Gantry Axis Research Articles

A 3D star shot to determine the gantry, collimator, and couch axes positions.

A linear accelerator has three independent axes that are nominally intersecting at the isocenter. Modern treatment techniques require the coincidence of these axes to lie within a 1‐mm diameter sphere. A solution to verify this requirement is to wrap a film on a cylindrical surface, align the cylinder to the linac's isocenter and gantry axis, and take multiple exposures of slits, rotating either the gantry, collimator, or couch between exposures. The resulting exposure pattern is the 3D equivalent of the 2D star shot and encodes sufficient information to determine each axis’ position in 3D. Moreover, this method uses a single sheet 8“x10” film, a standard film scanner, and a phantom that can be readily built in‐house, making a practical solution to this 3D‐measurement problem.

Optimization design of O-ring linear accelerator gantry and mechanical isocenter detection method.

With the development of the precise radiotherapy, the accuracy of radiotherapy equipment is gradually improved. The gantry, the carrier of the treatment head and various testing devices, is the most important component that determines the accuracy of the entire equipment. In this paper, the layout of the O-ring linear accelerator and the structure of the gantry are optimized to reduce the weight of the gantry by 50% and the moment of inertia by 60%. A mechanical isocenter detection method based on the laser tracker is proposed to conduct a rapid and accurate isocenter measurement for the optimized gantry. The experimental results show that the maximum deformation of the optimized gantry under load is 0.13mm during rotation and the maximum intersection distance between the gantry axis and the treatment head axis at the isocenter position is 0.21mm.

Validating robotic couch isocentricity with 3D surface imaging.

BackgroundA proton therapy system with 190° gantries uses robotic couch rotations to change the treatment beam laterality. Couch rotations are typically validated clinically with post‐rotation radiographic imaging.AimsThis study assesses the specificity and sensitivity of a commercial 3D surface imaging system, AlignRT (Vision RT, London UK) for validating couch rotations.Materials & MethodsIn clinical operation, a reference surface image of the patient is acquired after radiographic setup with couch at 270°, perpendicular to the gantry axis of rotation. The couch is then rotated ±90° to a typical treatment angle, and AlignRT reports a 3D displacement vector. Patient motion, changes in patient surface, non‐coincidence between AlignRT and couch isocenter, and mechanical couch run‐out all contribute to the 3D vector magnitude. To assess AlignRT sensitivity in detecting couch run‐out, volunteers were positioned orthogonal to the proton gantry and reference surface images were captured without x‐ray localization. Subjects were repeatedly rotated ±90⁰ to typical treatment angles and displacement vectors were recorded. Additionally, measurements were performed in which intentional translations of 2, 4, 6, and 8 mm were combined with the intended isocentric rotations. Data sets were collected using a phantom; subjects with a thoracic isocenter and no immobilization; and subjects with a cranial isocenter and thermoplastic immobilization. A total of 300 rotations were measured.ResultsDuring isocentric rotations, the mean AlignRT displacement vectors for the phantom, immobilized, and non‐immobilized volunteers were 0.1 ± 0.1 mm, 0.8 ± 0.1 mm, and 1.1 ± 0.2 mm respectively. 95% of the AlignRT measurements for the immobilized and non‐immobilized subjects were within 1 mm and 2 mm of the actual displacement respectively.DiscussionAfter characterizing the accuracy using phantoms and volunteers, we have shown that a three‐pod surface imaging system can be used to identify gross non‐isocentric patient rotations. Significant positional deviations, either due to improper couch rotation or patient motion, should be followed by radiographic imaging and repositioning.ConculsionAlignRT can be used to verify patient positioning following couch rotations that are applied after the initial x‐ray guided patient setup. Using a three‐pod AlignRt system, positional deviations exceeding 4 mm were flagged with sensitivity and specificity of 90% and 100% respectively.

The impact of isocentric shifts on delivery accuracy during the irradiation of small cerebral targets-Quantification and possible corrections.

PurposeTo assess the impact of isocenter shifts due to linac gantry and table rotation during cranial stereotactic radiosurgery on D98, target volume coverage (TVC), conformity (CI), and gradient index (GI).MethodsWinston‐Lutz (WL) checks were performed on two Elekta Synergy linacs. A stereotactic quality assurance (QA) plan was applied to the ArcCHECK phantom to assess the impact of isocenter shift corrections on Gamma pass rates. These corrections included gantry sag, distance of collimator and couch axes to the gantry axis, and distance between cone‐beam computed tomography (CBCT) isocenter and treatment beam (MV) isocenter. We applied the shifts via script to the treatment plan in Pinnacle 16.2. In a planning study, isocenter and mechanical rotation axis shifts of 0.25 to 2 mm were applied to stereotactic plans of spherical planning target volumes (PTVs) of various volumes. The shifts determined via WL measurements were applied to 16 patient plans with PTV sizes between 0.22 and 10.4 cm3.ResultsArcCHECK measurements of a stereotactic treatment showed significant increases in Gamma pass rate for all three measurements (up to 3.8 percentage points) after correction of measured isocenter deviations. For spherical targets of 1 cm3, CI was most severely affected by increasing the distance of the CBCT isocenter (1.22 to 1.62). Gradient index increased with an isocenter‐collimator axis distance of 1.5 mm (3.84 vs 4.62). D98 (normalized to reference) dropped to 0.85 (CBCT), 0.92 (table axis), 0.95 (collimator axis), and 0.98 (gantry sag), with similar but smaller changes for larger targets. Applying measured shifts to patient plans lead to relevant drops in D98 and TVC (7%) for targets below 2 cm3 treated on linac 1.ConclusionMechanical deviations during gantry, collimator, and table rotation may adversely affect the treatment of small stereotactic lesions. Adjustments of beam isocenters in the treatment planning system (TPS) can be used to both quantify their impact and for prospective correction of treatment plans.

Application of computed radiography in the quality assurance of linear accelerators in radiotherapy

Introduction: The two mechanisms, optical and radiation fields, operate individually and independently in a Linear Accelerator and cause changes with respect to each other. The standard method for performing the quality assurance (QA) test in radiotherapy involves the irradiation of a radiographic film. In this study, we made an attempt to examine how we could maximize the benefit from an impending filmless environment in the radiotherapy QA program. Aim: The aim is to study the feasibility of using computed radiographs (CRs) in the radiotherapy QA program. Materials and Methods: In this study, the QA tests were performed in Linear accelerators Clinac 600c and Clinac iX, both from Varian Medical Systems, Palo Alto, CA, which was commissioned during September 2004 and July 2011, respectively, were used. Optical and radiation field congruence, radiation isocenter for the gantry, collimator, and couch rotational axis verification, in two linear accelerators were done using gafchromic (EBT3) films and CRs. The standard Gafchromic® EBT3 film, utilized for routine QA were used. The errors estimated were compared and analyzed. Results: The mean error estimated in the QA with both linear accelerators using both QA tools (CR and Film) ranged between 0.053 mm and 0.069 mm, and the standard deviation was estimated to be within 0.062-0.164 mm. Conclusion: The results infer that the QA done with CR is in good agreement with the film. This study poses new challenges to the researchers, task groups, and the regulatory bodies to estimate the frequency of QAs for the newer and the older machines and also, the onset of frequent QAs, once the machine becomes older.

SU‐E‐T‐708: Spatial Accuracy of Small‐Target SRS Using Conebeam CT Guidance and Static Arcs with MLC‐Defined Fields

Purpose:To measure the spatial accuracy of dose delivery under conebeam CT guidance to small targets using static arcs with MLC‐defined fields.Methods:A plastic (PMMA) phantom simulating a small brain lesion was constructed. The lesion was a cylindrical air cavity having 3 mm length and 3.175 mm diameter. Gafchromic EBT 2 film passing through the cavity was pin pricked at the exact geometric cavity center. Following CT simulation treatment plans involving static arcs with MLC‐defined fields were designed, similar to radiotherapy with circular tertiary cones. 5×2 mm fields were generated by opening two centrally located leaf pairs of a high‐definition MLC to 2.0 mm. The target was irradiated with 6 MV photons on a STx linac under conebeam CT image guidance to 600 cGy at isocenter. Films from coronal and sagittal planes were scanned and evaluated using the ImageJ software. The distance between the centroid of the 50% isodose line and the center of the pin prick was the measure of geographic accuracy.Results:Single arcs resulted in ellipsoidal 50% isodose surfaces having diameters of 5.0 mm along the gantry axis and 7.7 mm in the cross‐sectional directions. 4‐arc delivery at couch angles of 0, 45, 90 and 315 degrees produced 50% isodose surfaces having diameters of 5.6, 5.7 and 7.5 mm diameters along the lateral, longitudinal and anterior‐posterior directions, respectively. In any of the experiments, the largest distance by which the centroid of the 50% isodose line missed the center of the target was 0.36 mm or less.Conclusion:MLC‐defined static arcs delivered under conebeam CT guidance provide precise irradiation of small targets. The method is very efficient since no special attachments or modifications of the accelerator are required. Since MLC leaves are stationary throughout the treatment, patient‐specific QA involving phantoms should not be required.

SU‐E‐T‐202: Comparison of 4D‐Measurement‐Guided Dose Reconstructions (MGDR) with COMPASS and OCTAVIUS 4D System

Purpose:To cross‐validate the MGDR of COMPASS (IBA dosimetry, GmbH, Germany) and OCTAVIUS 4D system (PTW, Freiburg, Germany).Methods:Volumetric‐modulated arc plans (5 head‐and‐neck and 3 prostate) collapsed to 40° gantry on the OCTAVIUS 4D phantom in QA mode on Monaco v5.0 (Elekta, CMS, Maryland Heights, MO) were delivered on a Elekta Agility linac. This study was divided into two parts: (1) error‐free measurements by gantry‐mounted EvolutionXX 2D array were reconstructed in COMPASS (IBA dosimetry, GmbH, Germany), and by OCTAVIUS 1500 array in Versoft v6.1 (PTW, Freiburg, Germany) to obtain the 3D doses (COM4D and OCTA4D). COM4D and OCTA4D were compared to the raw measurement (OCTA3D) at the same detector plane for which OCTAVIUS 1500 was perpendicular to 0° gantry axis while the plans were delivered at gantry 40°; (2) beam steering errors of energy (Hump=‐2%) and symmetry (2T=+2%) were introduced during the delivery of 5 plans to compare the MGDR doses COM4D_Hump (COM4D_2T), OCTA4D_Hump (OCTA4D_2T), with raw doses OCTA3D_Hump (OCTA3D_2T) and with OCTA3D to assess the error reconstruction and detection ability of MGDR tools. All comparisons used Υ‐criteria of 2%(local dose)/2mm and 3%/3mm.Results:Averaged Υ passing rates were 85% and 96% for COM4D,and 94% and 99% for OCTA4D at 2%/2mm and 3%/3mm criteria respectively. For error reconstruction, COM4D_Hump (COM4D_2T) showed 81% (93%) at 2%/2mm and 94% (98%) at 3%/3mm, while OCTA4D_Hump (OCTA4D_2T) showed 96% (96%) at 2%/2mm and 99% (99%) at 3%/3mm. For error detection, OCTA3D doses were compared to COM4D_Hump (COM4D_2T) showing Υ passing rates of 93% (93%) at 2%/2mm and 98% (98%), and to OCTA4D_Hump (OCTA4D _2T) showing 94% (99%) at 2%/2mm and 81% (96%) at 3%/3mm, respectively.Conclusion:OCTAVIUS MGDR showed better agreement to raw measurements in both error‐ and error‐free comparisons. COMPASS MGDR deviated from the raw measurements possibly owing to beam modeling uncertainty.

SU‐E‐T‐790: Validation of 4D Measurement‐Guided Dose Reconstruction (MGDR) with OCTAVIUS 4D System

Purpose:To validate the MGDR of OCTAVIUS 4D system (PTW, Freiburg, Germany) for quality assurance (QA) of volumetric‐modulated arc radiotherapy (VMAT).Methods:4D‐MGDR measurements were divided into two parts: 1) square fields from 2×2 to 25×25 cm2 at 0°, 10° and 45° gantry, and 2) 8 VMAT plans (5 nasopharyngeal and 3 prostate) collapsed to gantry 40° in QA mode in Monaco v5.0 (Elekta, CMS, Maryland Heights, MO) were delivered on the OCTAVIUS 4D phantom with the OCTAVIUS 1500 detector plane perpendicular to either the incident beam to obtain the reconstructed dose (OCTA4D) or the 0° gantry axis to obtain the raw doses (OCTA3D) in Verisoft 6.1 (PTW, Freiburg, Germany). Raw measurements of OCTA3D were limited to < 45° gantry to avoid >0.5% variation of detector angular response with respect to 0° gantry as determined previously. Reconstructed OCTA4D and raw OCTA3D doses for all plans were compared at the same detector plane using γ criteria of 2% (local dose)/2mm and 3%/3mm criteria.Results:At gantry 0° and 10°, the γ results for all OCTA4D on detector plane coinciding with OCTA3D were over 90% at 2%/2mm except for the largest field (25×25 cm2 ) showing >88%. For square field at 45° gantry, γ passing rate is > 90% for fields smaller than 15x 15cm2 but < 80% for field size of 20 x20 cm2 upward. For VMAT, γ results showed 94% and 99% passing rate at 2%/2mm and 3%/3mm, respectively.Conclusion:OCTAVIUS 4D system has compromised accuracy in reconstructing dose away from the central beam axis, possibly due to the off‐axis softening correction and errors of the percent depth dose data necessary as input for MGDR. Good results inVMAT delivery suggested that the system is relatively reliable for VMAT with small segments.

SU‐E‐T‐160: Characterization and Monitoring of Linear Accelerator Gantry Radiation Isocenter Motion

Purpose:To characterize the motion of the radiation isocenter, over time, as a function of gantry rotation for multiple linear accelerators (linacs). Two semi‐automated image‐based quality control (QC) test workflows were designed to achieve this goal.Methods:The full QC‐test workflow for motion characterization consisted of acquiring 16 megavoltage images at 8 gantry angles of a ball‐bearing suspended off the end of the couch. Performance constancy was assessed using a shortened QC‐test workflow which consisted of imaging a cube phantom placed on the couch (5 images at 4 gantry angles). Both workflows use an image processing algorithm to determine the field center and phantom position on each image and computed radiation isocenter motion as a function of gantry angle. Motion was characterized for 9 linacs of same model and performance monitored for 2 months on 3 linacs.Results:The maximum isocenter motion determined with the full‐workflow for 9 linacs was within 0.38–0.79 mm. The shortened‐workflow usually agreed within 0.1 mm with the full‐workflow and the time required for these methods was about 4 and 15 min, respectively. For all linacs, the isocenter motion perpendicular to the gantry rotation plane followed a consistent pattern with maximum amplitude of 0.36–0.59 mm. In the gantry rotation plane, the variation among linacs was higher and the beam axis described a circle of up to 0.6 mm radius around the gantry axis of rotation (2 linacs). The radiation isocenter motion was stable as a function of time for the monitored linacs and was within ±0.1 mm of the average.Conclusion:Radiation isocenter motion parallel and perpendicular to the gantry rotation plane was characterized. In the gantry rotation plane, beam spot positioning adjustment might be used to reduce the observed radiation isocenter motion. A shortened‐workflow was designed and enables performance monitoring over time.

Angular Dependence of the MOSFET Dosimeter and its Impact on In vivo Surface Dose Measurement in Breast Cancer Treatment

The focus of this study is the angular dependence of two types of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) dosimeters (MOSFET20 and OneDose/OneDosePlus) when used for surface dose measurements. External beam radiationat different gantry angles were delivered to a cubic solid water phantom with a MOSFET placed on the top surface at CAX. The long axis of the MOSFET was oriented along the gantry axis of rotation, with the dosimeter (bubble side) facing the radiation source. MOSFET-measured surface doses were compared against calibrated radiochromic film readings. It was found that both types of MOSFET dosimeters exhibited larger than previously reported angular dependence when measuring surface dose in beams at large oblique angles. For the MOSFET20 dosimeter the measured surface dose deviation against film readings was as high as 17% when the incident angle was 72 degrees to the norm of the phantom surface. It is concluded that some MOSFET dosimeters may have a strong angular dependence when placed on the surface of water-equivalent material, even though they may have an isotropic angular response when surrounded by uniform medium. Extra on-surface calibration maybe necessary before using MOSFET dosimeters for skin dose measurement in tangential fields.

Magnet Developments and Commissioning for the IBA Compact Gantry

The ProteusOne gantry will be the first IBA gantry with scanning upstream of the last bending magnet. This configuration offers the combined advantage of compactness and Pencil Beam Scanning (PBS) with reasonable source to gantry axis distance (SAD). This new proton therapy gantry required modifications on existing IBA beam line magnets as well as development of new ones. This paper presents the basic features and advantages of the gantry optics and details the magnets that needed to be designed. In addition, some specific issues raised by the gantry compactness will be detailed. The scanning magnet and the last, wide gap, bending magnet required special attention to ensure good quality beam optics. Indeed, due to the small distance between them, careful design is required to minimize their mutual influence. Parts of the gantry structural elements also had to be taken into account, adding complexity to the magnetic modeling and requiring very good collaboration between beam optics and mechanical design teams. Finally, some details of the commissioning of the magnets and model crosschecks are also presented.

3D tomodosimetry using long scintillating fibers: A feasibility study

3D dosimetry is recognized as an ideal for patient-specific quality assurance (QA) of highly conformal radiotherapy treatments. However, existing 3D dosimeters are not straightforward to implement in the clinic, as their read-out procedure is often tedious and their accuracy, precision, and∕or sample size exhibit limitations. The purpose of this work is to develop a 3D dosimeter based on the concept of tomodosimetry inside concentric cylindrical planes using long scintillating fibers for the QA of modern radiotherapy techniques such as intensity-modulated radiation therapy (IMRT) or intensity-modulated arc therapy (IMAT). Using a model-based simulation, scintillating fibers were modeled on three concentric cylindrical planes of radii 2.5, 5.0, and 7.5 cm, inside a 10 cm radius water-equivalent cylinder phantom. The phantom was set to rotate around its central axis, made parallel to the linac gantry axis of rotation. Light acquisitions were simulated using the calculated dose from the treatment planning software and reconstructed in each cylindrical plane at a resolution of 1 mm(2) using a total-variation minimization iterative reconstruction algorithm. The 3D dose was then interpolated from the reconstructed cylindrical plane doses at a resolution of 1 mm(3). Different scintillating fiber patterns were compared by varying the angle of each fiber in its cylindrical plane and introducing a light-tight cut in each fiber. The precision of the reconstructed cylindrical dose distribution was evaluated using a Poisson modeling of the acquired light signals and the accuracy of the interpolated 3D dose was evaluated using an IMRT clinical plan for a prostate case. Straight scintillating fiber patterns with light-tight cuts were the most accurate in cylindrical dose reconstruction, showing less than 0.5 mm distance-to-agreement in dose gradients and a mean local dose difference of less than 0.2% in the high dose region for a 10 × 10 cm(2) field. The precision attained with this fiber configuration was less than 0.9% in the high dose, low gradient region of an IMRT segment for light acquisitions of 0.1 MU over a 360 degree rotation of the cylinder phantom. 3D dose interpolation for the IMRT clinical plan yielded an overall dose difference with the reference input of less than 1%, except in high dose gradients. Using long scintillating fibers inside rotating, concentric cylindrical planes, the authors demonstrate that their tomodosimetry method has the potential for high resolution, precise, and accurate 3D dosimetry. Moreover, because of its water-equivalence and rotational symmetry, this design should find interesting application for both treatment QA and machine commissioning.

Cross Coupling Controller for Accurate Motion Synchronization of Dual Servo Systems

In order to attain higher manufacturing efficiency, “dual” (two) servo systems are widely used in advanced Computer Numerical Controlled (CNC) machine tools. A well-known example is the linear motor driven gantry type of micro machine tools where dual servos are employed to drive the heavier gantry axis. Recently, dual servos are also used in spindle systems. “Double sided milling” is an example where two spindles are required to cooperatively remove material on both sides of a workpiece. Synchronization of dual servo systems is crucial for achieving the desired manufacturing accuracy. This paper presents a crosscoupling controller to accurately synchronize dualmotor driven servo systems. Proposed cross coupling controller penalizes differential positioning error between dual servo drives by modifying the reference position and velocity commands. It improves motion synchronization without affecting the overall tracking bandwidth. A tuning method for the proposed controller is also presented for the control engineer’s practice.

Optimizing of the Tangential Technique and Supraclavicular Fields in 3 Dimensional Conformal Radiation Therapy for Breast Cancer

Radiotherapy plays an essential role in the management of breast cancer. Three-dimensional conformal radiation therapy (3D-CRT) is applied based on 3D image information of anatomy of patients. In 3D-CRT for breast cancer one of the common techniques is tangential technique. In this project, various parameters of tangential and supraclavicular fields are optimized. This project has been done on computed tomography images of 100 patients in Isfahan Milad Hospital. All patients have been simulated and all the important organs have been contoured by radiation oncologist. Two techniques in supraclavicular region are evaluated including: 1-A single field (Anterior Posterior [AP]) with a dose of 200 cGy per fraction with 6 MV energy. This is a common technique. 2-Two parallel opposed fields (AP-Posterior Anterior [PA]). The dose of AP was 150 cGy with 6 MV energy and PA 50 cGy with 18 MV. In the second part of the project, the tangential fields has been optimized with change of normalization point in five points: (1) Isocenter (Confluence of rotation gantry axis and collimator axis) (2) Middle of thickest part of breast or middle of inter field distance (IFD) (3) Border between the lung and chest wall (4) Physician's choice (5) Between IFD and isocenter. Dose distributions have been compared for all patients in different methods of supraclavicular and tangential field. In parallel opposed fields average lung dose was 4% more than a single field and the maximum received heart dose was 21.5% less than a single field. The average dose of planning tumor volume (PTV) in method 2 is 2% more than method 1. In general AP-PA method because of a better coverage of PTV is suggested. In optimization of the tangential field all methods have similar coverage of PTV. Each method has spatial advantages and disadvantages. If it is important for the physician to reduce the dose received by the lung and heart, fifth method is suggested since in this method average and maximum received dose to heart and lung have been reduced few percent in comparison to other methods. If a better coverage of PTV is important for the physician second method can be an optimized method. In this method, average and maximum received dose to PTV have been increased few percent in comparisons of physician's choice method and three other methods. In optimizing of supraclavicular field AP-PA method due to better coverage of PTV is suggested. In optimizing of tangential all methods are similar. Each method has special advantages and disadvantages. The physicians can change the depth of the normalization point in the breast to get the desired average dose.

Estimation of CT cone-beam geometry using a novel method insensitive to phantom fabrication inaccuracy: Implications for isocenter localization accuracy

Mechanical instabilities that occur during gantry rotation of on-board cone-beam computed tomography (CBCT) imaging systems limit the efficacy of image-guided radiotherapy. Various methods for calibrating the CBCT geometry and correcting errors have been proposed, including some that utilize dedicated fiducial phantoms. The purpose of this work was to investigate the role of phantom fabrication imprecision on the accuracy of a particular CT cone-beam geometry estimate and to test a new method to mitigate errors in beam geometry arising from imperfectly fabricated phantoms. The authors implemented a fiducial phantom-based beam geometry estimation following the one described by Cho et al. [Med Phys 32(4), 968-983 (2005)]. The algorithm utilizes as input projection images of the phantom at various gantry angles and provides a full nine parameter beam geometry characterization of the source and detector position and detector orientation versus gantry angle. A method was developed for recalculating the beam geometry in a coordinate system with origin at the source trajectory center and aligned with the axis of gantry rotation, thus making the beam geometry estimation independent of the placement of the phantom. A second CBCT scan with the phantom rotated 180 degrees about its long axis was averaged with the first scan to mitigate errors from phantom imprecision. Computer simulations were performed to assess the effect of 2D fiducial marker positional error on the projections due to image discretization, as well as 3D fiducial marker position error due to phantom fabrication imprecision. Experimental CBCT images of a fiducial phantom were obtained and the algorithm used to measure beam geometry for a Varian Trilogy with an on-board CBCT. Both simulations and experimental results reveal large sinusoidal oscillations in the calculated beam geometry parameters with gantry angle due to displacement of the phantom from CBCT isocenter and misalignment with the gantry axis, which are eliminated by recalculating the beam geometry in the source coordinate system. Simulations and experiments also reveal an additional source of oscillations arising from fiducial marker position error due to phantom fabrication imprecision that are mitigated by averaging the results with those of a second CBCT scan with phantom rotated. With a typical fiducial marker position error of 0.020 mm (0.001 in.), source and detector position are found in simulations to be within 250 microm of the true values, and detector and gantry angles less than 0.2 degrees. Detector offsets are within 100 microm of the known value. Experimental results verify the efficacy of the second scan in mitigating beam geometry errors, as well as large apparent source/detector isocenter offsets arising from phantom fabrication imprecision. The authors have developed and validated a novel fiducial phantom-based CBCT beam geometry estimation algorithm that does not require precise positioning of the phantom at machine isocenter and is insensitive to positional imprecision of fiducial markers within the phantom due to fabrication errors. The method can accurately locate source and detector isocenters even when using an imprecise phantom, which is very important for measurement of isocenter coincidence of the therapy and on-board imaging systems.

SU‐GG‐J‐31: Simultaneous Estimation of Beam Geometry and Radiation/Imaging Isocenter Coincidence in Cone‐Beam CT‐Guided Radiation Therapy

Purpose: To simultaneously characterize the geometric pose of an on‐board CBCT imaging system and measure the relative positions of the imaging and treatment machine radiation isocenters. Method and Materials: The method is based on an existing technique (Cho, et al, Med Phys 32(4), 968), which utilizes CBCT projections of a fiducial phantom to calculate the source and detector positions and orientations as a function of gantry angle. The algorithm was modified to localize the phantom pose relative to the kV source trajectory, and to transform beam geometry parameters from the phantom to imaging isocenter coordinate system. The method was applied to both CBCT and MV EPID projections acquired during the same imaging session without moving the phantom. The calculated positions of the phantom relative to the CBCT and EPID projection isocenters provide the relative displacement of the imaging and treatment isocenters. Two additional CBCT imaging experiments were then performed with the phantom displaced known distances. Results: Calculated CBCT geometric parameters were consistent over the three experiments, even with the phantom displaced from the machine isocenter by 10mm or misaligned with the gantry rotation axis by 1°. The CBCT beam parameters were close to nominal with small variations with gantry angle, with the exception of a 1 mm detector offset parallel to the gantry axis. Displacement of the CBCT imaging isocenter from the treatment isocenter was found to be (0.07, 1.24, 0.05 mm) in the (L‐R, A‐P, S‐I) direction. Conclusion: The method's output can assist in mitigation of geometric distortion‐related CBCT image artifacts and simultaneously provides the imaging‐to‐treatment isocenter offset. Supported by NCI Grant P01 CA 116602

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SU-FF-T-311: Exposure of An Ultrasound System for Prostate Localization to Neutrons

Purpose:Ultrasound systems are commonly used for confirming prostate localization. At our center, these devices are used for both photon and proton treatments. The question arose as to whether the neutron environment in the proton room would adversely affect their operation and, lacking any manufacturer's information regarding neutron exposure to the instrument, it was decided to determine the neutron dose around the ultrasound unit in a treatment room housing a 6/18 MV linear accelerator.Method and Materials: The ultrasound device is stored at the side of the room when not in use, close to the primary beam for a gantry angle of 90°. TLDs were placed on all four sides of the device, on the wall along the gantry rotation axis and on both sides of a short maze. The purpose of the TLDs on the walls was to provide a constancy check; this was particularly true for the TLD on the gantry axis since the readings should be independent of the gantry angle. Results: Six sets of TLDs were left in for periods of 16, 5.5, 4, 2, 1 and 2 days. For those periods, the number of MUs delivered to all the patients was computed using the record and verify system. The average number of MUs per day for both energies was 16,000 ± 20% and for 18 MV was 7,900 ± 9%. The variation is due to the change in the number of IMRT treatments delivered with time. The results are 0.366, 0.145, 0.140 and 0.176 mrem/MU (proximal/distal to source; proximal/distal to beam) for neutrons and 0.230, 0.083, 0.078 and 0.176 mrem/MU for photons.Conclusion: Based on use till now, the results establish a lower limit of 1650 rem on the neutron dose that the ultrasound system can tolerate.

SU‐FF‐T‐24: Comparison of Endobronchial HDR Brachytherapy Using CT Imaging and Conventional Simulator Filming

Purpose: To compare the clinical merit of endobronchial HDR based on CT imaging and conventional simulator films. Method and Materials: The tumor locations within the lung and trachea were first identified and defined with bronchoscopy (FigA). Next a catheter was inserted in conjunction with a dwell position x‐ray marker. Subsequently, the patient underwent either simulator filming or CT scanning for HDR treatment planning. For simulator filming, two orthogonal films were taken to define the tumor location and dose reference points (FigB&C). For CT, the area containing the bronchial target was scanned (FigD). Unlike simulator filming, 3D contouring of the tumor and the reference dose points can be obtained with CT in compliance with actual clinic requirements. Isodose distributions and tumor dose volume histograms (DVH) in both techniques were compared. Results: The simulator filming technique has an advantage of time efficiency. FigC shows the isodose distribution using simulator films. The treatment time durations on each dwell position are fairly constant. Since the tumor volume was defined by estimation, DVHs have little significance. Also, there is a magnification deviation of the tumor volume if the catheter is not always along the gantry axis. For large patients (lateral separation > 40cm), image quality diminishes resulting in uncertainty of isodose distribution coincidence to the tumor. FigE illustrates the CT images isodose distribution. The isodose distribution in each CT image changes from slice to slice and conformal to the tumor, thus reducing dose toxicity to adjacent tissues. It is in contrast to the monotonic one of the simulator films plan. The DVH is now more significance for analysis. Conclusions: Despite procedure time and cost saving with conventional simulator filming, CT imaging is favored for endobronchial HDR in terms of isodose conformity to the tumor and normal functional tissue toxicity reduction.

Commissioning and clinical implementation of a sliding gantry CT scanner installed in an existing treatment room and early clinical experience for precise tumor localization.

The primary objective of the present study is to demonstrate that a unique computed tomography (CT)-linear accelerator combination can be used to reduce uncertainties caused by organ motion and setup inaccuracy. The acceptance, commissioning, and clinical implementation of a sliding gantry CT scanner installed in an existing linear accelerator room are reported in this paper. A Siemens CT scanner was installed directly opposite to an existing accelerator. The scanner is movable on a pair of horizontal rails mounted parallel to the longitudinal axis of the treatment couch replaced with a carbon fiber tabletop. Acceptance and commissioning of the CT scanner were verified with phantom studies. For clinical implementation, quality assurance (QA) procedures have been instituted to ensure the integrity of the CT gantry axis alignment and the accuracy of its movement using a phantom designed in house. A clinical example employing the CT-Linac combination to correct the isocenter positioning caused by organ motion and setup inaccuracy was presented for a prostate irradiation. Dose calculations were performed to study the effects on tumor coverage without the adjustments of the isocenter. A summary of the isocenter adjustments for the first 30 patients is also presented. The geometric accuracy of the CT scanner is < or =1 mm. An isocenter deviation of > or =2 mm from the original plan can be detected. For the clinical example of a prostate patient, the average movement of the prostate gland was found to be approximately 3mm in the anterior-posterior (AP/PA) direction and 5 mm in the cephalic-caudal direction. Variations in the isocenter position may result in underdosage of the PTV if correction is not made for the change in the isocenter position. Our experience with the first 30 patients indicates that while the left-right adjustment of the isocenter is minimal, in the AP/PA direction, about 33% of treatments required an adjustment of 3-5 mm, and about 18% required a 5.1-mm to 10-mm adjustment. In the caudal-cephalic direction, about 26% required an adjustment of 3-5 mm, and 8% required a 5.1-mm to 10-mm adjustment. Retrofitting a CT scanner in an existing linear accelerator room requires careful planning and well-coordinated efforts from all personnel involved. Special QA procedures are needed to ensure the mechanical integrity and imaging accuracy of the CT scanner. A CT scan of the patient prior to irradiation provides valuable information on organ motion. Any deviations from treatment plan can be corrected before dose delivery. Significant deviation from the planning isocenter may occur due to daily variations in the rectal filling. The CT-Linac combination has significant implications for the treatment of prostate cancer.