Machine Isocenter Research Articles

Monte Carlo modeling of the origin of contaminant electrons on a 0.5T bi-planar Linac-MR.

In the last decade, hybrid linear accelerator magnetic resonance imaging (Linac-MR) devices have evolved into FDA-cleared clinical tools, facilitating magnetic resonance guided radiotherapy (MRgRT). The addition of a magnetic field to radiation therapy has previously demonstrated dosimetric and electron effects regardless of magnetic field orientation. This study uses Monte Carlo simulations to investigate the importance and efficacy of the magnetic field design in mitigating surface dose enhancement in the Aurora-RT, focusing specifically on contaminant electrons, their origin, and energy spectrum. The Aurora-RT 0.5 T Biplanar Linac-MR device was modeled using the BEAMnrc package using the updated EM macros, a magnetic field map generated from Opera 3D. Simulation generated phasespace data at the distal side of the first magnetic pole plate (89cm) and at machine isocenter (120cm) were analyzed with respect to electron energy spectra and electron creation origins, both with and without the static magnetic field. The presence of the main magnetic field was verified to affect the origin and distribution of contaminant electrons, removing them from the air column up to 60cm from the target, and focusing them along the CAX within the region below. Analysis of the remaining electron energy fluence reveals the net removal of electrons with energies>2 MeV and generation of electrons with energies<2 MeV in the presence of the static magnetic field as compared to no magnetic field. Moreover, in the presence of the magnetic field the integral energy contained in the contaminant electrons increases from 89cm to isocenter but is still 15% less overall than the integral energy contained in contaminant electrons without the magnetic field. This study provides an analysis of contaminant electrons in the Aurora-RT 0.5 T Linac-MR, emphasizing the role of magnetic field design in successfully minimizing electron contaminants.

Experimental determination of magnetic field quality conversion factors for eleven ionization chambers in 1.5 T and 0.35 T MR-linac systems.

The static magnetic field present in magnetic resonance (MR)-guided radiotherapy systems can influence dose deposition and charged particle collection in air-filled ionization chambers. Thus, accurately quantifying the effect of the magnetic field on ionization chamber response is critical for output calibration. Formalisms for reference dosimetry in a magnetic field have been proposed, whereby a magnetic field quality conversion factor kB,Q is defined to account for the combined effects of the magnetic field on the radiation detector. Determination of kB,Q in the literature has focused on Monte Carlo simulation studies, with experimental validation limited to only a few ionization chamber models. The purpose of this study is to experimentally measure kB,Q for 11 ionization chamber models in two commercially available MR-guided radiotherapy systems: Elekta Unity and ViewRay MRIdian. Eleven ionization chamber models were characterized in this study: Exradin A12, A12S, A28, and A26, PTW T31010, T31021, and T31022, and IBA FC23-C, CC25, CC13, and CC08. The experimental method to measure kB,Q utilized cross-calibration against a reference Exradin A1SL chamber. Absorbed dose to water was measured for the reference A1SL chamber positioned parallel to the magnetic field with its centroid placed at the machine isocenter at a depth of 10cm in water for a 10 × 10cm2 field size at that depth. Output was subsequently measured with the test chamber at the same point of measurement. kB,Q for the test chamber was computed as the ratio of reference dose to test chamber output, with this procedure repeated for each chamber in each MR-guided radiotherapy system. For the high-field 1.5 T Elekta Unity system, the dependence of kB,Q on the chamber orientation relative to the magnetic field was quantified by rotating the chamber about the machine isocenter. Measured kB,Q values for our test dataset of ionization chamber models ranged from 0.991 to 1.002, and 0.995 to 1.004 for the Elekta Unity and ViewRay MRIdian, respectively, with kB,Q tending to increase as the chamber sensitive volume increased. Measured kB,Q values largely agreed within uncertainty to published Monte Carlo simulation data and available experimental data. kB,Q deviation from unity was minimized for ionization chamber orientation parallel or antiparallel to the magnetic field, with increased deviations observed at perpendicular orientations. Overall (k=1) uncertainty in the experimental determination of the magnetic field quality conversion factor, kB,Q was 0.71% and 0.72% for the Elekta Unity and ViewRay MRIdian systems, respectively. For a high-field MR-linac, the characterization of ionization chamber performance as angular orientation varied relative to the magnetic field confirmed that the ideal orientation for output calibration is parallel. For most of these chamber models, this study represents the first experimental characterization of chamber performance in clinical MR-linac beams. This is a critical step toward accurate output calibration for MR-guided radiotherapy systems and the measured kB,Q values will be an important reference data source for forthcoming MR-linac reference dosimetry protocols.

AAPM Medical Physics Practice Guideline 8.b: Linear accelerator performance tests.

The purpose of this guideline is to provide a list of critical performance tests to assist the Qualified Medical Physicist (QMP) in establishing and maintaining a safe and effective quality assurance (QA) program. The performance tests on a linear accelerator (linac) should be selected to fit the clinical patterns of use of the accelerator and care should be given to perform tests which are relevant to detecting errors related to the specific use of the accelerator. Current recommendations for linac QA were reviewed to determine any changes required to those tests highlighted by the original report as well as considering new components of the treatment process that have become common since its publication. Recommendations are made on the acquisition of reference data, routine establishment of machine isocenter, basing performance tests on clinical use of the linac, working with vendors to establish QA tests and performing tests after maintenance and upgrades. The recommended tests proposed in this guideline were chosen based on consensus of the guideline's committee after assessing necessary changes from the previous report. The tests are grouped together by class of test (e.g., dosimetry, mechanical, etc.) and clinical parameter tested. Implementation notes are included for each test so that the QMP can understand the overall goal of each test. This guideline will assist the QMP in developing a comprehensive QA program for linacs in the external beam radiation therapy setting. The committee sought to prioritize tests by their implication on quality and patient safety. The QMP is ultimately responsible for implementing appropriate tests. In the spirit of the report from American Association of Physicists in Medicine Task Group 100, individual institutions are encouraged to analyze the risks involved in their own clinical practice and determine which performance tests are relevant in their own radiotherapy clinics.

Evaluation of the Role of Optical Surface Monitoring Systems in Aiding Pre-CBCT Setups for Head and Neck and Neurological Cancer Patients

Patients undergoing radiation therapy for head and neck (HN) or intracranial (IC) targets are immobilized using a thermoplastic mesh mask. One anterior and two lateral points marked on the mask are aligned with the treatment room lasers to position the patient relative to the machine isocentre. Following alignment, a cone-beam computed tomography (CBCT) dataset is acquired for verification of patient position prior to treatment. Any errors in patient position are corrected with couch motions (in 3 translation and 3 rotation axes) determined from matching the CBCT with the plan CT. A recently acquired optical skin monitoring system (OSMS) shows promise in aiding the patient setup, however these systems do not work when the skin is obscured by the mask. The purpose of this study is to determine if using OSMS to position the patient’s head prior to fitting the mask reduces the magnitude of corrections indicated by CBCT to plan CT matching. Secondly, this study seeks to determine whether the use of OSMS prior to mask fitting aids in reproducing the posture of HN patients. Lastly, this study reports therapists’ perception of the utility of using OSMS in this way.

An observational study of post-operative volumetric modulated arc radiotherapy (VMAT) of breast cancer by immobilizing patients on All-in-One (AIO™) solution breast and lung board without using thermoplastic mask: risk of collision of gantry with couch exists if acquisition of Cone Beam Computed Tomography (CBCT) is attempted for set-up verification

Automatic cone beam computed tomography (CBCT)–based image matching for set-up verification is recommended as compared with 2D image match for post-operative local/loco-regional radiotherapy of breast cancer patients by volumetric modulated arc therapy (VMAT) technique. However, in a supine position, off-midline peripheral body clinical target volume (CTV) of unilateral breast cancer patients immobilized on breast and lung board of All-in-One (AIO) positioning system may necessitate augmented movement of the couch in ‘x’ and ‘z’ axes thereby raising the risk of collision of x-ray sources/detector system with couch. VMAT was planned by a pair of partial arcs for the whole target volume for seven consecutive post-operative breast cancer patients (five post-mastectomy and two post-breast conservation patients). Tattoo-based set-up by the shift of treatment table in x-, y-, and z-axis as determined by treatment planning system followed by x-rays with planar image acquisition and online 2D image matching with Digitally reconstructed radiographs (DRRs) was performed for set-up verification. In-room 360° rotation of x-ray source and detector system of a linear accelerator (linac) was performed before x-ray planar image acquisition. Completion of 360° in-room rotation of x-ray source and detector system of linac around the machine isocentre was not possible in six out of seven patients due to the possibility of collision of the gantry with contralateral side of the couch. Performing CBCT for generating 3D images for computed tomography (CT) reconstruction may not be practical for patient set-up verification of post-operative radiotherapy of unilateral breast cancer patients positioned supine on breast and lung board.

Evaluation of the RUBY modular QA phantom for planar and non-coplanar VMAT and stereotactic radiations.

PurposeThis study evaluates the clinical use of the RUBY modular QA phantom for linac QA to validate the integrity of IGRT workflows including the congruence of machine isocenter, imaging isocenter, and room lasers. The results have been benchmarked against those obtained with widely used systems. Additionally, the RUBY phantom has been implemented to perform system QA (End‐to‐End testing) from imaging to radiation for IGRT‐based VMAT and stereotactic radiations at an Elekta Synergy linac.Material and MethodsThe daily check of IGRT workflow was performed using the RUBY phantom, the Penta‐Guide, and the STEEV phantom. Furthermore, Winston–Lutz tests was carried out with the RUBY phantom and a ball‐bearing phantom to determine the offsets and the diameters of the isospheres of gantry, collimator, and couch rotations, with respect to the room lasers and kV‐imaging isocenter. System QA was performed with the RUBY phantom and STEEV phantom for eight VMAT treatment plans. Additionally, the visibility of the embedded objects within these phantoms in the images and the results of CT and MR image fusions were evaluated.ResultsAll systems used for daily QA of IGRT workflows show comparable results. Calculated shifts based on CBCT imaging agree within 1 mm to the expected values. The results of the Winston–Lutz test based on kV imaging (2D planar and CBCT) or room lasers are consistent regardless of the system tested. The point dose values in the RUBY phantom agree to the expected values calculated using algorithms in Masterplan and Monte Carlo engine in Monaco within 3% of the clinical acceptance criteria.ConclusionAll the systems evaluated in this study yielded comparable results in terms of linac QA and system QA procedures. A system QA protocol has been derived using the RUBY phantom to check the IGRT‐based VMAT and stereotactic radiations workflow at an Elekta Synergy linac.

Comparing Setup Errors Using Portal Imaging in Patients With Gynecologic Cancers by Two Methods of Alignment

AimsAlignment tattoos on a lax abdomen contribute to misalignment of patients undergoing abdomino-pelvic radiotherapy (RT). The present study was undertaken to assess setup reproducibility in gynecologic cancer patients positioned identically but aligned for treatment to machine isocenter by two different ways. Materials and methodsA prospective study in 35 women treated with radical RT for gynecologic malignancy was undertaken. A RT planning contrast-enhanced computed tomography scan in the supine position using an foot and ankle positioning device was done, and three reference points tattooed on the reference plane, anteriorly at the mons pubis and one on each side laterally at a fixed table top-to-vertical height of 10 cm, whereas a fourth point was tattooed at the xiphoid in the anterior midline. Patients were aligned using either a field center, that is, conventional method (Arm I, n = 18) or by a new setup isocenter (Arm II, n = 17) defined by a cranial offset of 4 cm to the reference plane for daily treatment. Anterior and right lateral digitally reconstructed radiograph setup fields were created at the treatment isocenters and compared with orthogonal megavoltage portal images (PI) taken during initial 3 days of RT and subsequently twice weekly. Setup deviations-rotations and translations were analysed in mediolateral (ML), craniocaudal, and anteroposterior direction. No online and offline corrections were performed. Population systematic error and random error were calculated and planning target volume margins required were estimated using van Herk's formula. ResultsArm I had 209 PI while Arm II had 188 PI. Patients in arm II had a lesser systematic error in the ML direction. Patients with large pelvic girth (>95 cm) were susceptible for greater movements during treatment, more so in Arm I, major shifts (>5 mm) with respect to Arm II in the ML direction (37% vs. 22%, P = .001). A larger planning target volume expansion was required in Arm I (1.6 cm) compared with Arm II (0.9 cm). The margin expansion required from clinical target volume in anteroposterior direction was about 0.6 cm and about a cm in the craniocaudal direction in both the arm. ConclusionsAlignment of patient with anterior tattoo at the relatively immobile portion of lower abdomen (mons pubis) Arm II (setup) is superior to a more cranial location over the flabby abdomen during radiation treatment.

A Daily QA Phantom for Linear Accelerator with Image-Guided Radiosurgery Capability

Image guided radiosurgery and stereotactic body radiotherapy (IG-SRS/SBRT) have been widely adopted on linear accelerators. Ideally, a daily quality assurance program on such machines should provide the targeting accuracy from image guidance quantitatively and efficiently. A suitable phantom for such test requires sufficient landmarks for robust online image registration and a high contrast ball bearing (BB) to provide clear images for Winston Lutz (WL) test. Such phantoms, however, are not widely available. In this study, we investigated a prototype daily QA phantom that allows all major components of the IG-SRS/SBRT process to be tested. The phantom is a 12 cm cube with 3 pairs of AL rods at 3 corners, and 4 embedded AL ceramic BBs. The BB in the center of phantom is 5/16 inch in diameter, while the others are ¼ inch in size. On the treatment table, the phantom sits atop a holder with build-in pitch, roll, and pitch. The holder can be index to the couch at a preset starting position. A high resolution planning CT was acquired of the phantom on a level surface. A plan was then generated with isocenter placed precisely in the central BB of the phantom. The plan was delivered on a medical linear accelerator and a radiosurgery system, using cone beam CT (CBCT) and stereoscopic x-ray system for image guidance, respectively. Both imaging systems provide 6DoF deviations that were subsequently applied to the treatment couch. Afterwards, verification imaging was acquired with an on-board imaging (OBI) KV/MV pair on a medical linear accelerator and a 2nd pair of stereotactic x-ray images on a radiosurgery system, and WL tests were performed to measure the agreement between central BB and machine isocenter. The plan was delivered 12 times each on the two machines. The initial correction, deviation from verification imaging, and WL test results (μ ± σ) are provided in the table below, where R represents 3D vector length.The verification imaging indicated that all 6DoF corrections were applied correctly, and WL test showed that both systems are able to localize well within 1 mm accuracy on a phantom. The phantom incorporates necessary features to enable daily QA with the goal of simulating IG-SRS/SBRT workflow and quantitatively evaluating localization accuracy. Accuracy determined from this daily QA procedure should represent the upper limit of targeting accuracy for ideal conditions.Abstract 3613; Table 1Medical Linear Accelerator (n=12)Radiosurgery System (n=12)CorrectionVerificationWL testCorrectionVerificationWL testVRT (mm)-8.0±0.10.0±0.10.02±0.04-8.6±0.30.0±0.10.30±0.15LNG (mm)6.8±0.10.2±0.20.25±0.087.0±0.40.0±0.10.18±0.10LAT (mm)6.3±0.2-0.1±0.2-0.20±0.146.0±0.40.1±0.10.20±0.13R (mm)12.2±0.20.4±0.10.34±0.0912.6±0.40.2±0.10.45±0.11Yaw (o)1.4±0.1-0.2±0.1-1.4±0.20.1±0.1-Pitch (o)-1.8±0.10.1±0.1--1.7±0.10.0±0.1-Roll (o)-1.0±0.10.1±0.1--0.9±0.10.0±0.1- Open table in a new tab

Correcting geometric image distortions in slice-based 4D-MRI on the MR-linac.

The importance of four-dimensional-magnetic resonance imaging (4D-MRI) is increasing in guiding online plan adaptation in thoracic and abdominal radiotherapy. Many 4D-MRI sequences are based on multislice two-dimensional (2D) acquisitions which provide contrast flexibility. Intrinsic to MRI, however, are machine- and subject-related geometric image distortions. Full correction of slice-based 4D-MRIs acquired on the Unity MR-linac (Elekta AB, Stockholm, Sweden) is challenging, since through-plane corrections are currently not available for 2D sequences. In this study, we implement a full three-dimensional 3D correction and quantify the geometric and dosimetric effects of machine-related (residual) geometric image distortions. A commercial three-dimensional (3D) geometric QA phantom (Philips, Best, the Netherlands) was used to quantify the effect of gradient nonlinearity (GNL) and static-field inhomogeneity (B0I) on geometric accuracy. Additionally, the effectiveness of 2D (in-plane, machine-generic), 3D (machine-generic), and in-house developed 3D (machine-specific) corrections was investigated. Corrections were based on deformable vector fields derived from spherical harmonics coefficients. Three patients with oligometastases in the liver were scanned with axial 4D-MRIs on our MR-linac (total: 10 imaging sessions). For each patient, a step-and-shoot IMRT plan (3×20Gy) was created based on the simulation mid-position (midP)-CT. The 4D-MRIs were then warped into a daily midP-MRI and geometrically corrected. Next, the treatment plan was adapted according to the position offset of the tumor between midP-CT and the 3D-corrected midP-MRIs. The midP-CT was also deformably registered to the daily midP-MRIs (different corrections applied) to quantify the dosimetric effects of (residual) geometric image distortions. Using phantom data, median GNL distortions were 0.58mm (no correction), 0.42-0.48mm (2D), 0.34mm (3D), and 0.34mm (3D ), measured over a diameter of spherical volume (DSV) of 200mm. Median B0I distortions were 0.09mm for the same DSV. For DSVs up to 500mm, through-plane corrections are necessary to keep the median residual GNL distortion below 1mm. 3D and 3D corrections agreed within 0.15mm. 2D-corrected images featured uncorrected through-plane distortions of up to 21.11mm at a distance of 20-25cm from the machine's isocenter. Based on the 4D-MRI patient scans, the average external body contour distortions were 3.1mm (uncorrected) and 1.2mm (2D-corrected), with maximum local distortions of 9.5mm in the uncorrected images. No (residual) distortions were visible for the metastases, which were all located within 10cm of the machine's isocenter. The interquartile range (IQR) of dose differences between planned and daily dose caused by variable patient setup, patient anatomy, and online plan adaptation was 1.37Gy/Fx for the PTV D95%. When comparing dose on 3D-corrected with uncorrected (2D-corrected) images, the IQR was 0.61 (0.31)Gy/Fx. GNL is the main machine-related source of image distortions on the Unity MR-linac. For slice-based 4D-MRI, a full 3D correction can be applied after respiratory sorting to maximize spatial fidelity. The machine-specific 3D correction did not substantially reduce residual geometric distortions compared to the machine-generic 3D correction for our MR-linac. In our patients, dosimetric variations in the target not related to geometric distortions were larger than those caused by geometric distortions.

First Observation of an X-Ray Beam Following a New Geodesic When Gravitational Waves Deform Space-Time

By using X-rays of a linear accelerator (LINAC Siemens X rays, 6 MeV) for medical use, we were able to measure gravitational waves, GW, (amplitude = 56:385mm, frequency =1 = 3Hz, velocity = c and polarization) and its threedimensional effect on X-ray trajectories. The collimated X-ray beam, which is in the plane (X; Y); travels on the Z axis at the speed of light in air and passing through the machine isocenter, until it reaches the target and, ultimate, is recorded in a radiographic film. Apparently, there is an exceptional coincidence in the operation of LINAC and the presence of GW. This coincidence occurred in VIRGINIA, GPS (38.634 351 1, -77.282 523 9), UTC (12/06/2011: 12: 56: 01). This important event, but not sui generis, was recorded in the LINAC computer system, on a film for radiography, in the log file of the cancer treatment center and it was reported to SIEMENS in order to try to find an explanation of a possible hardware failure, some abnormality or any software issue. The physicist and Siemens service engineer on site concluded that such event should never happened because LINAC was not malfunctioning. Consequently, for the X-rays, there was a deviation of the isocenter of the LINAC (△X = (11:5 &amp;plusmn; 0:5)mm, △Y = (48 &amp;plusmn; 0:5)mm), by the action of the amplitude of GW. The tolerance of a LINAC is lower than these measurements, and the equipment will stop working if they are greater than &amp;plusmn;1:0mm for isocenter (zero position) and &amp;plusmn;2:0mm for other collimator leaf positions. Therefore, this constitutes a register of space-time alteration with a consequent variation of the path of the X-ray beam. Finally, the registered gravitational waves leave invariant the angle between the axes (X; Y), of the X-ray beam, indicating a constant polarization.

45 Quantification of geometric distortion on MR images and evaluation of the impact of distortion correction

Introduction Magnetic resonance (MR) images have proven very useful for target definition in radiotherapy. However, geometric distortions might be of concern. In this study, a GE software based solution to correct geometric distortions is evaluated. The amount of residual distortions on corrected MRI is then quantified and used to determine PTV margins. Methods Axial T1 gradient-echo sequences of an MR compatible phantom and of patients with brain metastases were acquired on a 3T MRI. All patients were injected with Gadolinium. MR images were corrected for geometric distortion using the default algorithm implemented on our acquisition software, leading to two sets of MR images per acquisition (corrected and uncorrected). The gross tumor volume (GTV) was then contoured by a physician on both corrected and uncorrected MR images of the patients. Acquisition parameters were the ones commonly used in clinical practice for intracranial tumors. The MR phantom external contour was drawn on both corrected and uncorrected MR images, and used to deform uncorrected images to match corrected images. The amplitude of the displacement vector field was used to identify the main regions of the image undergoing large geometric distortions (Figs. 1 and 2). The same work was done on patients using the external surface of the patients. Geometric distortions were quantified using visible markers on both CT and MR images (Fig. 3). Results The main regions affected by geometric distortion were identified using the vector displacement field. These regions are located apart from the machine isocenter in the transverse direction (Figs. 4 and 5). Maximum distortions measured on the phantom for uncorrected MR images ranged from 1 to 2 mm for slices located near the isocenter and until 3 mm for the most distant slices. Distortions remained below 1 mm in corrected MRI. Considering patient images, a maximum distance of 2 mm was observed between the GTVs contoured on both corrected and uncorrected MRI (Fig. 6). Conclusions Our results point out the importance of using a correction for geometric distortions in MR images. Without being corrected for, distortions might reach several millimeters. The correction algorithm we used significantly reduces the amount of distortion in the images, particularly in regions located away from the isocenter. Quantification of residual distortions was made on phantom. However, additional sources of distortions might be of interest when patients are considered as for example the difference in magnetic susceptibility among tissues. We are currently investigating a multimodal approach based on CT-MRI registration to quantify patient-specific distortions. Download : Download high-res image (570KB) Download : Download full-size image

On the total system error of a robotic radiosurgery system: phantom measurements, clinical evaluation and long-term analysis

The total system error (TSE) of a CyberKnife® system was measured using two phantom-based methods and one patient-based method. The standard radiochromic film (RCF) end-to-end (E2E) test using an anthropomorphic head and neck phantom and isocentric treatment delivery was used with the 6Dskull, Fiducial and Xsight® spine (XST) tracking methods. More than 200 RCF-based E2E results covering the period from installation in 2006 until 2017 were analyzed with respect to tracking method, system hardware and software versions, secondary collimation system, and years since installation. An independent polymer gel E2E method was also applied, involving a 3D printed head phantom and multiple spherical target volumes widely distributed within the brain. Finally, the TSE was assessed by comparing the delineated target in the planning computed tomography images of a patient treated for a thalamic functional target with the radiation-induced lesion defined on the six-month follow-up magnetic resonance (MR) images. Statistical analysis of the RCF-based TSE results showed mean ± standard deviation values of 0.40 ± 0.18 mm, 0.40 ± 0.19 mm, and 0.55 ± 0.20 mm for the 6Dskull, Fiducial, and XST tracking methods, respectively. Polymer gel TSE values smaller than 0.66 mm were found for seven targets distributed within the brain, showing that the targeting accuracy of the system is sustained even for targets situated up to 80 mm away from the center of the skull. An average clinical TSE value of 0.87 ± 0.25 mm was also measured using the FSE T2 and FLAIR post-treatment MR image data. Analysis of the long-term RCF-based E2E tests showed no changes of TSE over time. This study is the first to report long-term (>10 years) analysis of TSE, TSE measurement for targets positioned at large distances from the virtual machine isocenter, or a clinical assessment of TSE for the CyberKnife system. All of these measurements demonstrate TSE consistently < 1 mm.

Quantitative analysis of setup errors in lung SBRT with R624-SCF immobilization equipment

Objective KV-CBCT was utilized to evaluate the setup errors in lung SBRT with R624-SCF immobilization equipment, quantitatively analyze the percentage of all types of errors in the cumulative errors and unravel the main sources of setup errors. Methods The CBCT data weekly and QA data monthly from 32 patients diagnosed with lung neoplasms were collected to quantitatively analyze the setup errors. The cumulative errors were calculated by statistical model. The proportion and source of each type of setup error was analyzed. Results All 32 patients received a total of 420 times of CBCT.The setup errors of immobilization equipment in the lateral, supine-inferior, anterior-posterior directions were (0.03±0.72) mm, (0.73±1.16) mm and (-0.21±0.95) mm, respectively. The errors of tumor motion in three directions were (0.71±2.61) mm, (-0.80±2.60) mm and (0.075±1.77) mm, respectively. According to the calculation formula proposed by Vance Keeling, the proportion of the cumulative error was 54.55%, 9.21% for immobilization equipment, 12.97% for tumor motion, 2.55% for couch sagging, 5.70% for Gantry radiation isocenter, 4.73% for Collimator radiation isocenter, 4.61% for couch radiation isocenter and 5.70% for X-ray field isocenter, respectively. Conclusions The main factors of setup errors during SBRT treatment for lung cancer are setup random, tumor motion, immobilization equipment, couch sagging and machine isocenter. During radiotherapy, targeted control of tumor motion is of significance for minimizing the cumulative errors. Key words: Lung neoplasm/radiotherapy; Setup error; Cumulative error

AAPM Medical Physics Practice Guideline 8.a.: Linear accelerator performance tests.

PurposeThe purpose of this guideline is to provide a list of critical performance tests in order to assist the Qualified Medical Physicist (QMP) in establishing and maintaining a safe and effective quality assurance (QA) program. The performance tests on a linear accelerator (linac) should be selected to fit the clinical patterns of use of the accelerator and care should be given to perform tests which are relevant to detecting errors related to the specific use of the accelerator.MethodsA risk assessment was performed on tests from current task group reports on linac QA to highlight those tests that are most effective at maintaining safety and quality for the patient. Recommendations are made on the acquisition of reference or baseline data, the establishment of machine isocenter on a routine basis, basing performance tests on clinical use of the linac, working with vendors to establish QA tests and performing tests after maintenance.ResultsThe recommended tests proposed in this guideline were chosen based on the results from the risk analysis and the consensus of the guideline's committee. The tests are grouped together by class of test (e.g., dosimetry, mechanical, etc.) and clinical parameter tested. Implementation notes are included for each test so that the QMP can understand the overall goal of each test.ConclusionThis guideline will assist the QMP in developing a comprehensive QA program for linacs in the external beam radiation therapy setting. The committee sought to prioritize tests by their implication on quality and patient safety. The QMP is ultimately responsible for implementing appropriate tests. In the spirit of the report from American Association of Physicists in Medicine Task Group 100, individual institutions are encouraged to analyze the risks involved in their own clinical practice and determine which performance tests are relevant in their own radiotherapy clinics.

Instrumental Quality Control of Therapeutic Linear Accelerator Performance

The objective of the article was to assess therapeutic linear accelerator performance. Material & method used were quality control tools, direct measurement & theoretical calculation methods. The analysis of results showed that: shift of machine isocenter was 1 mm then increases up to 2 mm through the gantry angles 0 to 300° and 300 to 360 respectively. The diaphragm rotation isocenter clock & anti-clock wise was 1mm. the light and radiation fields showed concise matching up to 9×9 cm, then for 10×10, 14×14 and 16×16 cm there were incongruence by 0.25, 0.3 and 0.41 cm respectively. The increment of the field sizes (2×2, 4×4 - 20×20) cm following SSD increment fitted with the inverse square law significantly (R2 = 1). The theoretical (calculation method) field size was greater than the measured (practical) field size relative to SSD by 0.2 cm. The system output in Gy/Mu increases significantly (R2 = 0.9) as the field size increases in logarithmic equation; while it decreases as SSD increases. The measured output on phantom surface was greater (0.8Gy/MU) than that calculated theoretically which was (0.5 Gy/MU). A significant (R2 = 0.8) reduction in output reading following the increment of temperature for Linac 10 MV and 6 MV respectively, while the pressure lead to significant (0.6) increment of system output reading. TLD showed narrow penumbra extension as 0.32 and 0.2 cm for lianc 6MV and 10MV respectively compared with 0.5 and 0.3 cm at maximum depth dose when obtained from dose histogram.

Poster ‐ 35: Monitoring patient positioning during deep inspiration breath hold with a distance measuring laser

Purpose:The accuracy of treatment delivery for left breast/chest wall patients using deep inspiration breath hold (DIBH) is being monitored using a distance measuring laser (DML)Methods:A commercially available DML (DLS‐C15, Dimetix) was mounted behind a Varian TrilogyTM linac. Relative to the machine isocenter, the laser from the beam was offset by 8 cm to the left and by 1 cm in the superior direction. This position was selected because this point is situated on the sternum for the majority of the left breast/chest‐wall patients treated at our institution.The Varian Real‐Time Positioning Management™ (RPM) guided DIBH treatments at our institution is delivered by placing the system's tracking block on the patient's abdomen. The treatment beam is enabled only when the block is in between a predefined abdomen motion range as determined during the CT simulation process. A LabVIEW program was developed to record both beam status (i.e. on/off) and distance measurements. In this study the DML was only used to monitor the position of a single point on the chest and no clinical decisions/adjustments were made based on these measurements.Results and Conclusions:Thus far, 34 fractions have been recorded for 4 patients. As such, the performance of our DIBH treatment technique cannot be fairly evaluated at this point. However, deviations between expected and measured distances have been observed and if these are found to be reproducible, then modifications in our treatment procedures and policies will have to take place.

TU-FG-209-09: Mathematical Estimation and Experimental Measurement of Patient Free-In-Air Skin Entrance Exposure During a Panoramic Dental X-Ray Procedure

Purpose: To develop a simple mathematical model for estimating the patient free-in-air skin entrance exposure (SEE) during a panoramic dental x-ray that does not require the use of a head phantom. This eliminates issues associated with phantom centering and the mounting of a detector on the phantom for routine QC testing. Methods: We used a Sirona Orthophos XG panoramic radiographic unit and a Radcal Accu-Gold system for this study. A solid state detector was attached over the slit of the Orthophos’ sensor with the help of a custom-built jig. A single measurement of the free-in-air exposure at this position was taken over a full panoramic scan. A mathematical model for estimating the SEE was developed based upon this measurement, the system geometry, x-ray field beam width, and x-ray sweep angle. To validate the model, patient geometry was simulated by a 16 cm diameter PMMA CTDI phantom centered at the machine's isocenter. Measurements taken on the phantom's surface were made using a solid state detector with lead backing, an ion chamber, and the ion chamber with the phantom wrapped in lead to mitigate backscatter. Measurements were taken near the start position of the tube and at 90 degrees from the start position. Results: Using the solid state detector, the average SEE was 23.5+/−0.02 mR and 55.5+/−0.08 mR at 64 kVp and 73 kVp, respectively. With the lead-wrapping, the measurements from the ion chamber matched those of the solid state detector to within 0.1%. Preliminary results gave the difference between the mathematical model and the phantom measurements to be approximately 5% at both kVps. Conclusion: Reasonable estimates of patient SEE for panoramic dental radiography can be made using a simple mathematical model without the need for a head phantom.

Analysis of precision in tumor tracking based on optical positioning system during radiotherapy.

Tumor tracking is performed during patient set-up and monitoring of respiratory motion in radiotherapy. In the clinical setting, there are several types of equipment for this set-up such as the Electronic Portal imaging Device (EPID) and Cone Beam CT (CBCT). Technically, an optical positioning system tracks the difference between the infra ball reflected from body and machine isocenter. Our objective is to compare the clinical positioning error of patient setup between Cone Beam CT (CBCT) with the Optical Positioning System (OPS), and to evaluate the traditional positioning systems and OPS based on our proposed approach of patient positioning. In our experiments, a phantom was used, and we measured its setup errors in three directions. Specifically, the deviations in the left-to-right (LR), anterior-to-posterior (AP) and inferior-to-superior (IS) directions were measured by vernier caliper on a graph paper using the Varian Linear accelerator. Then, we verified the accuracy of OPS based on this experimental study. In order to verify the accuracy of phantom experiment, 40 patients were selected in our radiotherapy experiment. To illustrate the precise of optical positioning system, we designed clinical trials using EPID. From our radiotherapy procedure, we can conclude that OPS has higher precise than conventional positioning methods, and is a comparatively fast and efficient positioning method with respect to the CBCT guidance system.

SU‐F‐T‐547: Off‐Isocenter Winston‐Lutz Test for Stereotactic Radiosurgery/stereotactic Body Radiotherapy

Purpose:To perform a quantitative study to verify that the mechanical field center coincides with the radiation field center when both are off from the isocenter during the single‐isocenter technique in linear accelerator‐based SRS/SBRT procedure to treat multiple lesions.Methods:We developed an innovative method to measure this accuracy, called the off‐isocenter Winston‐Lutz test, and here we provide a practical clinical guideline to implement this technique. We used ImagePro V.6 to analyze images of a Winston‐Lutz phantom obtained using a Varian 21EX linear accelerator with an electronic portal imaging device, set up as for single‐isocenter SRS/SBRT for multiple lesions. We investigated asymmetry field centers that were 3 cm and 5 cm away from the isocenter, as well as performing the standard Winston‐Lutz test. We used a special beam configuration to acquire images while avoiding collision, and we investigated both jaw and multileaf collimation.Results:For the jaw collimator setting, at 3 cm off‐isocenter, the mechanical field deviated from the radiation field by about 2.5 mm; at 5 cm, the deviation was above 3 mm, up to 4.27 mm. For the multileaf collimator setting, at 3 cm off‐isocenter, the deviation was below 1 mm; at 5 cm, the deviation was above 1 mm, up to 1.72 mm, which is 72% higher than the tolerance threshold.Conclusion:These results indicated that the further the asymmetry field center is from the machine isocenter, the larger the deviation of the mechanical field from the radiation field, and the distance between the center of the asymmetry field and the isocenter should not exceed 3 cm in of our clinic. We recommend that every clinic that uses linear accelerator, multileaf collimator‐based SRS/SBRT perform the off‐isocenter Winston‐Lutz test in addition to the standard Winston‐Lutz test and use their own deviation data to design the treatment plan.

SU-G-TeP2-04: Comprehensive Machine Isocenter Evaluation with Separation of Gantry, Collimator, and Table Variables

Purpose: To develop and demonstrate application of a method that characterizes deviation of linac x-ray beams from the centroid of the volumetric radiation isocenter as a function of gantry, collimator, and table variables. Methods: A set of Winston-Lutz ball-bearing images was used to determine the gantry radiation isocenter as the midrange of deviation values resulting from gantry and collimator rotation. Also determined were displacement of table axis from gantry isocenter and recommended table axis adjustment. The method, previously reported, has been extended to include the effect of collimator walkout by obtaining measurements with 0 and 180 degree collimator rotation for each gantry angle. Twelve images were used to characterize the volumetric isocenter for the full range of available gantry, collimator, and table rotations. Results: Three Varian True Beam, two Elekta Infinity and four Versa HD linacs at five institutions were tested using identical methodology. Varian linacs exhibited substantially less deviation due to head sag than Elekta linacs (0.4 mm vs. 1.2 mm on average). One linac from each manufacturer had additional isocenter deviation of 0.3 to 0.4 mm due to jaw instability with gantry and collimator rotation. For all linacs, the achievable isocenter tolerance was dependent on adjustment of collimator position offset, transverse position steering, and alignment of the table axis with gantry isocenter, facilitated by these test results. The pattern and magnitude of table axis wobble vs. table angle was reproducible and unique to each machine. Conclusion: This new method provides a comprehensive set of isocenter deviation values including all variables. It effectively facilitates minimization of deviation between beam center and target (ball-bearing) position. This method was used to quantify the effect of jaw instability on isocenter deviation and to identify the offending jaw. The test is suitable for incorporation into a routine machine QA program. Software development was performed by Radiological Imaging Technology, Inc.