatter environment (i... For MV-CBCT. Vol. For commissioning purposes.. Image quality The general principles of image quality QA for CT-based IGRT technologies follow those of fan-beam computed tomography systems (single and multislice third and fourth generation scanners operated in axial or helical modes) that have been described comprehensively in AAPM report 74167 and reiterated in the AAPM TG-142 report. To verify stability. utilizing a megavolt imaging beam and=or a cone beam geometry. MVCBCT and kV-CBCT) require large area detectors. An error in the vertical position of the flat panel does not translate in an error in the reconstructed isocenter position. and with a recommended tolerance of 65 mm. phantom size or field size) may yield spurious deviations from the baselines. or bars.159 Deviations in scale and distance accuracy will affect image registration accuracy and may reduce patient positioning correction accuracy.15. Note that rather than using multiple small ROIs. such as a water bath or water-equivalent object.15.. adopting a 6-month schedule when stability has been demonstrated.: QA for image-guided radiation therapy utilizing CT-based technologies 1953 improvements expected in a few years. the IGRT scan is mainly used for localization of the preidentified and segmented structures. embedded in the image quality phantom. using a single larger ROI makes it difficult to decouple the effects of image noise and nonuniformity.17 The fan-beam MVCT system can resolve 13 mm diameter objects with 2% density differences from background. low contrast resolution. Image uniformity may reduce with increasing object size because of the subsequent increase of scattered-to-primary x-ray fluence.173 Spatial resolution has been shown to be independent of dose or location of the phantom with respect to the isocenter plane.159 Ring artifacts are often caused by detector element malfunction and require recalibration of the defective pixels map. the contrast difference between the prostate and the rectum is typically 2% while that between the normal breast tissue and a seroma cavity is 10–15%. Noise is characterized by the average signal variability over these ROIs and should also meet the vendor specifications. using a single volumetric image acquisition. Authors report that spatial resolution is on the order of 6–9 line-pairs=cm for kV-CBCT.g.18.18 Several authors have reported on the scale and distance accuracy. the AAPM TG-179 recommends the test frequencies and tolerances presented in Table II. Routine QA of the spatial resolution is. IV. monthly spatial resolution evaluation against established baselines is sufficient.19 and MV-CBCT can resolve 2 cm objects with 1% contrast. The visibility of 1% contrast objects that are 7 mm in diameter has been reported for kV-CBCT systems.175 Cupping artifacts (i. Vol. Low contrast detectability is tested by scanning a phantom containing objects with a variety of linear attenuation coefficients.171 These types of artifacts affect image uniformity.. These changes should be fixed by the geometric accuracy calibration procedure described in Sec. the center of the image of a uniform object appears darker than at the periphery) are mostly caused by scattered radiation.18 and of up to 4 line-pairs=cm for MV-CBCT. In fan-beam MVCT images.15. While the diagnostic CT scan is used to diagnose disease and identify the anatomy. Uniformity and noise Low contrast detectability requirements for a CT-based IGRT system are generally looser than for a diagnostic CT scanner. e.B.174 The gray scale window width should be selected to reveal clinically relevant artifacts: examples of such artifacts are provided in the AAPM TG-104 (Sec. April 2012 Nonuniformities and artifacts can be easily detected during a visual inspection of a volumetric image of a uniform density phantom. adopting a 6-month schedule when stability has been demonstrated. nevertheless. No.17.e. Uniformity is characterized by the variability of the average signal over several small regions of interest (ROI) and should meet vendor specifications. IV. Changes in low contrast detectability are likely related to changes in image noise and=or image uniformity. spatial resolution can be compromised. while fanbeam systems. so scanning a uniform density phantom can provide quantitative image uniformity and noise values. such as CT-on-rails and tomotherapy.4. A monthly test frequency is recommended initially. and image artifacts. 4.159. IV A.1953 Bissonnette et al. A monthly test frequency is recommended initially. rods.B.B. Spatial resolution Image scale and voxel size accuracy can be quantified by scanning objects of known sizes and comparing the object size in the image to the actual size. object size. While commercial systems have cupping artifact correction calibrations. Medical Physics.3. exhibit uniformity comparable to those of diagnostic CT scans. In general.170. offline. therefore allowing several parameters to be analyzed. useful because a reduction in spatial resolution may indicate changes in scanner geometry and=or gantry angle readout calibration. Cone-beam CT systems are more susceptible to this effect.1.6 mm high contrast object should be resolved. It is. CT-based IGRT systems have displayed distance accuracy well within 1 mm.2. and imaging technique. 39. important to obtain the test images using clinically relevant parameters and keep these parameters constant for quality control checks. Spatial resolution measurements are conducted by imaging a series of high contrast objects with suitable resolution objects. Contrast detectability depends on phantom size.20 These data are usually acquired by imaging one of the phantoms mentioned in the previous paragraph. uniformity.17. As the latter is more important for IGRT.B. these calibrations occasionally cause their own artifacts. spatial resolution.19 . the vendor’s specification indicates that a 1.5 mm in size. Low contrast resolution Most CT-based IGRT systems clinically operate at a spatial resolution that is substantially lower than their best performance due to the large size of the volumetric datasets that would be obtained at full resolution.58 The required contrast resolution for clinical scenarios depends on the anatomical region.160 A monthly test frequency is recommended initially. adopting a 6-month schedule when stability has been demonstrated. For example. There is an intrinsic tradeoff between spatial resolution and low contrast detectability in computed tomography imaging. reconstructed voxel size. Such deviations are likely to be caused by unintended changes in scanner geometry that could also degrade the spatial resolution.20 thus enabling visualization of high-contrast objects of 1–2. The low contrast visibility should be tested against a baseline image on a monthly basis. therefore. IV.18 For routine QA. Scale and distance accuracy IV. plates. Users are justified to reduce the frequency of those image-quality tests marked by an asterisk to semiannual or upon extensive service after the stability of their own CTbased IGRT systems has been demonstrated. III B 6). 183 We refer the reader to the AAPM Task Group 75 report on advice on managing image dose for IGRT.g. visualize bony anatomy to correct patient positioning) remains feasible and particularly indicated for smaller volumes containing high-contrast structures. kV-CBCT does not provide quantitative CT because small deviations from acceptance phantom conditions significantly affect CT numbers. the smaller amount of scatter and the reduced energy dependence of the photon interactions in the MeV x-ray range produce a cupping artifact that is predictable.182 At a minimum. For routine QA. Each of these strategies is the topic of current research.1954 Bissonnette et al.8 cGy=scan for MVCBCT.132. dose calculations performed on MVCBCT images agree with calculations conducted using conventional kVCT images within 61% and 1–3% on phantom and patient images.174 For fan-beam MVCT images. but individual scanners will exhibit inaccuracies in linear attenuation coefficient measurements. Simple measurement techniques and dose indices. For fan-beam MVCT. This allows for the use of simple nonpatient specific correction methods to improve the MV-CBCT image uniformity and provide accurate and stable CT numbers. tempting to use relatively high mAs imaging techniques such as those used for diagnostic imaging.160 monitoring relative deviations is more useful than measuring absolute contrast values.D.1 to 2 cGy=scan for kV-CBCT and 0.7 to 10.183 IV. This is not the case for either fan-beam MVCT132 or for CT-onrails. This may be especially important in treating patients with long post-treatment life expectancies. the calibration of MV-CBCT images is more elaborate than conventional fan-beam CT calibration. justified to reduce the image dose while the task at hand (e. therefore. 39.7 to 4 cGy and depend on the selected CT pitch and the imaged anatomy thickness. such as pediatric patients.159.176. Dosimetric CT-based imaging studies have been published10.142.18.172.21. For fan-beam MVCT images. the doses range from 0. comparing uniformity and noise with the established baselines is sufficient. therefore.E.187 IV. for example.184.24.176. April 2012 CT number accuracy becomes important when IGRT scans are used for dose calculations. CT numbers are defined as being proportional to linear attenuation coefficients.179 Image quality is intimately linked to imaging dose. Using postprocessed images.. Vol. a compromise solution between the risk estimated from image doses must be balanced with the benefit offered (high precision treatment leading to reduced high dose volumes) and should be contrasted with the estimated risk from the extra doses that can result from IMRT and volumetric arc therapy.: QA for image-guided radiation therapy utilizing CT-based technologies 1954 and such systems can hold the same generally accepted tolerances. reducing imaging frequency.143. usually variations on the CTDI Medical Physics.178. cumulate from 3 to 370 cGy over a course of treatment. each facility should evaluate the doses associated with each IGRT implementation and discuss the cost versus benefit with the radiation oncologists. This test is recommended only if images are used for dose calculations. Image registration Image registration is an important step in CT-based IGRT.161 IV. a maximum HU difference in peripheral and central ROIs of 25 HU is recommended in TG-148 if the images are to be used for dose calculations.119. However. A monthly test frequency is recommended initially. so faithfully reproducing test conditions is crucial for obtaining meaningful quality control check results.C.180.141. It is.174 For cone-beam CT systems.185 CT number accuracy is measured by scanning a phantom containing inserts with a wide range of electron densities and comparing the CT numbers in the image with the specifications for the inserts. Image nonuniformity also affects dose calculation accuracy. Image dose dose indices used in conventional CT. The same holds true when a phantom size deviates from calibration conditions. reducing the number of projections acquired for a whole scan or performing partial scans.159.177 and report dose ranging from 0. or minimizing the field-of-view to reduce integral dose. above the threshold doses reported in the literature for secondary malignancy occurrence.187 Once the corrections mentioned above become available in released CT-based IGRT products. adopting a 6-month schedule when stability has been demonstrated.24 Dose can. making MV-CBCT suitable for dose recalculation. Accuracy of CT numbers CT-based IGRT has progressed rapidly as experience has shown it to be a good means for identifying and correcting geometric errors prior to initiating radiation therapy.160 Research is ongoing to better correct for scatter contribution and thus improve HU integrity of CBCT scans. Daily imaging doses are generally small compared to therapeutic doses but are distributed over the entire imaged volume. HU for materials with densities close to waterlike densities should be within 30 HU from the calibration data and within 50 HU for lung and bonelike materials. The relationship between electron density and linear attenuation coefficient for human tissues is bilinear. For these reasons.161 Because of the cupping artifact produced by scatter radiation and beam hardening. 4.181 Still. therefore. Strategies to achieve doses that are reasonably low may include reducing tube mAs. without reaching equivalent image quality for kV-CBCT. respectively. have been suggested to describe imaging dose. regular monitoring of the daily patient dose based on on-board images acquired of the patient on the treatment table can become a powerful tool for tracking the progress and accuracy of the treatment.186 CT number accuracy for CBCT suffers from the sensitivity of scatter-to-primary x-ray fluence to object and=or field size. a monthly test of the HU calibration is recommended in TG-148.139–141. It is. Due to the nonrigid nature of patient’s anatomy and . repeating the uniformity measurements under conditions that were identical to acceptance testing will be important to be able to compare the results against the established baselines. especially for large and low-contrast volumes. No.22. such as head-and-neck. The rapid daily procedure described below inherently involves other routine QC items. the imaging equipment is positioned by powerful robotic arms that move near the patient.g. alignment goals.195 and the image-guidance system196 itself.e. The primary rationale for daily QC procedures is to identify any sudden performance changes or gross errors that could result from collisions. Therefore. and evaluation criteria. with adapted phantoms. such as warming up the x-ray tube. April 2012 This task group recommends that daily CT-based IGRT QC tests be performed. available couch rotations. for several commercial couches. As IGRT is employed to empower adaptive radiation therapy. a uniform consensus for image registration quality assurance has not been developed. or implanted fiducial surrogates.200 Typically. IV. A compromise between aligning soft tissue structures.7. the volumetric image registration algorithm should calculate both rotational and translational shifts. and verifying there is sufficient disk space for the work day. have been proposed for image-guidance systems.190 A couch with 6 degrees of freedom (3 translations and 3 rotations) is also commercially available. using a more efficient procedure than the lengthy but more precise monthly procedure described in Sec. etc. any couch translation larger than 1 cm or rotation exceeding 3 degrees) might warrant repeating the patient setup procedure. shifts that are too large for a specific treatment technology (e. or afterhours service.194 film. Table II does not specify additional recommended tests to those of TG-142. 4. This device typically involves a motorized. One benefit from the end-to-end QC test described in Sec.F Accuracy of remote-controlled couch imaging. 39. A tolerance of 62 mm has been demonstrated for such daily geometric accuracy assessment. but such time trends have not yet been reported. Care should be taken to optimally derive a correction from a registration result. automatic registration based on bony anatomy is used to identify and correct gross geometric discrepancies. It is also highly recommended that the user establish site-specific clinical protocols to explicitly describe the volume of interest. 62mm=1 )160 and should suffice for highprecision radiotherapy and SBRT. Another benefit of daily QC is to obtain a record of the geometric accuracy of the therapy equipment.159 In some implementations.193 optical navigation systems. These data suggest that couch motion accuracy is well within the vendor-provided specifications or the tolerances suggested by the AAPM TG142 report (i.159.163 and similar tests. assistance with this task might be provided by projecting contours defined on the planning scan onto the daily volumetric image. commercial vendors will have to develop reliable algorithms to account for deforming or moving anatomy over a single treatment or an entire radiation therapy course and suggest test methods with the collaboration of early adopters.G. Submillimeter couch position accuracy has been demonstrated.199. Ideally. No. the daily QC procedures also need to ensure the touch panels or motion interlocks are all functional. manual or automated soft-tissue registrations are used to refine registration to improve the measurement accuracy. bony.8 confirm source-toimager distance. the accuracy should be confirmed with orthogonal portal images of the phantom.: QA for image-guided radiation therapy utilizing CT-based technologies 1955 the limited correction methods. creating a potential for injury. In this approach. such as volumetric image orientation.15. using high-precision calipers. IV G is that the accuracy of couch motions is tested on a daily basis... remote-controlled couch that translates the patient along three-axes. However. IV A.198 and even assess dose. Cubic phantoms with a marker placed at the center can be aligned with the linac isocenter using room lasers or portal images. upgrades. volumetric phantom images would therefore measure the isocenter localization accuracy.8. Tracking the trends from repeating these tests over extended periods of time may guide appropriate test frequencies.192 Patient positioning corrections need to be both accurate and precise to realize the full potential of IGRT. Simple.) will also be required and will impact the registration method selection.1955 Bissonnette et al. and nearby critical structures. thereby speeding up the execution of the daily test at the cost of relating accuracy to the laser system rather than directly to the isocenter. The AAPM TG-179 recommends that the tolerances for accuracy of remote-controlled couches match those specified in TG-142. Vol. reporting certain warning messages and system interlocks.189 IV. Selection between available intervention methods (simple couch translations.191.8 A phantom with multiple markers at known positions would provide additional assessments. Thus.197. Subsequently. as well as what long-term accuracy is achievable.11 . At this writing. so this area falls outside the scope of the current task group.190 portal Medical Physics. It should be pointed out that difficulties in differentiating soft tissues can arise with CT-based image-guidance systems. although the AAPM TG-132 is currently designing protocols for patient specific image registration and fusion software acceptance testing and quality assurance. authors typically recommend repeating such procedures during regularly scheduled QA activities but fail to specify test frequency. When soft tissue localization is ambiguous. The accuracy and precision of correction movements should be assessed during commissioning. online replanning methods.188 Careful assessment of the target position and rotation in combination with organs at risk or other anatomical landmarks should be performed. may be required. it is prudent to adopt a two-staged image guidance approach. Daily operational issues A key component to any image-guidance system is the patient positioning device. integral tests have been described to check the overall CT-simulation processes accuracy. these phantoms are aligned to the accelerator isocenter using the room lasers. the “best” alignment may depend on the clinical case.197 assess image sharpness.198 A variety of methods have been proposed to achieve this goal. Users are cautioned to visually assess image registration accuracy not only over the target area but also in its surrounding volume. The shift is determined by comparing the localization and simulation images and a suggested couch shift is determined. Daily geometric tests need to be performed in short amounts of time. Several initiatives can be performed in parallel. submillimetric geometric accuracy that suffices to both conventional and hypofractionated radiotherapy fractionation schedules. The shift is applied and a verification image dataset is acquired and registered again to the reference CT dataset. After the displacement is applied. based on a 95% confidence interval. These technologies can achieve several aims. The residual error obtained with this simplified geometric accuracy check should be 0 6 2 mm. As described above. April 2012 consistent with the TG-101 and TG-104 reports for a number of reasons. again. 39.and intrafractional organ motion during radiotherapy. The displacement should be less than 2 cm in any cardinal direction because remote-controlled movements for most IGRT couches are currently limited to 2 cm to reduce the likelihood of patient–machine collision.1956 Bissonnette et al. the daily procedures described in this section provide rapid assessment of geometric accuracy. a localization image dataset is acquired to assess what couch motions are required to align the phantom to its nominal position.: QA for image-guided radiation therapy utilizing CT-based technologies 1956 A variant of the daily QA procedure can be implemented to assess communication between the image registration software and the remote-controlled couch. the target (fiducial or soft-tissue or bony matching). The displacement indicated from registering the verification dataset to the reference CT defines the residual couch correction error. or MUs). Vol. or measure the actual efficacy of immobilization accessories. Second. a displacement of (1. Multidisciplinary teams can build their confidence in the IGRT process. and the targeting method (manual or automatic). this rapidity is achieved at the cost of a reduced test precision relative to the geometric calibration procedures described at the beginning of this section. this displacement is independently measured. to verify couch motion accuracy. 4. submillimeter accuracy has been reported on phantom studies. from CT simulation to treatment delivery. manage inter. It is also useful for technical and clinical staff training as well as to provide a quick check after scheduled or unscheduled service events or other mishaps. leading to reduced doses to organs at risk and perhaps escalating dose. The “residual correction error” is a useful measure of the targeting and couch correction accuracy. mAs. help . For example. Finally. service. in agreement with the AAPM TG-101 report. No. and accuracy is compromised because the user manually aligns the phantom with room lasers and because the accelerator component flexes and torques are under sampled when only orthogonal portal images are used to assess coincidence of all isocenters. IV A can take up to 2 h and so is inappropriate for daily QC. a reference CT scan of the phantom is used as a surrogate of the CT simulation scan. the enhanced geometric accuracy can be used to review and perhaps reduce setup margins for PTV design.11 The AAPM TG-179 stresses that the complete geometric calibration procedure described in Sec. and A=P) directions would not expose an error where the coordinate axes were mismatched between a scan from the CT-based image-guidance system and a reference CT scan. The daily QA procedure should also avoid repeated. or clinically irrelevant displacement magnitudes. Verifying the geometric accuracy using the geometric calibration procedure described in Sec. manual image matching introduces further uncertainties in the test. To mimic patient treatments. Thus. Second.201 The image acquisition and registration software is typically independent of the treatment couch control system. Finally. V. trivial.1) cm in the (L=R. repeating this procedure whenever extensive service or upgrades are performed should maintain a high.11. the daily test aims to detect gross. using end-to-end tests where a phantom is treated exactly like a patient.159 The AAPM Task Group 179 is Medical Physics. kV or MV).202 However. such as a collision. or research activities performed after hours. the software provides an objective measure of the couch positioning accuracy and precision. This value can be estimated by first placing a phantom at isocenter and then displacing the phantom at predefined distances in three directions. COMMISSIONING THE IMAGE-GUIDED PROCESS Clinical experience with CT-based image-guidance technologies is steadily growing. IV A has proven the submillimeter accuracy of CT-based IGRT devices over extended periods of time. One strategy for implementing wide-scale IGRT is to build on several short-term successes.8. first-time users of this technology should ascertain which of these aims are desirable for their own clinical contexts and tailor their commissioning and QA programs accordingly. S=I. IGRT may also empower adaptive radiotherapy because clinicians can assess anatomical changes seen during a course of radiation and rationally respond to those changes. starting with acceptance testing of the first CT-based IGRT device to develop image-guidance protocols.11 Repeated measurements of this residual error define the systematic error (mean) and the uncertainty (standard deviation) of the targeting and correction system. Secondary aims of IGRT might include replacing film or portal imaging to document positional accuracy. system integrity.146 The AAPM TG-142 report recommends that daily tests of IGRT systems demonstrate an accuracy of 61 mm for SBRT techiniques160 while the AAPM TG-104 report does not distinguish between conventional and hypofractionated techniques. unintentional misalignments that may be caused by collisions. and to provide a rapid end-to-end test of the IGRT process. they can increase radiotherapy accuracy by verifying the patient position with respect to the treatment beam immediately prior to irradiation.11.1. the overall accuracy has been shown to depend on the IG modality (2D or 3D. hence. First. and functionality. This daily QA procedure creates confidence in the IGRT system accuracy and precision. for some systems. Such end-to-end tests simulate the process in a multidisciplinary environment.203 the x-ray technique (kVp. It should be noted that the tolerance supported by published data for daily geometric accuracy test is 0 6 2 mm to a 95% confidence interval. while making teams aware of the fiscal constraints that need to be met once the new IGRT processes are stabilized. (ii) sufficient patient volume to allow clinical team to learn without overloading it (about five patients initially on a treatment unit). over the course of a few months.96.99. Specific operating issues include obtaining soft-tissue contrast on volumetric datasets using doses lower than or comparable to portal imaging opens the possibility of frequent and accurate positioning of the patient at the onset of each treatment.105. and (iv) when image contrast gains offered by kilovoltage over portal imaging are immediately obvious (lung.205 Depending on the comfort and confidence levels of users. 39. selecting a few anatomical sites prior to broadening the use of imageguidance across all anatomical sites. April 2012 .1957 Bissonnette et al. Vol. the analysis of positional data from end-toend testing will help the team reassess setup margins. recommendation of appropriate imaging techniques for specific anatomical sites. therapists.05 full-time equivalent position Notes Operation Review of images Daily quality control tests Monthly quality control tests Annual quality control tests Continued clinical support First install only First install only First install only Each treatment with IGRT.110 Teams can test and benchmark CT-based IGRT against portal imaging guidance. Depending on the aims of individual clinics in implementing IGRT.5 days 2 days 2 days 2 days 5 mins=patient 10 min=patient 5 min=scan 10 min 1–2 h 2–4 h 0. No. this build clinicians’ confidence in the IGRT process accuracy. Clinical patient studies are introduced gradually. and administrators need to allow extra time for staff to learn the new process. (iii) difficulty of the imaging process.: QA for image-guided radiation therapy utilizing CT-based technologies 1957 identify and resolve issues.175 accounting for the goals and aims to be achieved with such guidance systems. identify appropriate immobilization accessories. Coordination meetings help definition of the initial positional accuracy requirements in the clinic. keep radiation doses low. includes image acquisition and evaluation Data transfers to imaging platform 0 when performed by therapists Ad hoc activity Medical Physics. tolerances for residual positional errors. and involvement of team members in the image-guidance process share the findings from various site groups and identify infrastructure issues.. 4. with clear statements of accuracy requirements. a QA program must be established to ensure the safe. be it simply the correction of geometric uncertainties to empowering an adaptive radiotherapy protocol for routine use. Activity Acceptance testing and commissioning Education Responsibility Physicists Physicists Therapists Dosimetrists Therapists Dosimetrists Physicians Therapists Physicists Physicists Physicists Time 2. Vendors have an obligation to provide user training. such studies involve verification imaging to assess the accuracy of the position correction.35. initial anatomical site groups can be selected based on (i) research or personal interests. and brain). defining an appropriate frequency of site-specific image guidance protocols.e. and medical physics for successful wide-scale implementation of CT-based image guidance technologies and processes. responsibilities.95. Each team should fully document site-specific image-guided processes. TABLE III. In parallel.204 Often. and physicians) should be involved in the development of site-specific IGRT techniques. and consistent operation of the CT-based image-guidance technology. Clinics are advised to plan for additional resources from radiation therapy. and identify obstacles or potential pitfalls for safe and efficient use of the technology. A team with representation from all disciplines (i. Subsequently. such as nomenclature. assessing performance under a clinical load. and defining the roles. documentation of the image-guidance protocols. These documents can then be used as learning tools for other staff. reliable. initially using rigid phantoms and end-to-end tests. More time is required when performing commissioning and quality control testing of 2D functions on some platforms. as influenced by visualization of soft tissues and mobility of internal organs.7 followed with patient studies. Other issues include the following: • • • • developing an appropriate nomenclature for clearly and systematically communicating and documenting imageguided measurements. and opportunities for dose escalation.36. and defining data archiving requirements. The team should develop an initial imageguidance procedure considering the following: • • • • • • A powerful motivating factor for multidisciplinary teams is the clinical research required to establish safe and efficient clinical operation. Table III summarizes the collected experiences of AAPM TG-179 members as an estimate of the resource needs to maintain image-guidance programs with CT-based technologies. the time and resource commitment to implement CT-based IGRT technologies differs between clinics around the world and between imaging systems. radiation oncology. disk space. and develop expertise. physicists.109. pediatrics. Estimated human resources required for image guidance using CT-based IGRT technologies. Estimates are obtained from the collected experiences of the task group members. document results from end-to-end image guidance testing. Radiat. J. I. D. A. Morin. Biol. Yin. Aubin. A. Khoo. G.” Med. L. 71. Dosim. C. Langen. and D. 17 C.on. G. P. Huber. 8. J. S. G. 3 T. 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