Gantry Rotation Speed Research Articles

SU-E-T-196: Commissioning for Volumetric Modulated Radiation Therapy on Varian Clinac 21EX.

Recently the volumetric arc therapy (VMAT) technology such as RapidArc is widely distributed in Japan. These technologies are normally provided by the high spec linear accelerator such as Trilogy, Novalis Tx, Synergy, et al. The specific DICOM-file is generally used for commissioning of these technologies. On the other hand, we had to apply RapidArc into historic linear accelerator. This title expresses an experience how we performed the commissioning of RapidArc with the old linear accelerator. Two Varian's linear accelerators "Clinac 21EX" equipped with Millenium multi-leaf collimator and a Varian's treatment planning system "Eclipse ver.8.9" were used for this study. The commissioning for RapidArc was performed in energy 4,6,10,15 MV (Max-DR: 250, 600, 400, 600 MU/min). Commissioning procedure composed two categories: the general machine QA for DMLC-IMRT procedure and the specific RapidArc QA procedure. In RapidArc QA procedure, we modified DICOM-file to apply into the potential spec of Clinac 21EX optimally. The specific MLC-motion sequence and the gantry rotation speed were created by the dedicated programs (Shaper and DicomEdit, Varian) for RapidArc QA procedure. Each tolerance value was defied by the data from daily/monthly QA and the paper by Ling et al. As the results of the general machine QA procedure, the variance of radiation output during static/dynamic gantry rotation was less than 1%. The deference of fence tests during static/dynamic gantry rotation and RapidArc were less than 1 mm in each. However, the results of the RapidArc QA were worse than the latest machine (especially variable gantry speed) and it was careful to define tolerance level. The procedure of commissioning for RapidArc on historic linear accelerator was proposed. Several minor revisions for DICOM-file should be required for suitable commissioning and it may ensure the tolerance limit for gantry/MLC-leaf motion speeds.

SU‐E‐T‐376: Report on Use of a Methodology for Commissioning and Quality Assurance of a VMAT System

Introduction: Results of use of methodology for VMAT commissioning and quality assurance, utilizing both control point tests and dosimetric measurements are presented. Methods: A generalizable, phantom measurement approach is used to characterize the accuracy of the measurement system. Correction for angular response of the measurement system and inclusion of couch structures are used to characterize the full range gantry angles desirable for clinical plans. A dose based comprehensive daily QA measurement approach is defined. Results: Agreement in the static vs. VMAT picket fence control point test was better than 0.5 mm. Control point tests varying gantry rotation speed, leaf speed and dose rate, demonstrated agreement with predicted values better than 1%. Angular dependence of the MatriXX array, varied over a range of 0.94 –1.06, with respect to the calibration condition. Phantom measurements demonstrated central axis dose accuracy for un‐modulated four field box plans was =2.5 % vs. 1% with and without angular correction respectively with better results for VMAT (0.4%) vs. IMRT (1.6%) plans. Daily QA results demonstrated average agreement of all three chambers within 0.4% over 9 month period with no false positives at a 3% threshold. Conclusions: The methodology described is simple in design and characterizes both the inherit limitations of the measurement system as well at the dose based measurements that may be directly related to patient plan QA.

SU‐C‐BRA‐01: 4D Cone‐Beam CT Acquisition Using Respiratory Phase Predication Technique

Purpose: Respiration‐correlated CBCT reduces respiratory motion artifacts at the cost of increased view‐aliasing artifacts due to the gaps in projection data that are sorted to other phases. Respiratory‐gated acquisitions can cover the full scanning range but is inefficient (∼5 minutes per phase). In this study, we developed a 4D CBCT acquisition method using respiratory phase predication technique that can potentially acquire full projection data for each phase within a clinically acceptable (∼10 minutes or less) time. Methods: The proposed 4D CBCT continuously scans the target until sufficient number of projections is acquired for all phases. The respiratory phase predication technique consists of the following iterative steps: (1) training using respiratory signal for the first few (e.g., five) breathing cycles; (2) initial optimization of the gantry rotation speed; (3) adaptive control of the rotation speed and image acquisition during the scan (4) sorting the projection images into different phases. The method was validated using the 4D XCAT digital lung phantom with the motion driven by a sinusoid wave and a real patient respiration signal acquired with RPM system. CBCT images were reconstructed for each phase using the simultaneous arithmetic reconstruction technique and compared with the mixed‐phase reconstruction. Results: Sufficient projection images (an angular interval less than 1 degree) were acquired for all 10 phases after 10 gantry rotations. The acquisition time was estimated from 10–12 minutes assuming each gantry rotation took ∼1 minute. The CBCT images reconstructed from the mixed‐phase projections were blurred in comparison to those for individual phases. Conclusion: A respiratory prediction technique for 4D CBCT has been developed and validated with real patient breathing pattern. We plan to further reduce the number of gantry rotations and therefore the scanning time by improving the control of adaptive image acquisition and taking advantage of data redundancy as in 4D CT.

Least-error projection sorting to optimize retrospectively gated cardiac micro-CT of free-breathing mice

To develop and characterize a technique for optimizing image quality by eliminating streaking artifacts in retrospectively gated microcomputed tomography (micro-CT) images of mice caused by insufficient and irregular angular sampling. A least-error sorting technique was developed to minimize streak artifacts in retrospectively gated cardiac micro-CT images. To ensure complete filling of projection space, for each angular position, the projection acquired closest to the desired cardiac phase is used to reconstruct a volumetric image. An acrylic slanted-edge phantom undergoing cyclic motion was used to characterize the system's resolution. The phantom was scanned using a volumetric micro-CT scanner equipped with a flat-panel detector mounted on a slip-ring gantry. Projection images of the moving phantom were collected over a period of 60 s using a variety of acquisition protocols with the rotation period of the gantry ranging from 1 to 5 s. The modulation transfer function (MTF) of the reconstructed images was measured for many combinations of acquisition and reconstruction parameters. The use of the least-error technique was also demonstrated in vivo. The motion blurring introduced into the images at physiologically significant velocities of 6 cm∕s agreed well with predicted values; limiting resolution (frequency at 10% MTF) degraded from 2.5 to 1.0 mm(-1) for a velocity of 6 cm∕s and 5 s∕rotation gantry speed. Faster gantry rotation speeds led to improved temporal resolution but the scanner's data storage and transfer rates and field of view limitations made scanning at gantry speeds faster than 2 s∕rotation impractical. The least-error technique effectively eliminates streaking artifact caused by missing views and allows for optimization of image quality in retrospectively gated micro-CT.

Use of cone beam computed tomography in otolaryngologic treatments

Cone beam computed tomography (CBCT) allows us to evaluate 3-dimensional (3D) morphology of the maxillofacial skeleton and also used in dentomaxillofacial imaging to solve complex diagnostic and treatment planning problems such as craniofacial fractures, temporamandibular dysfunctions or sinus imaging. CBCT uses a rectangular or round 2D detector, which allows a single rotation of the gantry to generate a scan of the entire region of interest. Technological and application-specific factors such as development of compact, relatively low-cost, high-quality, large, flat-panel detector arrays; the availability of low-cost computers with processing power sufficient for cone beam image reconstruction; the fabrication of highly efficient radiograph tubes capable of multiple exposures necessary for cone beam scanning at prices lower than those currently used for fan beam CT; and limited volume scanning (e.g., head and neck) eliminating the need for subsecond gantry rotation speeds make this possible. The objective of this study is to review published evidence for CBCT having an important role in ORL treatments. We aimed to review all the available literature about the CBCT imagination in ORL treatments. Systematic literature search was performed using PubMed and Ovid. Additional literature was retrieved from reference lists in the articles. Systematic analysis of the literature from 1998 to 2010 was performed. A total of 40 abstracts were evaluated independently by two members of the project group, and 38 articles were included in the review.

Extraction of tumor motion trajectories using PICCS-4DCBCT: A validation study

As a counterpart of 4DCT in the treatment planning stage of radiotherapy treatment, 4D cone beam computed tomography (4DCBCT) method has been proposed to verify tumor motion trajectories before radiation therapy treatment delivery. Besides 4DCBCT acquisition using slower gantry rotation speed or multiple rotations, a new method using the prior image constrained compressed sensing (PICCS) image reconstruction method and the standard 1-min data acquisition were proposed. In this paper, the PICCS-4DCBCT method was combined with deformable registration to validate its capability in motion trajectory extraction using physical phantom data, simulated human subject data from 4DCT and in vivo human subject data. Two methods were used to validate PICCS-4DCBCT for the purpose of respiratory motion delineation. The standard 1-min gantry rotation Cone Beam CT acquisition was used for both methods. In the first method, 4DCBCT projection data of a physical motion phantom were acquired using an on-board CBCT acquisition system (Varian Medical Systems, Palo Alto, CA). Using a deformable registration method, the object motion trajectories were extracted from both FBP and PICCS reconstructed 4DCBCT images, and compared against the programmed motion trajectories. In the second method, using a clinical 4DCT dataset, Cone Beam CT projections were simulated by forward projection. Using a deformable registration method, the tumor motion trajectories were extracted from the reconstructed 4DCT and PICCS-4DCBCT images. The performance of PICCS-4DCBCT is assessed against the 4DCT ground truth. The breathing period was varied in the simulation to study its effect on motion extraction. For both validation methods, the root mean square error (RMSE) and the maximum of the errors (MaxE) were used to quantify the accuracy of the extracted motion trajectories. After the validation, a clinical dataset was used to demonstrate the motion delineation capability of PICCS-4DCBCT for human subjects. In both validation studies, the RMSEs of the extracted motion trajectories from PICCS-4DCBCT images are less than 0.7 mm, and their MaxEs are less than 1 mm, for all three directions. In comparison, FBP-4DCBCT shows considerably larger RMSEs in the physical phantom based validation. PICCS-4DCBCT also shows insensitivity to the breathing period in the 4DCT based validation. For the in vivo human subject study, high quality 3D motion trajectory of the tumor was obtained from PICCS-4DCBCT images and showed consistency with visual observation. These results demonstrate accurate delineation of tumor motion trajectory can be achieved using PICCS-4DCBCT and the standard 1-min data acquisition.

SU‐E‐J‐143: Respiratory‐Correlated Cone‐Beam CT for Potential Use in Measuring Lung Tumor Trajectory

Purpose: Respiratory‐correlated (4D) CT scans produce a set of images corresponding to different phases of the breathing cycle. In lung cancer treatment, a potential application of 4D cone‐beam CT (CBCT) scans taken on the treatment unit is to measure the tumor trajectory before each treatment, to verify whether the motion remains stable over the course of treatment. This presentation describes techniques we are developing for tumor trajectory measurement with 4D CBCT on our Varian linacs. Methods: A CBCT scan of a phantom placed on a periodically moving platform is taken on a Varian iX unit. The Varian RPM system is used to record the “breathing” phase vs time, and this information is used to sort the projection images into phase bins. For each phase bin an image is reconstructed using conventional Feldkamp‐Davis‐Kress filtered backprojection. The “tumor” region is contoured manually on one phase image, and a deformable registration algorithm is used to calculate the shift in points on the regionˈs surface from this phase to each other phase. The centroid of these surface points is used as a measure of the tumor position. Results: The deformable registration algorithm succeeds on test data consisting of clinical 4DCT scans of lung cancer patients taken on a CT simulator. Tumor motions of a few mm are measured. Preliminary 4D CBCT phantom images taken at the normal gantry rotation speed of 6 degrees/second show significant streak artifacts since the angular spacing between projections is large (>10 degrees); this low image quality is not suitable for the registration algorithm. Conclusions: Deformable image registration between phases is a practical method to measure lung tumor trajectory. In a 4D CBCT scan, reducing the gantry rotation speed is necessary to improve image quality: we plan to investigate this on a Varian TrueBeam unit.

SU‐E‐T‐236: Evaluation of Electronic Portal Imaging Device in the Quality Assurance of Volumetric Modulated Arc Therapy

Purpose: To evaluate the feasibility of engaging electronic portal imaging device (EPID) in the routine VMAT QA programs. Methods: A small radiation field of 1.6×1.6cm was used to irradiate an 8mm diameter ball bearing positioned precisely at machine isocenter and projected images at different gantry angles were recorded using EPID. A dynamic‐MLC based picket fence pattern was created both in (a) static mode and (b) VMAT mode. In either mode, a total of 40 control points were evenly divided into ten strips of 2cm in width. The ratio of radiation fluence between two neighboring strips was set at 2. Elekta linear accelerators operate with binned dose rates. The dose rate transition is instant and in strict synchronization with the gantry rotation speed and MLC movement. The entire delivery of the picket fence pattern was recorded in accumulation mode with Elekta iView. Image analysis was performed using an in‐house program written in MATLAB. Results: During a full gantry rotation, EPID panel was found to move up to 1.3 mm in the gantry‐target direction but less than 0.5mm in the lateral direction. The MLC speed and positioning were well controlled in both static mode and VMAT mode. At any time point during radiation delivery, the positions of individual leaves in each bank were within 0.5mm of the planned position in static mode. Comparing the picket fence pattern in VMAT mode with that in static mode, we noted minimal discrepancy (<0.5mm) between the corresponding leaf edges. In addition, we observed a negligible impact of gantry rotation speed and MLC travel speed on the accuracy of leaf positioning. Conclusions: Through this study, we demonstrate that EPID is indeed a feasible tool in routine VMAT QA program given that the motion of EPID panel per se is carefully considered.

SU‐E‐E‐16: Quality Assurance Check on Tomotherapy and Precalculation of Time

Purpose: In order to find treatment time prior to the calculation since tomotherapy takes very long time for calculation and checking quality assurance on input parameters like modulation factor, field size, pitch experimentally. Methods: Cheese phantom was scanned and the target, organ at risk(inner circle and outer ring) were delineated concentrically. The target was sandwiched between OAR(organ at risk) and calculated the gantry rotation time and treatment time for various input parameters modulation factor(M), pitch(P) and field size(fs). Experimentally verified all these input parameters from the gantry speed and treatment time. The relation of dR/dT(rotation time/treatment time) against pitch and dM/dR(mod factor/rotation) against pitch were verified. Moreover by evaluating constancy c in the formula D = c Dr R/(PM) helps to calculating treatment time prior to planning in order to know the time in advance. Results: The field size verification from the graph dR/dT against pitch was done and obtained experimentally 2.52, 1.15, 4.97 for the field sizes set at the beginning of the experiment 2.48, 1 and 5. dM/dR with respect to pitch also verified which shows smooth power fit. The constancy c value obtained when gantry rotation speed saturates at 15s at the pitch value 0.143. Further decrease in pitch only alters treatment time, not the gantry speed. When R and t are constant and c is linearly proportional to M and P by referring above formula. If we evaluate experimentally the second order partial differentiation of c with respect to modulation factor and pitch, is given by 0.0151. The c is given by when rotation period is more than15s is 0.008=M*P*0.0151 and c varies when speed attains 15 according M and P variation. Conclusions: The quality assurance of input parameters can be assured. Calculation sheet was developed for particular thickness by knowing c and will vary if gantry speed attains constant. I am working as Medical Physicist in Kuwait Cancer Control Center. I worked this project by taking interest and spent my money for everything, boarding and lodging. I am very much thankful to Dr. Anil Sharma who has helped me in this project. I can work any sort of Medical Physics project.

SU‐E‐T‐801: VMAT for the Treatment for Cervical Carcinoma and Para‐Aortic Nodes

Purpose: The PTV of cervical carcinoma and para‐aortic nodes is unique for its large width and length which pose a challenge to VMAT because of the MLC leaf length, speed and gantry rotation speed restrictions. This study establishes a field arrangement for VMAT and assesses the differences between VMAT and IMRT plans. Methods: Six patients were randomly selected for the study. The CTV included the areas of subclinical risk around the GTV and associated lymph node drainage sites. The PTV margin includes a 0.7cm expansion around the lymph nodes and 0.5cm margin elsewhere. Organs at risk included rectum, bladder, small bowel and kidneys. Six plans, each consisting of three arcs (collimator angle 90 degrees on one and 0 on others) and 9 field IMRT plans were generated with Eclipse treatment planning system and delivered with a 6MV beam and 120 leaf MLC. Planning goals for both VMAT and IMRT plans were: V100 of the PTV covered by 98% prescription dose with a maximum dose <112%, V15 < 33% for both kidneys, V40<60% for bladder and rectum and mean small bowel dose < 30Gy. Treatment time was measured from when the beam data was loaded into the treatment console to the end of beam delivery. Results: The average PTV was 1850cm3. The PTV coverage met the constraints on all plans. The average rectal V40 (VMAT: 56%; IMRT: 62%), bladder V40 (VMAT: 55%; IMRT: 59%) and mean small bowel dose (VMAT: 23Gy; IMRT: 24Gy) were comparable. The mean left and right kidneys V15 were substantially reduced with VMAT (VMAT: 15%; IMRT: 28.1%) and (VMAT: 14.1%; IMRT: 29.5%). VMAT reduced the average MU from 2581 to 662 resulting mean delivery time of 791s to 292s. Conclusions: VMAT results substantial kidney sparing for the same coverage while reducing treatment time and monitor units.

The role of multidetector CT in cardiovascular diagnosis: the CT coronary angiography. Current applications

Currently, computed tomographic (CT) imaging of the heart is mainly used for the quantification of coronary artery calcification as an indirect measure of coronary plaque burden and, less frequently, for minimally invasive coronary angiography. Recently, so-called multislice spiral computed tomographic (MSCT) scanners with gantry rotation speeds fast enough to produce diagnostic images of the heart under certain conditions have become widely available. As a consequence, cardiac CT imaging, most often performed for the purpose of calcium scoring, is increasingly applied. In this paper we provide a discussion of technical issues, applications, advantages, and limitations, after which we offer recommendations for current and future uses. To accomplish this, we conducted a comprehensive review of a study (Stacul et al. 2009) that was done to analyse the costs of 64-slice computed tomography (CT) coronary angiography and conventional coronary angiography and determine the cost-effectiveness of the two modalities.

小児ＣＴ検査における ａｄａｐｔｉｖｅ ｄｏｓｅ ｓｈｉｅｌｄの有用性について

The spiral scan with a wide detector row such as the 64-detector row computed tomography (CT) system may increase radiation exposure for infants because the irradiation range is wider than the planned range. The adaptive dose shield (ADS) prevents radiation exposure greater than the planned range. We examined the usefulness of the protection effect of the ADS for the infant inner ear CT. To confirm the protection effect of the ADS, we scanned X-ray films by using the 64-detector row CT system and measured the difference of the planned range and the irradiation range. The result of that is that when the planned range was small, the protection effect for the scan ending side was inferior to the scan starting side. And also, when the gantry rotation speed and pitch factor (PF) were high values, the protection effect was inferior to a low gantry rotation speed and low PF. There was a combination of gantry rotation speed and PF at which the protection effect decreases. Due to changes of the scanning direction and PF for the infant inner ear, the crystalline lens radiation exposure dose decreased from 11.89 mGy to 4.37 mGy. In conclusion, the ADS can reduce the radiation exposure dose of an adjacent organ. Therefore, it was thought that the ADS was a useful radiation exposure reduction function for infants in the 64-detector row CT system.

Generating arbitrary one-dimensional dose profiles using rotational therapy

Conformal radiation therapy can be delivered using several methods: intensity-modulated radiotherapy (IMRT) at fixed gantry angles, through the continuous gantry rotation of linac (rotational arc therapy), or by a dedicated treatment unit such as tomotherapy. The recently developed volumetric modulated arc therapy (VMAT), a form of rotational arc therapy, has attracted lots of attention from investigators to explore its capability of generating highly conformal dose to the target. The main advanced features of VMAT are the variable dose rate and gantry rotation speed. In this paper, we present a theoretical framework of generating arbitrary one-dimensional dose profiles using rotational arc therapy to further explore the new degree of freedom of the VMAT technique. This framework was applied to design a novel technique for total body irradiation (TBI) treatment, where the desired dose distribution can be simplified by a one-dimensional profile. The technique was validated using simulations and experimental measurements. The preliminary results demonstrated that the new TBI technique using either dynamic MLC only, variable dose rate only, or a combination of dynamic MLC and variable dose rate can achieve arbitrary dose distribution in one dimension, such as uniform dose to target and lower dose to critical organ. This technique does not require the use of customized compensators, nor large treatment rooms as in the conventional extended SSD technique.

A Study on the Dose Scalability of Volumetric Modulated Arc Therapy (VMAT) Fields

Volumetric modulated arc therapy (VMAT) is often a preferred treatment technique due to its capability in achieving conformal dose distribution and efficiency in treatment delivery. In the planning process, it is a common necessity to scale the machine monitor units (MUs) of an arc field from what was originally planned. However, it is not certain if the dose will scale accordingly in real delivery environment, as the MLC speed, gantry rotation speed, and beam dose rate may not follow a straightforward linear pattern.

Temporal resolution improvement in cardiac CT using PICCS (TRI‐PICCS): Performance studies

The recently proposed prior image constrained compressed sensing (PICCS) method has been applied in cardiac MDCT to improve the temporal resolution by approximately a factor of 2, by using projection data acquired from half of the standard short-scan angular range to reconstruct images with improved temporal resolution. The method was referred to as temporal resolution improvement using PICCS (TRI-PICCS). The primary purpose of this article is to study (1) the relationship between the performance of the TRI-PICCS algorithm and the angular range of projection data used in image reconstruction; (2) the relationship between the performance of the TRI-PICCS algorithm and the motion orientations and motion patterns of moving objects; and (3) the relationship between the performance of the TRI-PICCS algorithm and various heart rates. A hybrid phantom consisting of realistic cardiac anatomy and eight moving objects with known motion profiles to simulate coronary arteries was constructed by superimposing the analytical projection data of eight simulated moving vessels to the in vivo projection data from a cardiac MDCT scan. The motion profiles of the moving objects may independently change orientations, period, and amplitude. A prior image was reconstructed using a short-scan filtered backprojection method from a gated short-scan data set for each given motion profile. The TRI-PICCS method was applied to improve temporal resolution for each configuration of given motion profiles of moving objects and given active angular range specified by the target temporal resolution. To quantitatively study the performance, figures of merit were introduced to quantify signal intensity deficit, image distortion, and residual motion artifacts, respectively. The performance of the TRI-PICCS method is the same when the projection data are taken from 100 degrees to 120 degrees. The performance of the TRI-PICCS method is independent of location and motion orientations. The performance of the TRI-PICCS method does not significantly degrade for heart rates up to 100 bpm with a gantry rotation speed of 350 ms per rotation. The TRI-PICCS method can be used to systematically improve temporal resolution for MDCT cardiac imaging by a factor of 2-2.3 and the performance of the TRI-PICCS method is insensitive to motion locations and motion orientations. The TRI-PICCS method enables a single-source MDCT scanner with 350 ms or faster gantry speed to scan patients with heart rates as high as 100 bpm.

A method for optimizing LINAC treatment geometry for volumetric modulated arc therapy of multiple brain metastases

Volumetric modulated arc therapy (VMAT) is a rotational delivery technique in which MLC shapes, dose rate, and gantry rotation speed are optimized to produce conformal dose distributions. The aim of this work is to develop a beam projection method for deriving the optimal table and collimator angles for multilesion treatment planning. The method consists of four steps. The first step is to define the vector of beam-eye-view (BEV)-Y-axis in the treatment planning CT coordinates. The second step is to project each target onto the BEV-Y-axis vector. In the third step, the best table and collimator angle are found with a brute-force optimization technique that minimizes MLC leaf sharing between lesions. The fourth step is to generate an optimized VMAT plan with appropriate table/collimator angles and evaluate the plan quality. The authors tested the method on three example cases with targets of various locations in the brain and sizes ranging from 1.18 to 17.86 cm(3). Applying the optimized geometric parameter to generate VMAT plan, a reduction of the 12 Gy volume was more than 6.1% for all cases; the plan homogeneity (D2%-D95%) was improved from 5.88 +/- 1.21 to 5.21 +/- 0.93 Gy vs a VMAT plan with the manufacturer recommended table and collimator angles. The authors conclude that the use of the projection method minimizes the sharing of MLC leaves between lesions and improves the plan quality for multilesion VMAT delivery.

MO-SAM-BRB-01: Principles and Clinical Application of Volumetric Modulated Arc Therapy

Intensity modulated arc therapy (IMAT), first proposed by C. Yu in 1995, is a multiple arc technique in which the aperture defined by the multileaf collimator(MLC) of a conventional accelerator changes dynamically while gantry rotation speed and dose rate remain constant. IMAT was shown to deliver highly conformal dose distributions in times comparable to other conformal techniques; however, widespread adoption of IMAT was limited by the lack of commercial systems. Recently, volumetric modulated arc therapy (VMAT), in which gantry speed and dose rate are modulated in addition to MLC aperture, has been demonstrated and become commercially available. Commercial VMAT delivery systems are available from Elekta AB (Stockholm, Sweden) and Varian Medical Systems, Inc. (Palo Alto, CA). Although the dose distributions produced by VMAT are similar to those produced by fixed gantry intensity modulated radiation therapy(IMRT), VMAT is a distinct delivery technique with unique clinical considerations. We will describe the clinical implementation of VMAT, including commissioning, treatment planning, and patient and machine specific quality assurance. We will also review the elements of starting a VMAT program. Learning Objectives: 1. Describe volumetric modulated arc therapy (VMAT). 2. Survey VMAT treatment planning systems and planning techniques. 3. Describe commissioning of a VMAT delivery system. 4. Review patient‐ and machine‐specific VMAT quality assurance. 5. Summarize the key elements of starting a VMAT program. Conflict of Interest (only if applicable): Richard Popple and Cedric Yu each have a sponsored research agreement with Varian. David Shepard has a sponsored research agreement with Elekta.

SU‐GG‐J‐76: Validation of a Proposed Optimal Slow Gantry Rotation Technique for On‐Board Four‐Dimensional Imaging Using Lung Patient Data

Purpose: Substantial efforts have been made in recent years to develop on‐board four‐dimensional (4D) imaging techniques using gantry mounted cone‐beam imaging devices. This study validates a slow gantry rotation 4D imaging protocol using lung patient data. Methods & Materials: The proposed 4D imaging protocol determines optimal gantry rotation speeds and frame rates based on patients' respiratory cycles and desired phase windows to optimize projection distributions in phase bins by reducing numbers of tightly spaced projections and avoiding large gaps between projections. The effect of frame rate on projection distributions was tested using data from three lung patients. Projections were acquired using an on‐board kV imager (OBI)® (Varian Medical Systems, Palo Alto, CA). Gantry rotation speeds and frame rates were 0.59–0.73 °/s and 4.9–7.3 fps. Subsets of data were extracted to represent frame rates that were lower (50% and 75% of the predicted optimal) equal to and higher than (125%, 150%) predicted optimal frame rates. The protocol predicts that large gaps between projections will occur in the low (50 and 75%) frame rate data sets and excessive tightly spaced projections will occur in high (125 and 150%) frame rate data sets. Results: As predicted, large gaps occurred in the 50 and 75% frame rate sets. This effect was worse for the 50% data sets than for the 75%. Up to 23.8% of the intervals between projections in the 50% data sets exhibited large gaps, while up to 1.2% presented in the 75% frame rate sets. Percentages of tightly spaced projections averaged 6, 39 and 61% for 50, 100 and 150% frame rate data sets. Conclusion: A slow gantry rotation protocol for determining optimal acquisition parameters was validated using lung patient data and may serve as a foundation for further on‐board 4D imaging investigations. Conflict of Interest: Research sponsored by Varian Corporation.

Patient‐specific motion artifacts in 4DCT

Four-dimensional computed tomography (4DCT) has enhanced images of the thorax and upper abdomen during respiration, but intraphase residual motion artifacts will persist in cine-mode scanning. In this study, the source and magnitude of projection artifacts due to intraphase target motion is investigated. A theoretical model of geometric uncertainty due to partial projection artifacts in cine-mode 4DCT was derived based on ideal periodic motion. Predicted artifacts were compared to measured errors with a rigid lung phantom attached to a programmable motion platform. Ideal periodic motion and actual patient breathing patterns were used as input for phantom motion. Reconstructed target dimensions were measured along the direction of motion and compared to the actual, known dimensions. Artifacts due to intraphase residual motion in cine-mode 4DCT range from a few mm up to a few cm on a given scanner, and can be predicted based on target motion and CT gantry rotation time. Errors in ITV and GTV dimensions were accurately characterized by the theoretical uncertainty at all phases when sinusoidal motion was considered, and in 96% of 300 measurements when patient breathing patterns were used as motion input. When peak-to-peak motion of 1.5 cm is combined with a breathing period of 4 s and gantry rotation time of 1 s, errors due to partial projection artifacts can be greater than 1 cm near midventilation and are a few mm in the inhale and exhale phases. Incorporation of such uncertainty into margin design should be considered in addition to other uncertainties. Artifacts due to intraphase residual motion exist in 4DCT, even for ideal breathing motions (e.g., sine waves). It was determined that these motion artifacts depend on patient-specific tumor motion and CT gantry rotation speed. Thus, if the patient-specific motion parameters are known (i.e., amplitude and period), a patient-specific margin can and should be designed to compensate for this uncertainty.

Slow gantry rotation acquisition technique for on‐board four‐dimensional digital tomosynthesis

Four-dimensional cone-beam computed tomography (4D CBCT) has been investigated for motion imaging in the radiotherapy treatment room. The drawbacks of 4D CBCT are long scan times and high imaging doses. The aims of this study were to develop and investigate a slow gantry rotation acquisition protocol for four-dimensional digital tomosynthesis (4D DTS) as a faster, lower dose alternative to 4D CBCT. This technique was implemented using an On-Board Imager kV imaging system (Varian Medical Systems, Palo Alto, CA) mounted on the gantry of a linear accelerator. The general procedure for 4D DTS imaging using slow gantry rotation acquisition consists of the following steps: (1) acquire projections over a limited gantry rotation angle in a single motion with constant frame rate and gantry rotation speed; (2) generate a respiratory signal and temporally match projection images with appropriate points from the respiratory signal; (3) use the respiratory signal to assign phases to each of the projection images; (4) sort projection images into phase bins; and (5) reconstruct phase images. Phantom studies were conducted to validate theoretically derived relationships between acquisition and respiratory parameters. Optimization of acquisition parameters was then conducted by simulating lung scans using patient data. Lung tumors with approximate volumes ranging from 0.12 to 1.53 cm3 were studied. A protocol for slow gantry rotation 4D DTS was presented. Equations were derived to express relationships between acquisition parameters (frame rate, phase window, and angular intervals between projections), respiratory cycle durations, and resulting acquisition times and numbers of projections. The phantom studies validated the relationships, and the patient studies resulted in determinations of appropriate acquisition parameters. The phase window must be set according to clinical goals. For 10% phase windows, we found that appropriate frame rates ranging from 2 to 5 frames/s, gantry rotation speeds ranging from 0.44 to 1.03 degrees/s, and aiming for an approximate maximum angular interval of 3.4 degrees between projections in phase bins were appropriate for dose, scan time, and tumor visibility optimization. Adequate tumor visibility was achieved for coronal 4D DTS images of all three lung tumors with acquisition times ranging from 0.45 to 2.12 min. vs. 1.84 to 4.24 min for 4D CBCT. 4D DTS imaging doses ranged from 0.12 to 0.72 times the dose of a standard CBCT scan vs 0.48 to 1.44 times the dose of a standard CBCT scan for 4D CBCT. A slow gantry rotation acquisition technique for 4D DTS was developed and investigated. Study results indicated that 4D DTS is a feasible technique for imaging lung tumor motion in the treatment room and requires shorter acquisition times and less imaging dose than 4D CBCT for larger tumors that do not require large scan angles for sagittal views and for situations where only coronal views are needed to meet clinical needs.