Couch Speed Research Articles

Development of an external system for monitoring the couch speed in radiotherapy using continuous bed movement.

Total body irradiation before bone marrow transplantation for hematological malignancies using Radixact, a high-precision radiotherapy machine, can potentially reduce side effects and the risk of secondary malignancies. However, stable control of couch speed is critical, and direct assessment methods outlined in quality assurance guidelines are lacking. This study aims to develop a real-time couch speed verification system for the Radixact. The developed system used a linear encoder to measure couch speed directly. Accuracy was verified via a linear stage, comparing measurements with a laser distance sensor. After placing a phantom simulating the human body on the Radixact couch, the couch speed was verified using predefined speed plans. Operating the linear stage at 0.1, 0.5, and 1.0mm/s revealed that the maximum position error of the developed verification system compared to the laser distance sensor was nearly equivalent to the distance resolution of the system (0.05mm/pulse), with negligible average speed error. When the Radixact couch operated at 0.1, 0.5, and 1.0mm/s, the values obtained by the verification system agreed with the theoretical values within the sampling period (0.01s) and distance resolution (0.05mm). The verification system developed provides real-time monitoring of the speed of the Radixact table, ensuring treatment effectiveness and patient safety. It would guarantee the couch speed's soundness and contribute to the "visualization" of safety.

Considerations for CT Number Calibration and Imaging Parameter Selection for Adaptive Planning on a Novel On-Board Helical kVCT System

<h3>Purpose/Objective(s)</h3> To evaluate the impact of CT number calibration and imaging parameters on adaptive dose calculation accuracy relative to the CT planning process for an on-board helical kVCT imaging system used in helical tomotherapy treatments. <h3>Materials/Methods</h3> Helical kVCT calibrations were performed with thorax protocols at 120 kVp for <i>Fine</i> and <i>Normal</i> imaging modes using a standard electron density phantom. For this system, <i>Mode</i> determines the longitudinal beam width at isocenter, couch speed, and views per rotation, which impact the scatter signal, scan time, and patient dose, respectively, and offers <i>Fine</i> (50 mm beam width, long relative scan time, high relative mAs/rotation), <i>Normal</i> (100 mm, moderate scan time, moderate mAs/rotation), and <i>Coarse</i> (∼140 mm, short scan time, low mAs/rotation) mode options. An SBRT treatment plan was simulated for CT simulation scans of an anthropomorphic lung phantom and transferred to on-board helical kVCT scans with varying imaging and calibration protocols, and clinically relevant DVH metrics and dose-difference maps were used to compare the calculated dose distributions. <h3>Results</h3> Use of <i>Fine</i> mode for helical kVCT calibration resulted in smaller dose-volume deviations relative to the planning CT, with the magnitude of these deviations dependent on the mode used to acquire the scan of the lung phantom. For scans acquired under <i>Fine</i> mode, use of <i>Fine</i> mode calibration showed average differences in target volume DVH metrics of ± 1.0% (-1.7% max difference) compared to ± 2.5% (-3.6% max difference) with application of <i>Normal</i> mode calibration for the same images. Likewise, for phantom scans acquired under <i>Normal</i> mode, these differences were ± 1.2% (+ 2.1%) for <i>Fine</i> calibration compared to ± 1.4% (-2.1%) for <i>Normal</i> calibration, while for phantom scans acquired under <i>Coarse</i> mode, differences were ± 2.0% (-4.1%) and ± 3.3% (-6.2%), respectively. Dose-difference maps confirmed the greatest deviations in the calculated dose distributions occurred in the high-dose, target volume region, though some differences were also observed at the tissue-air interface of the chest wall. Visual differences relative to the planning CT were lessened with use of <i>Fine</i> mode acquisitions of both the calibration and lung phantoms. <h3>Conclusion</h3> Use of <i>Fine</i> mode for helical kVCT number calibration and image acquisition resulted in dose calculations most consistent with those calculated on diagnostic-quality CT images. If excess patient dose from additional imaging is a concern, <i>Normal</i> or <i>Coarse</i> mode image acquisitions with application of <i>Fine</i> mode calibration provided dose calculation accuracies within a reasonable tolerance as well. Also, while <i>Fine</i> mode increases scan times relative to other modes, this additional time is still significantly lessened compared to on-board CBCT or MVCT systems currently used for CT-based online adaptive planning.

A comprehensive method for calculating total body irradiation

Toprovide meansfor calculating the dose received by various tissues of the patient, calculate lung shield, and verify received dose using a phantom as a tool for quality assurance for a planned Total Body Irradiation (TBI) procedure in radiotherapy. Using Microsoft Visual Basic, MATLAB, and Python, a program for Total Body Irradiation Calculation in Radiotherapy (TBICR) is constructed. It uses patient translation and beam zone method for total body irradiation calculations to compute the proper dose received by the patient and determine the lung shield thickness. There are three main user-friendly interfaces in the application. The first one allows the user to upload the TBI topography and estimate the distances needed for TBI calculations. The second one enables the user to count the number of beam zones needed for each point and estimate the effective area (Aeff) for each level. The third interface estimates the velocity required to deliver the relative dose depending on patient separation, Monitor Units (MU), couch speed and travel distance. It allows the user to compute the required lung shield thickness, read any patient's CT DICOM file and acquire dose in any distinct location using machine learning model to predict the dose. The TBICR software has been successfully validated by reproducing all of the manual calculations in an exact and timely manner. TBICR generated more accurate results and confirmed the absorbed dose to patient through measurements on Anderson phantom. A computer program for the calculation of total body irradiation (TBI) is described in full. The dose received at each point on the patient, the calculation of lung shield and the determination of the velocity and time required for the couch movement are all made possible using the software. The ease of use, precision, data storage and printing are some important features of the present software.

Designing Robust and Automated Treatment Plans Utilizing Dynamic Couch Motion With Automated Treatment Planning: Optimizing Couch Motion for Precise Delivery ― Part A

<h3>Purpose/Objective(s)</h3> This study finds a relationship among couch speed, dose rate, and MLC, allowing gradual start and stop positioning of the couch for treatments utilizing dynamic couch-based dose delivery at static gantry positions. <h3>Materials/Methods</h3> We introduce a gantry static couch motion (GsCM) based technique, which delivers the dose using a dynamic couch and static gantry positions. It is imperative to study couch motion behavior for techniques like GsCM, where the patient is a moving target throughout the dose delivery. Sudden changes in couch speed can result in a shift in position and discomfort for the patient. Therefore, it is necessary to implement a smooth start, a constant speed in the middle, and deceleration at the couch's stopping positioning. We studied the relationship among the following parameters: dose rate, MLC speed, and couch motion. To fulfill this, we simulated a single treatment arc manually in the treatment planning system that includes multiple couch positions at static gantry positions. All the fields were merged into a single deliverable arc while exporting into the Extensible Markup Language (XML) format using the TPS scripting API. Developer mode on a Linac performed the treatment delivery. The second part of this study looked into the various ways to control the couch motion. Methods tried included a fixed couch speed versus controlling the couch speed by limiting the dose rate. Trajectory log files helped analyze the couch positional error (RMSE). <h3>Results</h3> We found couch speed optimization by limiting the dose rate is better than the MLC speed or couch speed. A slow ramping up and down of the speed with a constant speed in the middle of an arc field is achievable. Linac log file analysis showed minimum couch position error (RMSE: median 0.035⁰, range 0.02-0.0.045⁰) for 1° couch speed as compared to system defined couch speed (RMSE: median 0.07⁰, range 0.05-0.12⁰), and (RMSE: median 0.07⁰, range 0.06-0.09⁰) for 2° couch speed respectively. Even though 1° couch speed provided a minimum couch RMSE error but at the expense of a longer treatment time which is not suitable for keeping the patient longer on the treatment table. For the dose rate limiting option, couch positioning error at the start and stop location (RMSE: median 0.008⁰, range 0.006-0.02⁰) showed a gradual and accurate ramping up and down of the couch, and at the middle of the arc remained the same as noticed for the system defined couch speed (RMSE: median 0.09⁰, range 0.065-0.12⁰). <h3>Conclusion</h3> This feasibility study serves as an end-to-end test and is probably the first of its kind that thoroughly validates various ways to control the couch speed and its integration with patient motion during the dose delivery. Treatments utilizing dynamic couch-based dose delivery must include patient immobilization. Our study takes patient motion into account and provides gradual start and stop motions of the couch, which helps minimize the patient motion for treatments utilizing dynamic couch motion.

Patient motion tracking for non-isocentric and non-coplanar treatments via fixed frame-of-reference 3D camera.

PurposeAs C‐arm linac radiation therapy evolves toward faster, more efficient delivery, and more conformal dosimetry, treatments with increasingly complex couch motions are emerging. Monitoring the patient motion independently of the couch motion during non‐coplanar, non‐isocentric, or dynamic couch treatments is a key bottleneck to their clinical implementation. The goal of this study is to develop a prototype real‐time monitoring system for unconventional beam trajectories to ensure a safe and accurate treatment delivery.MethodsAn in‐house algorithm was developed for tracking using a couch‐mounted three‐dimensional (3D) depth camera. The accuracy of patient motion detection on the couch was tested on a 3D printed phantom created from the body surface contour exported from the treatment planning system. The technique was evaluated against a commercial optical surface monitoring system with known phantom displacements of 3, 5, and 7 mm in lateral, longitudinal, and vertical directions by placing a head phantom on a dynamic platform on the treatment couch. The stability of the monitoring system was evaluated during dynamic couch trajectories, at speeds between 10.6 and 65 cm/min.ResultsThe proposed monitoring system agreed with the ceiling mounted optical surface monitoring system in longitudinal, lateral, and vertical directions within 0.5 mm. The uncertainty caused by couch vibration increased with couch speed but remained sub‐millimeter for speeds up to 32 cm/min. For couch speeds of 10.6, 32.2, and 65 cm/min, the uncertainty ranges were 0.27– 0.73 mm, 0.15–0.87 mm, and 0.28–1.29 mm, respectively.ConclusionBy mounting a 3D camera in the same frame‐of‐reference as the patient and eliminating dead spots, this proof of concept demonstrates real‐time patient monitoring during couch motion. For treatments with non‐coplanar beams, multiple isocenters, or dynamic couch motion, this provides additional safety without additional radiation dose and avoids some of the complexity and limitations of room mounted systems.

Fast, Low-Dose Megavoltage-Topogram Localization on TomoTherapy: Initial Clinical Experience With Mesothelioma Patients

PurposeThis study aimed to evaluate the potential of megavoltage-topogram (MV-topogram)–based alignment as an alternative to megavoltage computed tomography (MVCT) in reducing setup time and imaging dose for patients with malignant pleural mesothelioma who are receiving TomoTherapy. Methods and MaterialsTwelve patients were enrolled in an ongoing institutional review board approved clinical trial at our institute. Patients were set up with a clinical protocol using red lasers. Anteroposterior (AP) and lateral (LAT) MV-topograms were acquired using gantry angles of 0°/90° with a 1 mm collimator opening, all multileaf collimator leaves open, a couch speed of 4 cm/s, and a 12.5-second scanning time. Routine MVCT scans were performed immediately afterward. The MV-topograms were reconstructed and enhanced using contrast-limited adaptive histogram equalization. Anteroposterior and LAT kilovoltage digital reconstructed topogram images were reconstructed based on TomoTherapy geometry from computed tomography simulation scans. Registrations between MV-topograms and kilovoltage-digital reconstructed topogram images were performed manually, and patients’ daily shifts were recorded. Results were compared against the corresponding daily MVCT shifts. MV-topogram and MVCT doses were measured and recorded using an ion chamber on a cheese phantom with depths between 1 and 14 cm, as well as the times required to acquire the 2 image modalities. ResultsThe mean and standard deviation of shift discrepancies between MV-topogram and MVCT were 0.74 ± 2.08, –0.09 ± 4.46, and 0.45 ± 3.57 mm in the LAT, longitudinal, and vertical directions, respectively. The MVCT imaging doses measured were 14.74 to 26.92 times higher than the MV-topogram doses, depending on depth. On average, MV-topograms with a mean scan length of 50 cm achieved a 5-fold image acquisition time savings over MVCT, with a mean scan length of 38 cm. ConclusionsMV-topograms has the potential to provide alignment performance equivalent to that of MVCT for patients with mesothelioma, with a significant reduction in imaging dose and acquisition time.

Quality assurance of non-coplanar, volumetric-modulated arc therapy employing a C-arm linear accelerator, featuring continuous patient couch rotation

PurposeTo perform quality assurance of non-coplanar, volumetric-modulated arc therapy featuring continuous couch rotation (CCR-VMAT) using a C-arm linear accelerator.MethodsWe planned and delivered CCR-VMAT using the TrueBeam Developer Mode. Treatment plans were created for both a C-shaped phantom and five prostate cancer patients using seven CCR trajectories that lacked collisions; we used RayStation software (ver. 4.7) to this end. Subsequently, verification plans were generated. The mean absolute error (MAE) between the center of an MV-imaged steel ball and the radiation field was calculated using the Winston–Lutz test. The MAEs between planned and actual irradiation values were also calculated from trajectory logs. In addition, correlation coefficients (r values) among the MAEs of gantry angle, couch angle, and multi-leaf collimator (MLC) position, and mechanical parameters including gantry speed, couch speed, MLC speed, and beam output, were estimated. The dosimetric accuracies of planned and measured values were also assessed using ArcCHECK.ResultsThe MAEs ±2 standard deviations as revealed by the Winston–Lutz test for all trajectories were 0.3 ± 0.3 mm in two dimensions. The MAEs of the gantry, couch, and MLC positions calculated from all trajectory logs were within 0.04°, 0.08°, and 0.02 mm, respectively. Deviations in the couch angle (r = 0.98, p < 0.05) and MLC position (r = 0.86, p < 0.05) increased significantly with speed. The MAE of the beam output error was less than 0.01 MU. The mean gamma passing rate ± 2 SD (range) of the 3%/3 mm, 3%/1 mm, and 5%/1 mm was 98.1 ± 1.9% (95.7–99.6%), 87.2 ± 2.8% (80.2–96.7%), and 96.3 ± 2.8% (93.9–99.6%), respectively.ConclusionsCCR-VMAT delivered via the TrueBeam Developer Mode was associated with high-level geometric and mechanical accuracy, thus affording to high dosimetric accuracy. The CCR-VMAT performance was stable regardless of the trajectory chosen.

Intensity-Modulated Radiation Therapy for Multiple Targets with Tomotherapy Using Multiple Sets of Static Ports From Different Angles - Pseudo Dynamic-Jaw and Dynamic-Couch Technique

Helical tomotherapy is suitable for irradiating multiple targets at once. However, treatment time becomes long when the targets are separated in craniocaudal direction, since the couch moves slowly at a uniform pace. The dynamic-jaw and dynamic-couch (DJDC) mode offers dynamic jaw alignment throughout the treatment and variable couch speeds allowing faster movement in unirradiated parts of the body, but it is not yet available for clinical use. To solve the problem of long treatment time, we invented a new technique using multiple static-port tomotherapy with the dynamic-jaw mode and named it pseudo-DJDC (pDJDC) technique. We compared the pDJDC plans and helical tomotherapy plans using the dynamic-jaw mode (HDJ) for multiple targets. In pDJDC plans, we use a beam set with 4-6 ports to the targets at the same level in the craniocaudal direction and employ another beam set for other targets using different port angles (9-12 angles in total). The couch moves fast during the intervals between the different beam sets. In 7 patients, 2 plans using the pDJDC and HDJ techniques were compared. Four of the 7 patients had multiple (n=2-6) distant metastases including those to the rib and lymph node and peritoneal dissemination. Two patients had esophageal cancer with 3 targets including a primary tumor and regional metastases. The other patient had 2 pleural disseminations from a thymic tumor. For multiple distant metastases, 25-40 Gy in 2- to 5-Gy fractions were prescribed for the planning target volumes at D50%. For esophageal cancer, 20-22 Gy in 2-Gy fractions were prescribed as boost plans. For thymic tumor, 60 Gy in 7.5-Gy fractions were prescribed. Conformity index, uniformity index (D5%/D95%), dose distribution in the lung, and beam-on time were evaluated using Wilcoxon signed-rank test. The median conformity index of all 7 patients was 3.0 for the pDJDC plans and 2.4 for the HDJ plans (p = .031). The median uniformity index of the planning target volume (n=25) for the 2 plans was 1.048 and 1.057, respectively (p = .10). For 5 patients with thoracic targets, the median of the mean lung dose was 2.6 Gy and 2.4 Gy, respectively (p = .63). The median V5Gy and V20Gy of the lung in the 5 patients were 11.8% and 8.5% (p = .63) and 1.6% and 2.1% (p = .32), respectively. The pDJDC plans reduced the beam-on time by 52% compared to the HDJ plans (median; 421 and 883 seconds, respectively, p = .016) The pDJDC technique enables treatment of multiple targets in half time compared to the HDJ technique. The pDJDC plans were comparable to the HDJ plans in dose distribution except the conformity index.

Application of semiconductor matrix in synchronization test in helical tomotherapy

Objective To investigate how to use the ArcCHECK to complete synchronization test in helical tomotherapy (HT). Methods According to the synchronization test suggested by AAPM TG148, three aspects should be tested.(1) Gantry angle consistency: ArcCHECK device in the long axis was perpendicular to the rotation plane using virtual isocenter positioning. A delivery sequence was defined with a slice width of 2.5 cm, a pitch of 0.1, and a minimum of 40 rotations. The control sinogram was set to open the middle two leaves (No. 32 and 33) at projections centered at 0°, 120°, and 240°.(2) Couch speed uniformity: The ArcCHECK was fixed on the treatment couch by positioning the long axis in the movement direction of couch. An irradiation was done with a static gantry in the 0° position, the collimation set to 1 cm, and all MLC leaves open. A couch travel distance of 20 cm was programmed using a clinically common couch speed (0.3-0.5 mm/s for 2.5 cm treatment slice width) in the course of irradiation.(3) Synchronization of couch translation and gantry rotation: In this test, a rotational irradiation was used with the nominal 1.0 cm beam and a pitch of 1 for 13 rotations. The control sonogram was set to open all the leaves for a half rotation on the second, seventh, and twelfth rotations. The initial collected data were used as the baseline data of parameters. The subsequent measurements were compared with the baseline data using the point to point absolute dose pass rates of DD (dose difference, 1%). Results The gantry angle consistency could check the correct initial angles at the beginning of the treatment and the ability to reproduce the projection every rotation. There were six parallel and straight high-dose regions in ArcCHECK software. In the couch speed uniformity test, the relative dose in the long axis in the high-dose regions was changed less than 2%. In the synchronization test of couch translation and gantry rotation, three parallel high-dose regions with the same separation spacing were present in the ArcCHECK software. The synchronization tests were done twenty-eight times in the second, seventh, and twelfth irradiation rotations and the average pass rates were 93.2%±1.5%, 93.7%±1.1%, and 93.5%±1.3%, respectively. Conclusions The ArcCHECK device is a suitable alternative method for performing the synchronization test in HT due to its lower workload and costs compared with the film. Key words: Helical tomotherapy; Synchronicity test; Semiconductor matrix

Sensitivity evaluation of two commercial dosimeters in detecting Helical TomoTherapy treatment delivery errors

PurposeTo assess the sensitivity of two commercial dosimetry systems in detecting Helical TomoTherapy (HT) delivery errors. MethodTwo commercial dosimeters i) MatriXXEvolution and ii) ArcCHECK® were considered. Ten retrospective nasopharynx HT patients were analysed. For each patient, error plans were created by independently introducing systematic offsets in: a) Jaw width error ±1, ±1.5 and ±2mm, b) Couch speed error ±2%, ±2.5, ±3% and ±4%, and c) MLC Leaf Open Time (MLCLOT) errors (3 separate MLC errors: either leaf 32 open or leaf 42 remains open during delivery, and 4% random reductions in MLCLOT). All error plans, along with the no error plan for each patient, were measured using both dosimeters in the same session. Gamma evaluation (3%/3mm) was applied to quantitatively compare the measured dose from each dosimeter to the treatment planning system. The error sensitivity was quantified as the rate of decrease in gamma pass rate. ResultsThe gamma pass rate decreases with increase in error magnitude for both dosimeters. ArcCHECK was insensitive for couch speed error up to 2.5% and jaw width error up to −1.5mm while MatriXXEvolution was found to be insensitive to couch speed error up to 2% and couch speed up to −1mm. Both of the detectors show similar sensitivity to all the MLCLOT errors that were clinically relevant. ConclusionNo statistically significant (p>0.05) differences exist in detecting the simulated delivery errors between MatriXXEvolution and ArcCHECK dosimeter systems for HT plans. Both dosimeters were able to pick up clinically relevant delivery errors.

Filmless methods for quality assurance of Tomotherapy using ArcCHECK.

Tomotherapy delivers an intensity-modulated radiation therapy (IMRT) treatment by the synchronization of gantry rotation, multileaf collimator (MLC), and couch movement. This dynamic nature makes the quality assurance (QA) important and challenging. The purpose of this study is to develop some methodologies using an ArcCHECK for accurate QA measurements of the gantry angle and speed, MLC synchronization and leaf open time, couch translation per gantry rotation, couch speed and uniformity, and constancy of longitudinal beam profile for a Tomotherapy unit. Four test plans recommended by AAPM Task Group 148 (TG148) and the manufacturer were chosen for this study. Helical and static star shot tests are used for checking the leaves opened at the expected gantry angles. Another helical test is to verify the couch traveled the expected distance per gantry rotation. The final test is for checking the couch speed constancy with a static gantry. ArcCHECK can record the detector signal every 50 ms as a movie file, and has a virtual inclinometer for gantry angle measurement. These features made the measurement of gantry angle and speed, MLC synchronization and leaf open time, and longitudinal beam profile possible. A shaping parameter was defined for facilitating the location of the beam center during the plan delivery, which was thereafter used to calculate the couch translation per gantry rotation and couch speed. The full width at half maximum (FWHM) was calculated for each measured longitudinal beam profile and then used to evaluate the couch speed uniformity. Furthermore, a mean longitudinal profile was obtained for constancy check of field width. The machine trajectory log data were also collected for comparison. Inhouse programs were developed in MATLAB to process both the ArcCHECK and machine log data. The deviation of our measurement results from the log data for gantry angle was calculated to be less than 0.4°. The percentage differences between measured and planned leaf open time were found to be within 0.5% in all the tests. Our results showed mean values of MLC synchronization of 0.982, 0.983, and 0.995 at static gantry angle 0°, 45°, and 135°, respectively. The mean value of measured couch translation and couch speed by ArcCHECK had less than 0.1% deviation from the planned values. The variation in the value of FWHM suggested the couch speed uniformity was better than 1%. The mean of measured longitudinal profiles was suitable for constancy check of field width. Precise and efficient methods for measuring the gantry angle and speed, leaf open time, couch translation per gantry rotation, couch speed and uniformity, and constancy of longitudinal beam profile of Tomotherapy using ArcCHECK have been developed and proven to be accurate compared with machine log data. Estimation of the Tomotherapy binary MLC leaf open time is proven to be precise enough to verify the leaf open time as small as 277.8 ms. Our method also makes the observation and quantification of the synchronization of leaves possible.

Clinical significance of treatment delivery errors for helical TomoTherapy nasopharyngeal plans – A dosimetric simulation study

PurposeDevelop a framework to characterize helical TomoTherapy (HT) machine delivery errors and their clinical significance. Method and materialsTen nasopharynx HT plans were edited to introduce errors in Jaw width (JW), couch speed (CS), gantry period (GP), gantry start position (GSP), multi leaf collimator leaf open times (MLC LOT). In case of MLC LOT only, both systematic and random delivery errors were investigated. Each error type was simulated independently for a range of magnitudes. Dose distributions for the clinical reference plans and the error simulated plans were compared to establish the magnitude for each error type which resulted in a change in clinical tolerance, defined as 5% variation in D95 of PTV70, D0.1cc of spinal cord, D0.1cc of brainstem and the smallest value of either a 10% or 3.6Gy dose variation in mean parotid dose. ResultsDose variation from systematic delivery errors in JW ±0.5mm, CS ranges between −1% to 1.5%, GP ±1s, GSP ranges between −20 to 2.50 and MLC LOT random error up to 2% from the planned value relative to the clinical reference plan was within the set tolerance values for all the patient cohorts. GSP errors and the random MLC LOT errors with up to 10% standard deviation were found to be relatively insensitive compared to other delivery errors. ConclusionThis work has established a framework to characterize HT machine delivery errors. This framework could be applied to any patient dataset to determine clinically relevant HT QA tolerances.

Fast Megavoltage Computed Tomography: A Rapid Imaging Method for Total Body or Marrow Irradiation in Helical Tomotherapy

Megavoltage computed tomographic (MVCT) imaging has been widely used for the 3-dimensional (3-D) setup of patients treated with helical tomotherapy (HT). One drawback of MVCT is its very long imaging time, the result of slow couch speeds of approximately 1mm/s, which can be difficult for the patient to tolerate. We sought to develop an MVCT imaging method allowing faster couch speeds and to assess its accuracy for image guidance for HT. Three cadavers were scanned 4 times with couch speeds of 1, 2, 3, and 4mm/s. The resulting MVCT images were reconstructed using an iterative reconstruction (IR) algorithm with a penalty term of total variation and with a conventional filtered back projection (FBP) algorithm. The MVCT images were registered with kilovoltage CT images, and the registration errors from the 2 reconstruction algorithms were compared. This fast MVCT imaging was tested in 3 cases of total marrow irradiation as a clinical trial. The 3-D registration errors of the MVCT images reconstructed with the IR algorithm were smaller than the errors of images reconstructed with the FBP algorithm at fast couch speeds (2, 3, 4mm/s). The scan time and imaging dose at a speed of 4mm/s were reduced to 30% of those from a conventional coarse mode scan. For the patient imaging, faster MVCT (3mm/s couch speed) scanning reduced the imaging time and still generated images useful for anatomic registration. Fast MVCT with the IR algorithm is clinically feasible for large 3-D target localization, which may reduce the overall time for the treatment procedure. This technique may also be useful for calculating daily dose distributions or organ motion analyses in HT treatment over a wide area. Automated integration of this imaging is at least needed to further assess its clinical benefits.

SU‐F‐T‐457: A Filmless Method for Measurement of Couch Translation Per Gantry Rotation and Couch Speed for Tomotherapy Using ArcCheck

Purpose:To develop a filmless methodology based on an ArcCheck for QA measurement of the couch translation per gantry rotation and couch speed of a Tomotherapy unit.Methods:Two test plans recommended by TG148 were chosen for this study. A helical plan with 1 cm field size opened the leaves for 180 degrees at the 2nd, 7th and 12th of total 13 rotations and was used to verify if the couch travelled the expected distance per gantry rotation. The other test plan was a static plan with the gantry at 0°, 1cm field width and constant couch movement speed of 0.5mm/s. It was used for couch speed measurement. The ArcCheck was isocentrically set up and recorded movie files which took a snapshot exposure every 50ms. Due to the spiral pattern of diodes distribution, when one of the diodes of the ArcCheck located at the beam center, the dose profile as measured by the row of diodes which surrounded the center diode should have a symmetrical pattern. We divided the profile into left half A and right half B. Then a shape parameter was defined as S=Σ|(A−B)|/Σ(A+B). By searching the local minimum of S parameter, the beam center at different time could be located. The machine trajectory log data were also collected and analyzed for comparison.Results:The mean value of measured couch translation and couch speed by ArcCheck had less than 0.05% deviation from the planned values. For couch speed measurement, our result showed a mean value of 0.5002 with an uncertainty ±0.0031, which agreed very well with both the plan setting of 0.5 mm/s and the machine log data of 0.5005 mm/s.Conclusion:Couch translation measured using ArcCheck is accurate and comparable to the film‐based measurement. This filmless method also provides a convenient and independent way for measuring couch speed.

WE‐DE‐BRA‐09: Fast Megavoltage CT Imaging with Rapid Scan Time and Low Imaging Dose in Helical Tomotherapy

Purpose:Megavoltage computed tomography (MVCT) imaging has been widely used for daily patient setup with helical tomotherapy (HT). One drawback of MVCT is its very long imaging time, owing to slow couch speed. The purpose of this study was to develop an MVCT imaging method allowing faster couch speeds, and to assess its accuracy for image guidance for HT.Methods:Three cadavers (mimicking closest physiological and physical system of patients) were scanned four times with couch speeds of 1, 2, 3, and 4 mm/s. The resulting MVCT images were reconstructed using an iterative reconstruction (IR) algorithm. The MVCT images were registered with kilovoltage CT images, and the registration errors were compared with the errors with conventional filtered back projection (FBP) algorithm. Moreover, the fast MVCT imaging was tested in three cases of total marrow irradiation as a clinical trial.Results:Three‐dimensional registration errors of the MVCT images reconstructed with the IR algorithm were significantly smaller (p &lt; 0.05) than the errors of images reconstructed with the FBP algorithm at fast couch speeds (3, 4 mm/s). The scan time and imaging dose at a speed of 4 mm/s were reduced to 30% of those from a conventional coarse mode scan. For the patient imaging, a limited number of conventional MVCT (1.2 mm/s) and fast MVCT (3 mm/s) reveals acceptable reduced imaging time and dose able to use for anatomical registration.Conclusion:Fast MVCT with IR algorithm maybe clinically feasible alternative for rapid 3D patient localization. This technique may also be useful for calculating daily dose distributions or organ motion analyses in HT treatment over a wide area.

SU‐F‐J‐181: An Alternative Patient Alignment Tool On TomoTherapy: The First In‐ Human Megavoltage‐Topogram Acquisition

Purpose:To show the first in‐human Megavoltage (MV)‐Topogram acquisition for the evaluation of the potential for MV‐Topogram‐based alignment as an alternative to MVCT for reducing dose and imaging time.Methods:A lung cancer patient was enrolled in an ongoing IRB‐approved clinical trial at our institute. The patient was set up using the clinical protocol employing positioning lasers. 3.2mm diameter tungsten spheres were placed on the patient's skin at their alignment tattoos to check surface‐based marker concordance between topograms and MVCT. Anterior‐Posterior (AP) and lateral (LAT) MV‐Topograms were acquired using gantry angles of 0°/90° with a 1mm collimator opening, all MLC leafs open, 4cm/s couch speed, and 12.5s scanning time. The topogram acquisition was immediately followed by the normal MVCT scan acquisition. MV‐Topograms were reconstructed from the detector exit‐data using in‐house developed software. The topograms were also enhanced using contrast‐limited adaptive histogram equalization (CLAHE). The MV‐Topograms were registered to reference kV‐based digitally reconstructed topograms. The localization results were compared against results obtained comparing the clinical MVCT to the kVCT simulation.Results:The shifts using the unenhanced Topograms, enhanced Topograms, and MVCT were (LAT, LONG, VERT, ROLL) (5.8mm, 2.6mm, −5.6mm, 0.34°), (3.9mm, 2.5mm, −2.2mm, 0.65°) and (2.4mm, 1.5mm, −3.0mm, 0.5°), respectively. The magnitude alignment differences between the enhanced Topograms and MVCT were within 1.5 mm and 0.15°. The average MVCT and total Topogram acquisition times were 272.9s ± 31.5s and 46s, respectively.Conclusion:MV‐Topograms have the potential for providing equivalent performance with less dose and acquisition time than the traditional MVCT technique. We are evaluating other sites as well as adding patients to develop statistically significant analyses regarding the alignment quality differences. MV‐Topograms are likely to be most clinically useful for bony anatomy and radiopaque marker‐based alignments.The study was supported by an Accuray Grant.

Geometric validation of MV topograms for patient localization on TomoTherapy

Our goal was to geometrically validate the use of mega-voltage orthogonal scout images (MV topograms) as a fast and low-dose alternative to mega-voltage computed tomography (MVCT) for daily patient localization on the TomoTherapy system. To achieve this, anthropomorphic head and pelvis phantoms were imaged on a 16-slice kilo-voltage computed tomography (kVCT) scanner to synthesize kilo-voltage digitally reconstructed topograms (kV-DRT) in the Tomotherapy detector geometry. MV topograms were generated for couch speeds of 1–4 cm s−1 in 1 cm s−1 increments with static gantry angles in the anterior-posterior and left-lateral directions. Phantoms were rigidly translated in the anterior-posterior (AP), superior-inferior (SI), and lateral (LAT) directions to simulate potential setup errors. Image quality improvement was demonstrated by estimating the noise level in the unenhanced and enhanced MV topograms using a principle component analysis-based noise level estimation algorithm. Average noise levels for the head phantom were reduced by 2.53 HU (AP) and 0.18 HU (LAT). The pelvis phantom exhibited average noise level reduction of 1.98 HU (AP) and 0.48 HU (LAT). Mattes Mutual Information rigid registration was used to register enhanced MV topograms with corresponding kV-DRT. Registration results were compared to the known rigid displacements, which assessed the MV topogram localization’s sensitivity to daily positioning errors. Reduced noise levels in the MV topograms enhanced the registration results so that registration errors were <1 mm. The unenhanced head MV topograms had discrepancies <2.1 mm and the pelvis topograms had discrepancies <2.7 mm. Result were found to be consistent regardless of couch speed. In total, 64.7% of the head phantom MV topograms and 60.0% of the pelvis phantom MV topograms exactly measured the phantom offsets. These consistencies demonstrated the potential for daily patient positioning using MV topogram pairs in the context bony-anatomy based procedures such as total marrow irradiation, total body irradiation, and cranial spinal irradiation.

The sensitivity of ArcCHECK-based gamma analysis to manufactured errors in helical tomotherapy radiation delivery.

Three‐dimensional measurement arrays are an efficient means of acquiring a distribution of data for patient plan delivery QA. However, the tie between plan integrity and traditional gamma‐based analysis of these data are not clear. This study explores the sensitivity of such analysis by creating errors in Helical Tomotherapy delivery and measuring the passing rates with an ArcCHECK cylindrical diode array. Errors were introduced in each of the couch speed, leaf open time, and gantry starting position in increasing magnitude while the resulting gamma passing rates were tabulated. The error size required to degrade the gamma passing rate to 90% or below was on average a 3% change in couch speed, 5° in gantry synchronization, or a 5 ms in leaf closing speed for a 3%/3 mm Van Dyk gamma analysis. This varied with plan type, with prostate plans exhibiting less sensitivity than head and neck plans and with gamma analysis criteria, but in all cases the error magnitudes were large compared to actual machine tolerances. These findings suggest that the sensitivity of ArcCHECK‐based gamma analysis to single‐mode errors in tomotherapy plans is dependent upon plan and analysis type and at traditional passing thresholds unable to detect small defects in the plan.PACS number: 87.55.Qr

Multi-institutional Feasibility Study of a Fast Patient Localization Method in Total Marrow Irradiation With Helical Tomotherapy: A Global Health Initiative by the International Consortium of Total Marrow Irradiation

To develop, characterize, and implement a fast patient localization method for total marrow irradiation. Topographic images were acquired using megavoltage computed tomography (MVCT) detector data by delivering static orthogonal beams while the couch traversed through the gantry. Geometric and detector response corrections were performed to generate a megavoltage topogram (MVtopo). We also generated kilovoltage topograms (kVtopo) from the projection data of 3-dimensional CT images to reproduce the same geometry as helical tomotherapy. The MVtopo imaging dose and the optimal image acquisition parameters were investigated. A multi-institutional phantom study was performed to verify the image registration uncertainty. Forty-five MVtopo images were acquired and analyzed with in-house image registration software. The smallest jaw size (front and backup jaws of 0) provided the best image contrast and longitudinal resolution. Couch velocity did not affect the image quality or geometric accuracy. The MVtopo dose was less than the MVCT dose. The image registration uncertainty from the multi-institutional study was within 2.8 mm. In patient localization, the differences in calculated couch shift between the registration with MVtopo-kVtopo and MVCT-kVCT images in lateral, cranial-caudal, and vertical directions were 2.2 ± 1.7 mm, 2.6 ± 1.4 mm, and 2.7 ± 1.1 mm, respectively. The imaging time in MVtopo acquisition at the couch speed of 3 cm/s was <1 minute, compared with ≥15 minutes in MVCT for all patients. Whole-body MVtopo imaging could be an effective alternative to time-consuming MVCT for total marrow irradiation patient localization.

Poster — Thur Eve — 38: Feasibility of a Table‐Top Total Body Irradiation Technique using Robotic Couch Motion

Purpose:To develop and test the feasibility of a table‐top implementation for total body irradiation (TBI) via robotic couch motion and coordinated monitor unit modulation on a standard C‐arm linac geometry.Methods:To allow for collision free delivery and to maximize the effective field size, the couch was rotated to 270° IEC and dropped to 150 cm from the vertical radiation source. The robotic delivery was programmed using the TrueBeam STx Developer Mode using custom XML scripting. To assess the dosimetry of a sliding 30×20 cm2 field, irradiation on a solid water phantom of varying thickness was analyzed using EDR2 radiographic film and OSLDs. Beam modulation was achieved by dividing the couch path into multiple segments of varying dose rates and couch speeds in order to deliver 120 cGy to the midline.Results:The programmed irradiation in conjunction with coordinated couch motion was successfully delivered on a TrueBeam linac. When no beam modulation was employed, the dose difference between two different phantom sections was 17.0%. With simple beam modulation via changing dose rates and couch speeds, the desired prescription dose can be achieved at the centre of each phantom section within 1.9%. However, dose deviation at the junction was 9.2% due to the nonphysical change in the phantom thickness.Conclusions:The feasibility of robotic table‐top TBI on a C‐arm linac geometry was experimentally demonstrated. To achieve a more uniform dose distribution, inverse‐planning allowing for a combination of dose rate modulation, jaw tracking and MLC motion is under investigation.