Real-time Position Management System Research Articles

Intrafraction Verification of Gated RapidArc by Using Beam-Level Kilovoltage X-Ray Images

To verify the geometric accuracy of gated RapidArc treatment using kV images acquired during dose delivery. Twenty patients were treated using the gated RapidArc technique with a Varian TrueBeam STx linear accelerator. One to 7 metallic fiducial markers were implanted inside or near the tumor target before treatment simulation. For patient setup and treatment verification purposes, the internal target volume (ITV) was created, corresponding to each implanted marker. The gating signal was generated from the Real-time Position Management (RPM) system. At the beginning of each fraction, individualized respiratory gating amplitude thresholds were set based on fluoroscopic image guidance. During the treatment, we acquired kV images immediately before MV beam-on at every breathing cycle, using the on-board imaging system. After the treatment, all implanted markers were detected, and their 3-dimensional (3D) positions in the patient were estimated using software developed in-house. The distance from the marker to the corresponding ITV was calculated for each patient by averaging over all markers and all fractions. The average 3D distance between the markers and their ITVs was 0.8 ± 0.5 mm (range, 0-1.7 mm) and was 2.1 ± 1.2 mm at the 95th percentile (range, 0-3.8 mm). On average, a left-right margin of 0.6 mm, an anterior-posterior margin of 0.8 mm, and a superior-inferior margin of 1.5 mm is required to account for 95% of the intrafraction uncertainty in RPM-based RapidArc gating. To our knowledge, this is the first clinical report of intrafraction verification of respiration-gated RapidArc treatment in stereotactic ablative radiation therapy. For some patients, the markers deviated significantly from the ITV by more than 2 mm at the beginning of the MV beam-on. This emphasizes the need for gating techniques with beam-on/-off controlled directly by the actual position of the tumor target instead of external surrogates such as RPM.

A real-time respiration position based passive breath gating equipment for gated radiotherapy: A preclinical evaluation

To develop a passive gating system incorporating with the real-time position management (RPM) system for the gated radiotherapy. Passive breath gating (PBG) equipment, which consists of a breath-hold valve, a controller mechanism, a mouthpiece kit, and a supporting frame, was designed. A commercial real-time positioning management system was implemented to synchronize the target motion and radiation delivery on a linear accelerator with the patient's breathing cycle. The respiratory related target motion was investigated by using the RPM system for correlating the external markers with the internal target motion while using PBG for passively blocking patient's breathing. Six patients were enrolled in the preclinical feasibility and efficiency study of the PBG system. PBG equipment was designed and fabricated. The PBG can be manually triggered or released to block or unblock patient's breathing. A clinical workflow was outlined to integrate the PBG with the RPM system. After implementing the RPM based PBG system, the breath-hold period can be prolonged to 15-25 s and the treatment delivery efficiency for each field can be improved by 200%-400%. The results from the six patients showed that the diaphragm motion caused by respiration was reduced to less than 3 mm and the position of the diaphragm was reproducible for difference gating periods. A RPM based PBG system was developed and implemented. With the new gating system, the patient's breath-hold time can be extended and a significant improvement in the treatment delivery efficiency can also be achieved.

Comparison of three methods to delineate internal gross target volume of the primary hepatocarcinoma based on four-dimensional CT simulation images

To compare the position and magnitude of internal target gross volume (IGTV) of primary hepatocarcinoma delineated by three methods based on four-dimensional computed tomography (4D-CT) and to investigate the relevant factors affecting the position and magnitude. Twenty patients with primary hepatocarcinoma after transcatheter arterial chemoembolization (TACE) underwent big bore 4D-CT simulation scan of the thorax and abdomen using a real-time position management (RPM) system for simultaneous record of the respiratory signals. The CT images with respiratory signal data were reconstructed and sorted into 10 phase groups in a respiratory cycle, with 0% phase corresponding to end-inhale and 50% corresponding to end-exhale. The maximum intensity projection (MIP) image was generated. IGTVs of the tumor were delineated using the following three methods: (1) The gross tumor volume (GTV) on each of the ten respiratory phases of the 4D-CT image set was delineated and fused ten GTV to produce IGTV10; (2) The GTVs delineated separately based on 0% and 50% phase were fused to produce IGTV(IN+EX); (3) The visible tumor on the MIP image was delineated to produce IGTV(MIP). Twenty patients were divided into groups A and B based on the location of the target center,and were divided into groups C and D based on the tumor maximum diameter. The patients were divided into groups E and F based on the three-dimensional (3D) motion vector of the target center. The position of the target center, the volume of target, the degree of inclusion (DI) and the matching index (MI) were compared reciprocally between IGTV10, IGTV(IN+EX) and IGTV(MIP), and the influence of the tumor position and 3D motion vector on the related parameters were compared based on the grouping. The average differences between the position of the center of IGTVs on direction of X, Y and Z axes were less than 1.5 mm, and the difference was statistically not significant. The volume of IGTV10 was larger than that of IGTV(IN+EX), but the difference was not significant (t = 0.354, P = 0.725). The volume of IGTV10 was larger than that of IGTV(MIP) but the difference was not significant (t = -0.392, P = 0.697). The ratio of IGTV(IN+EX) to IGTV10 was 0.75 +/- 0.15 and the ratio of IGTV(MIP) to IGTV10 was 0.78 +/- 0.14. The DI of IGTV(IN+EX) in IGTV10 was (74.85 +/- 15.09)% and that of IGTV(MIP) in IGTV10 was (68.87 +/- 13.69)%. The MI between IGTV10 and IGTV(IN+EX), IGTV10 and IGTV(MIP) were 0.75 +/- 0.15 and 0.67 +/- 0.13, respectively. The median of ratio of IGTV(IN+EX)/ IGTV10 was 0.57 in group A versus 0.87 in group B, statistically with a significant difference between the groups A and B (Z = -3.300,P = 0.001). The median of ratio of IGTV(MIP)/IGTV10 was 0.51 in the group A and 0.72 in group B, with a significant difference between the groups A and B (Z = -3.413, P = 0.001). The median of ratio of IGTV(IN+EX)/IGTV10 was 0.79 in group C versus 0.74 in group D, with a difference not significant (Z = -0.920, P = 0.358). The median of ratio of IGTV(MIP)/IGTV10 was 0.85 in group C versus 0.80 in group D, with a non-significant difference (Z = -0.568, P = 0.570). The median of ratio of IGTV(IN+EX)/IGTV10 was 0.87 in group E versus 0.68 in group F, with a significant difference between the two groups (Z = -2.897, P = 0.004). The median of ratio of IGTV(MIP)/IGTV10 was 0.85 in the group E versus 0.81 in the group F, with a non-significant difference (Z = -0.568, P = 0.570). The center displacement of the IGTVs delineated separately by the three techniques based on 4D-CT images is not obvious. IGTV(IN+EX) and IGTV(MIP) can not replace IGTV10, however, IGTV(IN+EX) is more close to IGTV10 comparing with IGTV(MIP). The ratio of IGTV10 and IGTV(MIP) is correlated to the 3D motion vector of the tumor. When the tumor is situated in the upper part of the liver and with a 3D motion vector less than 9 mm, IGTV10 should be the best IGTV.

Real-time Position Management Systemを用いた呼吸同期による肺の体幹部定位放射線治療の品質保証

In this study, we investigated comprehensive quality assurance (QA) for respiratory-gated stereotactic body radiation therapy (SBRT) in the lungs using a real-time position management system (RPM). By using the phantom study, we evaluated dose liberality and reproducibility, and dose distributions for low monitor unite (MU), and also checked the absorbed dose at isocenter and dose profiles for the respiratory-gated exposure using RPM. Furthermore, we evaluated isocenter dose and dose distributions for respiratory-gated SBRT plans in the lungs using RPM. The maximum errors for the dose liberality were 4% for 2 MU, 1% for 4-10 MU, and 0.5% for 15 MU and 20 MU. The dose reproducibility was 2% for 1 MU and within 0.1% for 5 MU or greater. The accuracy for dose distributions was within 2% for 2 MU or greater. The dose error along a central axis for respiratory cycles of 2, 4, and 6 sec was within 1%. As for geometric accuracy, 90% and 50% isodose areas for the respiratory-gated exposure became almost 1 mm and 2 mm larger than without gating, respectively. For clinical lung-SBRT plans, the point dose at isocenter agreed within 2.1% with treatment planning system (TPS). And the pass rates of all plans for TPS were more than 96% in the gamma analysis (3 mm/3%). The geometrical accuracy and the dose accuracy of TPS calculation algorithm are more important for the dose evaluation at penumbra region for respiratory-gated SBRT in lung using RPM.

Comparison of the sensitivities of two 4DCT techniques: RPM and bellows system

In modern radiation therapy, the target motion caused by respiration could result in a geometric miss of the target and severe damage to the surrounding critical organs and normal tissues. To tackle this issue, various 4D Radiation Therapy (4DRT) techniques have recently been proposed and developed, among which the Real-time Position Management System (RPM) and the Bellows system are the most commonly used 4DRT approaches. Nevertheless, a systematic comparison of these two has not been conducted. In this study, we compared the clinical performance of the RPM and the Bellows system by using a newly developed breathing phantom.

A programmable motion phantom for quality assurance of motion management in radiotherapy

A commercially available motion phantom (QUASAR, Modus Medical) was modified for programmable motion control with the aim of reproducing patient respiratory motion in one dimension in both the anterior-posterior and superior-inferior directions, as well as, providing controllable breath-hold and sinusoidal patterns for the testing of radiotherapy gating systems. In order to simulate realistic patient motion, the DC motor was replaced by a stepper motor. A separate 'chest-wall' motion platform was also designed to accommodate a variety of surrogate marker systems. The platform employs a second stepper motor that allows for the decoupling of the chest-wall and insert motion. The platform's accuracy was tested by replicating patient traces recorded with the Varian real-time position management (RPM) system and comparing the motion platform's recorded motion trace with the original patient data. Six lung cancer patient traces recorded with the RPM system were uploaded to the motion platform's in-house control software and subsequently replicated through the phantom motion platform. The phantom's motion profile was recorded with the RPM system and compared to the original patient data. Sinusoidal and breath-hold patterns were simulated with the motion platform and recorded with the RPM system to verify the systems potential for routine quality assurance of commercial radiotherapy gating systems. There was good correlation between replicated and actual patient data (P 0.003). Mean differences between the location of maxima in replicated and patient data-sets for six patients amounted to 0.034 cm with the corresponding minima mean equal to 0.010 cm. The upgraded motion phantom was found to replicate patient motion accurately as well as provide useful test patterns to aid in the quality assurance of motion management methods and technologies.

Four‐dimensional magnetic resonance imaging (4D‐MRI) using image‐based respiratory surrogate: A feasibility study

Four-dimensional computed tomography (4D-CT) has been widely used in radiation therapy to assess patient-specific breathing motion for determining individual safety margins. However, it has two major drawbacks: low soft-tissue contrast and an excessive imaging dose to the patient. This research aimed to develop a clinically feasible four-dimensional magnetic resonance imaging (4D-MRI) technique to overcome these limitations. The proposed 4D-MRI technique was achieved by continuously acquiring axial images throughout the breathing cycle using fast 2D cine-MR imaging, and then retrospectively sorting the images by respiratory phase. The key component of the technique was the use of body area (BA) of the axial MR images as an internal respiratory surrogate to extract the breathing signal. The validation of the BA surrogate was performed using 4D-CT images of 12 cancer patients by comparing the respiratory phases determined using the BA method to those determined clinically using the Real-time position management (RPM) system. The feasibility of the 4D-MRI technique was tested on a dynamic motion phantom, the 4D extended Cardiac Torso (XCAT) digital phantom, and two healthy human subjects. Respiratory phases determined from the BA matched closely to those determined from the RPM: mean (± SD) difference in phase: -3.9% (± 6.4%); mean (± SD) absolute difference in phase: 10.40% (± 3.3%); mean (± SD) correlation coefficient: 0.93 (± 0.04). In the motion phantom study, 4D-MRI clearly showed the sinusoidal motion of the phantom; image artifacts observed were minimal to none. Motion trajectories measured from 4D-MRI and 2D cine-MRI (used as a reference) matched excellently: the mean (± SD) absolute difference in motion amplitude: -0.3 (± 0.5) mm. In the 4D-XCAT phantom study, the simulated "4D-MRI" images showed good consistency with the original 4D-XCAT phantom images. The motion trajectory of the hypothesized "tumor" matched excellently between the two, with a mean (± SD) absolute difference in motion amplitude of 0.5 (± 0.4) mm. 4D-MRI was able to reveal the respiratory motion of internal organs in both human subjects; superior-inferior (SI) maximum motion of the left kidney of Subject #1 and the diaphragm of Subject #2 measured from 4D-MRI was 0.88 and 1.32 cm, respectively. Preliminary results of our study demonstrated the feasibility of a novel retrospective 4D-MRI technique that uses body area as a respiratory surrogate.

Investigation on the impact to beam characteristics of a linear accelerator related to duty cycle of respiratory gating

Abstract Background and objective Respiratory-gated techniques are based on controlling the linear accelerator and limiting the beam-on time to only a small portion of the patient’s breathing cycle. For each beam-on, the machine must spend a start-up time (ST) to set up a stable and correct RF field in the waveguide and get the right beam current, etc. That might cause an obvious variation on the beam dosimetric characteristic, including the beam quality and profile, when the duration of ST is un-ignorable to the beam-on time. The purpose of this study is to investigate this kind of impact for various duty cycles (DC) of breathing-gated operation. Materials and method A respiratory motion phantom and a Real-time Position Management System (RPM) were used to simulate the patient respiration and to control the beam gating. Measurements were performed on a Varian 600C/D linac with 6 MV photons, using different DCs of 5%, 7%, 10%, 12%, 20%, 50%, 75%, and 100% to a 6-s breathing cycle (BC) simulation. The beam quality, flatness, symmetry were evaluated for these conditions. Output reproducibility and dose linearity were measured and evaluated for different MUs at 400 MU/min dose rate and 30% DC as well. Results For gated delivery in the above selected DCs and BC, the maximum changes in beam quality, symmetry, flatness were −0.181%, 0.124%, −0.17%, compared with non-gated data respectively. The coefficient of dispersion of output was less than 0.3% and good dose linearity was obtained for the tested gating operation condition. Conclusion For situation similar to our experiment, the duty cycle effect to beam characteristics is ignorable that the beam modeling based on non-gated delivery can be directly used for respiratory gating therapy. Otherwise, the beam characteristics should be carefully checked individually.

The “Air-Bag System”: A Novel Abdominal Compression Device Collaborated with RPM System

We developed a novel abdominal compression device which monitors the patient's body volume change as an external respiration signal collaborated with the Real-Time Position Management System (RPM, Varian Medical System, Palo Alto, CA). The “Air-Bag System” consists of a non-elastic air bag connected with an elastic air bag. A non-elastic air bag is placed between the patient's abdomen and the stereotactic fixation frame. Air can be injected into this system to give pressure on the patient's abdomen. A current meter located between two bags measures the air flow back and forth between these bags. This air flow data are processed by personal computer and translated into swinging decoy markers displayed on the computer screen so that an infrared camera of RPM system can capture the marker instead of an infrared reflective marker. The impedance model was adopted for describing the behavior of the air flow in this system. The phase gap between the respiratory motion and the monitoring signal is defined as θ = arctan (X/R). Where R is the resistive part, and X is the reactive part of the impedance. The resistive part represents the viscous forces at the air/wall interface. The reactive part, which is a respiratory frequency dependent parameter, represents the ability of air to store the kinetic energy as potential energy since air is a compressible medium. An elastic air bag also stores potential energy by inflation and deflation. Both values of R and X were obtained experimentally and the theoretical equation was evaluated with the clinical data of 25 patients. 4D-CT data of 93 patients (49 lung cancer patients and 44 liver cancer patients) were acquired by the RPM system with the “Air-Bag System” in order to observe the motion of the diaphragm while free breathing. Both values of R and X were obtained by least squares method with strong correlations (R; r2 = 0.965, X; r2 = 0.973). The theoretical equation was given by θ = arctan(X/R) = arctan((25.178f-1/0.604f)/23.780). Where f is the respiratory frequency. Comparing the 25 clinical data to the theoretical equation, the phase gaps were resulted in good correlations on each respiratory frequency. Diaphragm motion was reduced to 7.1 ± 3.8mm (1.2 - 23.5mm) in the superior to inferior direction with 4D-CT data of 93 patients. The novel abdominal compression device which presses on the patient's abdomen with the air bag reduces the movement of diaphragm without loading additional stress on the patient. This device can also be used for respiratory monitoring which monitors the patient's body volume change as an external respiration signal. The phase gap between the respiratory motion and the monitoring signal was negligibly small under average respiratory frequency.

SU‐E‐J‐159: Correlation of Respiration‐Induced Motion of an External Surrogate and Implanted Internal Markers

Purpose: To investigate the correlation between the respiratory motion of implanted internal markers (IM) and external RPM traces for lung cancer patients in dynamic imaging. Methods: Under an IRB‐approved research protocol, 3 patients with VISICOIL marker (10 mm long) implants near/in their lung tumor were scanned with a daily single‐slow gantry rotation arc 4D‐CBCT acquisition. The Varian Real‐time Position Management system (RPM) was used as an external respiratory motion surrogate, with the marker block placed on the patient abdominal surface near the diaphragm. The IM motion was segmented in all 2D projections with an in‐house built Matlab program. The positions in the 3D spatial domain of the markers were extracted to (LR: left‐right, CC: cranial‐caudal, AP: anterior‐posterior). A Pearson‐type auto‐correlation function of the RPM AP trace and cross‐ correlation functions between RPM AP motion and the IM motions in three orthogonal directions was used to analyze the time series. Results: Our data shows that there is a strong positive correlation between the RPM AP motion and the IM motion in the CC direction. The correlation between the external motion and the IM motions in LR and AP directions varied from fraction to fraction. In addition, phase shift is observed in LR and AP directions. Different implanted markers in the same patient presented the very similar correlations, especially in the CC direction. This result suggests that multiple implanted markers can be used to provide supplementary information of the tumor motion. When the patient coughs during their respiratory cycle, internal motion and external motion may become uncorrelated. Conclusions: The RPM respiratory trace provides a good surrogate of the internal respiratory motion in the CC direction when the patient maintains a normal breathing pattern.Supported by NIH grant P01 CA 116602.

SU‐E‐T‐459: Measurement of Gated Rapidarc Accuracy

Purpose: Recently, Kyung Hee university hospital at Gangdong installed the Varian 21iX linac which has the volumetric imrt (RapidArc) option and a real‐time position management (RPM) system. One of advantage of rapidarc is a short treatment time comparing with other treatments. Thus, the rapidarc treatment with the RPM system for the lung and liver cases, can reduce the treatment time comparing with the intensity modulation radiotherapy (IMRT) and 3 dimensional radiotherapy (3DRT) with gating system. Therefore, we estimate the accuracy of the gated rapidarc treatment. Methods: 10 patients data are used for this study. Each patient was planned by rapidarc technique using varian ECLISPE v8.6 planning machine. For the patient Quality Assurence (QA), a MATRIXX detector and IˈM RT software are used. At bottom and top of MATRIXX detector, 5×30×30cm̂3 of solid water phantoms are install for build up. A MATRIXX detector is fixed but the infrared reflecting box is periodically moved to provide the gating signal to the RPM system. Each patients dosimetric distribution is measured and the treatment summary which is provided by linac operating system is recoded to estimate the accuracy of the gated rapidarc treatment. Results: The dosimetric comparison between the measurement and the treatment plan is matched well for the 10 patients. With 3% of delta dose and 3mm of delta distance, more than 98% of area gives the gamma value as less than 1. Conclusions: We found that the patient dose measurement is matched well with the RTP planˈs dose distribution for the gated rapidarc treatment cases in the mechanical point of view. This study shows the gated rapidarcˈs dose reproducibility during the patient QA but this result has a limitation for the dose confirming for the real patients which has more soft and flexible body than the solid phantom.

WE‐D‐220‐02: Relation of External Surface to Diaphragm Motion Based on Automatic Analysis of Ultrasound Images

Purpose: The accuracy of delivering gated‐radiation therapy to lung tumors using an external respiratory surrogate relies on the degree of correlation between the marker serving as a surrogate from breathing and motion of the anatomic structures. The irregular breathing during an imaging and treatment session and from one session to another may cause the dose distribution actually delivered to be different from the intended dose. The purpose of this work was to use the diaphragm images acquired from Ultrasound system and abdominal motion signal from Real‐time Position Management (RPM) system, to study the relation between diaphragm and external surface motion during uncontrolled breathing cycles. Methods: The ultrasound images of diaphragm were captured at the same time as RPM signal acquisition for three volunteers from our staffs. After capture, one point at the image of diaphragm was selected and tracked for the whole breathing cycles using a proposed automatic tracking method which combines moving average strategy with Gaussian filter. Then, the diaphragm motion was compared to the respiratory signal from RPM system. Results: On three normal volunteer data sets: (1) the phase shifts were measured between the RPM signal in the abdominal region and diaphragm motion. Three volunteers showed the diaphragm motion either leading or delay abdominal motion with the mean time around 0.2–0.4 second; (2) the phase shifts at difference breathing cycle for the same people were different; (3) there is no significant difference between the phases at the end of inhale and exhale. Conclusions: We conclude that the external monitor can be used to predict internal respiratory motion. However, it may be important to check with Ultrasound images or fluoroscopy of diaphragm for possible time delays. We found that the Ultrasound can be a more reliable non‐ionizing method than RPM system used for respiratory gating in radiation therapy.

TH-E-220-06: Selection of External Surrogate for 4DCT Sorting: Does It Impact Target Delineation?

Purpose: To evaluate differences observed between two different external surrogate acquisition systems: Varianˈs Real-Time Position Management (RPM) and Philips Medical Systems pneumatic bellows, in the context of 4DCT sorting and tumor delineation. Methods: Eight patient displacement curves from RPM data were inputted into the motion platform with a variety of amplitudes (0.5 to 3 cm). Simultaneous 4DCT acquisition was performed with both bellows and RPM block placed on the platform. The bellows signal was used for on-line retrospective phase-based sorting, while raw 4DCT data was reconstructed off-line using RPM signal on a research version of the Extended Brilliance Workspace (Philips Healthcare). Curves were resampled, normalized, and analyzed to determine the association between different external surrogates, amplitude differences, and latency between systems. 4DCT data was imported into Eclipse, and a lung window/level was used for all contouring. The volumes and centroids were compared among techniques. Results: Pearson r correlations between the RPM and bellows-acquired breathing traces were 0.987– 0.999. Slight discrepancies were observed with low amplitude displacement, where bellows were not as sensitive as RPM to slight variations in motion platform position. Target volumes were 200.27 +/− 12 cc and 199.96 +/− 13.3 cc for RPM and bellows targets, respectively, which was not significantly different (t(7) = 0.33, p = 0.75). Small centroid variations were observed between bellows and RPM contoured MIP targets: 0.025 +/− 0.05 mm, 0.013 +/− 0.04 mm, and 0.25 +/− 0.32 mm in lateral, anterior-posterior, and superior- inferior directions, respectively. Largest volume difference was observed for an RPM target 2.5 cc (1%) less than bellows target, which showed the largest difference in centroid displacement (0.9 mm). Overall, RPM data lagged behind the bellows data ∼126 ms. Conclusions: We have demonstrated comparable results between Philips bellows and Varianˈs RPM system, particularly in the management of 4D treatment planning. Acknowledgment: This work is supported in part by Philips Healthcare Conflict of interest: Henry Ford Health Systems holds a research agreement with Philips Healthcare.

Evaluation of integrated respiratory gating systems on a Novalis Tx system

The purpose of this study was to investigate the accuracy of motion tracking and radiation delivery control of integrated gating systems on a Novalis Tx system. The study was performed on a Novalis Tx system, which is equipped with Varian Real‐time Position Management (RPM) system, and BrainLAB ExacTrac gating systems. In this study, the two systems were assessed on accuracy of both motion tracking and radiation delivery control. To evaluate motion tracking, two artificial motion profiles and five patients' respiratory profiles were used. The motion trajectories acquired by the two gating systems were compared against the references. To assess radiation delivery control, time delays were measured using a single‐exposure method. More specifically, radiation is delivered with a 4 mm diameter cone within the phase range of 10%–45% for the BrainLAB ExacTrac system, and within the phase range of 0%–25% for the Varian RPM system during expiration, each for three times. Radiochromic films were used to record the radiation exposures and to calculate the time delays. In the work, the discrepancies were quantified using the parameters of mean and standard deviation (SD). Pearson's product‐moment correlational analysis was used to test correlation of the data, which is quantified using a parameter of r. The trajectory profiles acquired by the gating systems show good agreement with those reference profiles. A quantitative analysis shows that the average mean discrepancies between BrainLAB ExacTrac system and known references are 1.5 mm and 1.9 mm for artificial and patient profiles, with the maximum motion amplitude of 28.0 mm. As for the Varian RPM system, the corresponding average mean discrepancies are 1.1 mm and 1.7 mm for artificial and patient profiles. With the proposed single‐exposure method, the time delays are found to be 0.20±0.03 seconds and 0.09±0.01 seconds for BrainLAB ExacTrac and Varian RPM systems, respectively. The results indicate the systems can track motion and control radiation delivery with reasonable accuracy. The proposed single‐exposure method has been demonstrated to be feasible in measuring time delay efficiently.PACS numbers: 87.56.bd, 87.56.‐v, 87.55.‐x

Temperature-Controlled Cooled-Tip Radiofrequency Linear Ablation of the Atria Guided by a Realtime Position Management System

Due to the difficulty in producing a transmural linear lesion and the possibility of complications such as thrombus formation leading to thromboembolism, the catheter-based maze procedure remains problematic. We tested, in pigs, the possibility of using a temperature-controlled cooled-tip radiofrequency (RF) ablation system together with a realtime position management (RPM) system to create a transmural linear lesion uncomplicated by thrombus formation.Nine pigs underwent insertion of two electrode catheters (each with two ultrasound electrodes), one into the coronary sinus (CS) and one into the right ventricular apex (references for ultrasound-based non-fluoroscopic three-dimensional mapping). A cooled-tip catheter (with two ultrasound electrodes) was introduced into the right atrium. Linear right atrial ablation was performed with a custom radiofrequency (RF) generator. The catheter was perfused with 0.66 mL/second of saline. RF was delivered for 60 seconds at a target temperature of 40°C. A linear ablation line was created between the superior vena cava and inferior vena cava. Three-dimensional isochronal maps were created during CS pacing before and after ablation. In 4 of the 9 pigs, a transmural linear ablation line was confirmed by three-dimensional mapping and postmortem macroscopic examination. No endocardial thrombus formation was noted. Temperature-controlled cooled-tip RF linear ablation guided by an RPM system appears to have potential for creating linear lesions in the atria. Further studies are needed to determine whether such an ablation technique and the parameters used will facilitate successful completion of the catheter-based maze procedure.

The “Air-bag System”: A Novel Respiratory Monitoring Device Collaborated with RPM System

Respiratory motion is an issue that is becoming increasingly important in the era of image-guided radiotherapy. The Real-Time Position Management System (RPM, Varian Medical System, Palo Alto, CA) is commercially available gating equipment which is popular around the world. This system uses a plastic box with a reflective marker which is placed on the patient's abdomen to detect the vertical motion by an infrared camera. However, this method with such a single point detector has several limitations to obtain the accurate respiratory motion. We developed the novel substitution device for this plastic box of the RPM system to monitor the patient's body volume change as an external respiration signal. The “Air-bag System”, a novel respiratory monitoring device, consists of a non-elastic air bag connected with an elastic air bag. A non-elastic air bag is placed between the patient's body and the stereotactic fixation frame. A current meter located between two bags measures the air flows back and forth between two bags. The data are processed by personal computer and translated the respiratory motion into swinging decoy markers displayed on the computer screen so that an infrared camera of RPM system can capture the swinging marker instead of an infrared reflective marker. 4D-CT data of 15 lung cancer patients were acquired by the RPM system with the “Air-bag System”. On each phase of the 4D data, correlations between the lung volume and the body volume were evaluated. 4D-CT data of another 3 lung cancer patients were acquired by the RPM system recording with two different respiratory motions. One is an abdominal respiratory motion using an infrared reflective marker placed on the patient's abdomen, the other one is a thorax respiratory volume shift using the air bag set on the patient's chest. The respiratory phases (8 phases) at each instant in time based on modeling two different motion amplitudes were compared. There was a strong correlation (r2 = 0.976) between the lung volume and the body volume on each respiratory phase. Comparing the reconstructed 4D-CT images on each respiratory phase recorded by using an infrared reflective marker to those recorded by using the “Air-bag System” resulted in no difference on all respiratory phases. The novel respiratory gating device which monitors the patient's body volume as an external respiration signal was developed. Recording with this “Air-bag System”, applicability of RPM system is not limited owing to enough amplitude and more accurate determination of the magnitude of respiratory motion. Moreover, abdominal compression with the air bag reduces the movement of diaphragm which can decrease the intra-thoracic organ motion without loading additional stress on the patient.

Commissioning and quality assurance for a respiratory training system based on audiovisual biofeedback

A respiratory training system based on audiovisual biofeedback has been implemented at our institution. It is intended to improve patients' respiratory regularity during four‐dimensional (4D) computed tomography (CT) image acquisition. The purpose is to help eliminate the artifacts in 4D‐CT images caused by irregular breathing, as well as improve delivery efficiency during treatment, where respiratory irregularity is a concern. This article describes the commissioning and quality assurance (QA) procedures developed for this peripheral respiratory training system, the Stanford Respiratory Training (START) system. Using the Varian real‐time position management system for the respiratory signal input, the START software was commissioned and able to acquire sample respiratory traces, create a patient‐specific guiding waveform, and generate audiovisual signals for improving respiratory regularity. Routine QA tests that include hardware maintenance, visual guiding‐waveform creation, auditory sounds synchronization, and feedback assessment, have been developed for the START system. The QA procedures developed here for the START system could be easily adapted to other respiratory training systems based on audiovisual biofeedback.PACS number: 87.56.Fc

SU‐GG‐J‐94: Can We Predict the Severity of Artifacts on 4DCT from the Respiratory Irregularity during 4DCT Image Acquisition?

Purpose: To evaluate the correlation between the severity of artifacts on 4DCT and patient's respiratory irregularity. Method and Materials: The 4DCT from 10 different patients were studied. These patients went through two consecutive procedures in a single session: 4DCT with free breathing (FB) followed by another 4DCT with audio and visual (AV) instructions. Phillips big bore Multislice CT scanner with 4D CT capabilities were used to acquire the 4DCT scans. Real Time Position Management system by Varian was used to capture patient's breathing trace during 4DCT acquisition. Describe your methods to quantify 4DCT artifact and respiratory irregularity. Results: Our results of all ten patients indicate that there is an obvious correlation between severity of artifact and breathing irregularity. For each individual patient, the severity of the artifact increases with the breathing irregularity for 4D CT with free breathing. However, it is difficult to use a universal function to describe the correlation among all patients. The most possible cause can lay on the unique dependence of internal organ motion on external motion of each patient. Our results also indicate that the AV coaching helps to reduce the breathing irregularity during the CT acquisition and therefore results in mitigated artifact in 4DCT. Conclusion: We conclude that it is feasible to predict the severity of the artifacts on 4DCT by quantification of breathing irregularity. We also found coaching is helpful to reduce the breathing irregularity. Conflict of Interest: Research sponsored by Philips.

Evaluation of delivered monitor unit accuracy of gated step‐and‐shoot IMRT using a two‐dimensional detector array

To overcome the problem of organ motion in intensity-modulated radiation therapy (IMRT), gated IMRT is often used for the treatment of lung cancer. In this study, the authors investigated the accuracy of the delivered monitor units (MUs) from each segment during gated IMRT using a two-dimensional detector array for user-specific verification purpose. The authors planned a 6 MV photon, seven-port step-and-shoot lung IMRT delivery. The respiration signals for gated IMRT delivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the Millennium 120 MLC. The MatriXX (IBA Dosimetry GmbH, Germany) was validated through consistency and reproducibility tests as well as comparison with measurements from a Farmer-type ion chamber. The authors delivered beams with varying dose rates and duty cycles and analyzed the MatriXX data to evaluate MU delivery accuracy. There was quite good agreement between the planned segment MUs and the MUs computed from the MatriXX within +/- 2% error. The beam-on times computed from the MatriXX data were almost identical for all cases, and they matched well with the RPM beam-on and beam-off signals. A slight difference was observed between them, but it was less than 40 ms. The gated IMRT delivery demonstrated an MU delivery accuracy that was equivalent to ungated IMRT, and the delivered MUs with a gating signal agreed with the planned MUs within +/- 0.5 MU regardless of dose rate and duty cycle. The authors can conclude that gated IMRT is able to deliver an accurate dose to a patient during a procedure. The authors believe that the methodology and results can be transferred to other vendors' devices, particularly those that do not provide MLC log data for a verification purpose.

Real-time dynamic MLC tracking for inversely optimized arc radiotherapy

Background and purpose Motion compensation with MLC tracking was tested for inversely optimized arc radiotherapy with special attention to the impact of the size of the target displacements and the angle of the leaf trajectory. Materials and methods An MLC-tracking algorithm was used to adjust the MLC positions according to the target movements using information from an optical real-time positioning management system. Two plans with collimator angles of 45° and 90°, respectively, were delivered and measured using the Delta 4® dosimetric device moving in the superior–inferior direction with peak-to-peak displacements of 5, 10, 15, 20 and 25 mm and a cycle time of 6 s. Results Gamma index evaluation for plan delivery with MLC tracking gave a pass rate higher than 98% for criteria 3% and 3 mm for both plans and for all sizes of the target displacement. With no motion compensation, the average pass rate was 75% for plan 1 and 70% for plan 2 for 25 mm peak-to-peak displacement. Conclusion MLC tracking improves the accuracy of inversely optimized arc delivery for the cases studied. With MLC tracking, the dosimetric accuracy was independent of the magnitude of the peak-to-peak displacement of the target and not significantly affected by the angle between the leaf trajectory and the target movements.