Respiratory Gating System Research Articles

Respiratory-related evoked potential measures of respiratory sensory gating

The purpose of this study was to demonstrate a neural respiratory gating system using a paired stimuli paradigm. The N1 peak of the respiratory-related evoked potential (RREP) represents early perceptual processing of respiratory sensory information. This is similar to the N100 peak shown with tactile sensation, where the second peak amplitude (S2) of the N100 peak from the somatosensory evoked potential (SEP) was smaller than the first peak amplitude (S1) when the stimuli were presented 500 ms apart. We hypothesized that paired inspiratory occlusions would result in a reduced amplitude of the S2 N1 RREP peak amplitude, indicating respiratory central neural gating. Twenty healthy subjects (10 men and 10 women; 25.8 +/- 6.5 yr old) completed the paired inspiratory occlusion (RREP) trial. Thirteen of the subjects also completed the paired mouth air puffs [mouth-evoked potential (MEP) trial], and the paired hand air puffs (SEP) trial. All paired presentations were separated by 500 ms. The N1 peak amplitudes of the RREP trial and the N100 peak amplitudes of the MEP and SEP trials for S1 and S2 and the S2/S1 ratios were determined. The S1 RREP N1 peak amplitude was significantly greater than S2, and the S2/S1 ratio was 0.43. The S1 MEP and SEP N100 peak amplitudes were significantly greater than S2, and the N100 ratio was 0.49 and 0.49, respectively. These results are consistent with central neural gating of respiratory afferent input. The RREP gating response is similar to somatosensory mechanoreceptor gating.

Time-of-flight sensor for respiratory motion gating

In this technical note we present a system that uses time-of-flight (ToF) technology to acquire a real-time multidimensional respiratory signal from a 3D surface reconstruction of the patient's chest and abdomen without the use of markers. Using ToF sensors it is feasible to acquire a 3D model in real time with a single sensor. An advantage of ToF sensors is that their high lateral resolution makes it possible to define multiple regions of interest to compute an anatomy-adaptive multidimensional respiratory signal. We evaluated the new approach by comparing a ToF based respiratory signal with the signal acquired by a commercially available external respiratory gating system and achieved an average correlation coefficient of 0.88.

SU-GG-T-214: On Technical Aspects of the Implementation of a New Respiratory Gating System with the Option of Three-Dimensional Tracking of a Surrogate's Motion Via a Wall-Mounted Camera

Purpose: To optimize performance of a new respiratory gating system with 3D tracking of a surrogate and to evaluate the performance of the system in clinical conditions. Method and Materials: An accuracy of the system was checked by calibrating the system using water parallel surface and then locating a surrogate at different positions on the surface and detecting its position. A clinically representative bending profile of a couch was found. While respecting restrictions resulting from the calibration procedure, three points on the profile that ensure the profile's optimal linear fit were selected. The system was recalibrated by positioning a surrogate into the found positions and the system's performance was checked. The longitudinal speed and the AP position of a surrogate were recorded simultaneously to quantify observed artifacts when the surrogate was moving in the longitudinal direction. Results: The values of the AP position of a surrogate on the leveled plane change less than ±1 mm (SD of a readout is 0.18 mm). The measured bending profiles exhibit non‐linearity and changes in the AP position are up to 8 mm (weight of 60 kg). After the proposed calibration, couch induced changes in the AP position during a CT acquisition are less than ±1 mm. The AP position readout varies linearly with the longitudinal speed of a surrogate (approx. 0.6 mm change at the speed of 10 mm/s). Conclusion: The new system enables an accurate readout of a surrogate's position. However, its accuracy is limited by the presumption that the surrogate's motion induced by a couch happens within a plane. A mean how to bring the model closest to the clinical conditions was proposed. Dependence of the AP position readout on the surrogate's longitudinal speed is an inconvenient property of the system, which has a little impact in clinical conditions though.

SU-GG-T-378: Development of Error Analysis Program for Phase-Based Respiratory Gating Radiation Therapy

Purpose: The RPM respiratory gating system (Varian, USA) establishes irradiation region using the amplitude-based or phase-based method. Although phase-based method is preferred because of the stability in the real treatment conditions, it has some limits to explain the exact correlation between the marker motion and organ motion. Even when the variation of amplitude which can introduce target motion considered as an error is produced, the phase-based method has the possibility to irradiate including the error positions. In this study, the error analysis program was developed for the verification of the tumor position's variation correlated with the variation of marker's amplitude which can be occurred during a phase-based respiratory gating treatment. Method and Materials: The analyzed program was created with the IDL5.5 (Research Systems Inc., USA) in order to evaluate the errors based on the log files which were recorded during a respiratory gated radiation therapy with The analysis program was verified with a virtual treatment record file and with a record file using moving phantom which were modified considering the irregular amplitude's variation simulating the real clinical situations. Results: In both cases, accurate discrimination of error points and error calculation were produced. When the treatment record files of a real patient were analyzed with the program, the accurate recognition and calculation of the error points were also verified. Conclusion: The analysis program developed in this study can be evaluated as a useful tool for the analysis of errors due to the irregular variation of patients' respiration during the phase-base respiratory gating radiation treatment.

TU-EE-A3-02: Multi-Dimensional Prediction of Respiration Motion

Purpose: The purpose of this study was to develop a technique to dynamically predict tumor position and uncertainty in three dimensions in real time. Method and Measurements: A novel multi-dimensional time delay kernel regression (MD-TDKR) technique was developed to predict motion up to 1.5 seconds into the future. This technique uses historical data and the correlations between the multiple dimensions to predict target position, and allows for continuous memory updates to dynamically retrain the model. This technique could be used in a variety of motion prediction applications. Results: In one example, the algorithm inaccurately predicted tumor position, but was able to identify the corresponding uncertainty increase. When the model was unable to accurately predict position, the 95 percent uncertainty interval becomes quite large. Likewise, the predictions are accurate where the uncertainty is small. This information can be used in determining when to temporarily trigger the beam on-and-off. The average percent error was computed at different latencies. For predictions 1.5 seconds into the future the error was 7.83%, for 1 second it was 6.55%, and for 0.5 seconds it was only 5.27%. The average uncertainty for the predictions at 0.5, 1, and 1.5 seconds into the future was 4.99%, 6.18%, and 7.12%, respectively. Conclusion: This study shows that MD-TDKR can learn complex relationships in multidimensional respiration data. Because of its ease of building and updating the memory matrix, the model can adjust to new operating conditions, such as changes in a patients breathing pattern. It was also able to determine the uncertainty for a given query vector, allowing for the ability to hold the beam off when the prediction is unsure. These factors indicate that MD-TDKR has great potential for use in radiation oncology. It is also anticipated that the MD-TDKR paradigm could be easily implemented into commercial respiratory gating systems.

Examination of geometric and dosimetric accuracies of gated step‐and‐shoot intensity modulated radiation therapy

Due to the complicated technical nature of gated radiation therapy, electronic and mechanical limitations may affect the precision of delivery. The purpose of this study is to investigate the geometric and dosimetric accuracies of gated step-and-shoot intensity modulated radiation treatments (SS-IMRT). Unique segmental MLC plans are designed, which allow quantitative testing of the gating process. Both ungated and gated deliveries are investigated for different dose sizes, dose rates, and gating window times using a commercial treatment system (Varian Trilogy) together with a respiratory gating system [Varian Real-Time Position Management system]. Radiographic film measurements are used to study the geometric accuracy, where it is found that with both ungated and gated SS-IMRT deliveries the MLC leaf divergence away from planned is less than or equal to the MLC specified leaf tolerance value for all leafs (leaf tolerance being settable from 0.5-5 mm). Nevertheless, due to the MLC controller design, failure to define a specific leaf tolerance value suitable to the SS-IMRT plan can lead to undesired geometric effects, such as leaf motion of up to the maximum 5 mm leaf tolerance value occurring after the beam is turned on. In this case, gating may be advantageous over the ungated case, as it allows more time for the MLC to reach the intended leaf configuration. The dosimetric precision of gated SS-IMRT is investigated using ionization chamber methods. Compared with the ungated case, it is found that gating generally leads to increased dosimetric errors due to the interruption of the "overshoot phenomena." With gating the average timing deviation for intermediate segments is found to be 27 ms, compared to 18 ms for the ungated case. For a plan delivered at 600 MU/min this would correspond to an average segment dose error of approximately 0.27 MU and approximately 0.18 MU for gated and ungated deliveries, respectively. The maximum dosimetric errors for individual intermediate segments are found to deviate by up to approximately 0.64 MU from their planned value when delivered at 600 MU/min using gating, this compares to only approximately 0.32 MU for the ungated case.

Quality assurance device for four‐dimensional IMRT or SBRT and respiratory gating using patient‐specific intrafraction motion kernels

Emerging technologies such as four‐dimensional computed tomography (4D CT) and implanted beacons are expected to allow clinicians to accurately model intrafraction motion and to quantitatively estimate internal target volumes (ITVs) for radiation therapy involving moving targets. In the case of intensity‐modulated (IMRT) and stereotactic body radiation therapy (SBRT) delivery, clinicians must consider the interplay between the temporal nature of the modulation and the target motion within the ITV. A need exists for a 4D IMRT/SBRT quality assurance (QA) device that can incorporate and analyze customized intrafraction motion as it relates to dose delivery and respiratory gating. We built a 4D IMRT/SBRT prototype device and entered (X, Y, Z)(T) coordinates representing a motion kernel into a software application that transformed the kernel into beam‐specific two‐dimensional (2D) motion “projections,”previewed the motion in real time, anddrove a precision X–Y motorized device that had, atop it, a mounted planar IMRT QA measurement device. The detectors that intersected the target in the beam's‐eye‐view of any single phase of the breathing cycle (a small subset of all the detectors) were defined as “target detectors” to be analyzed for dose uniformity between multiple fractions. Data regarding the use of this device to quantify dose variation fraction‐to‐fraction resulting from target motion (for several delivery modalities and with and without gating) have been recently published. A combined software and hardware solution for patient‐customized 4D IMRT/ SBRT QA is an effective tool for assessing IMRT delivery under conditions of intrafraction motion. The 4D IMRT QA device accurately reproduced the projected motion kernels for all beam's‐eye‐view motion kernels. This device has been proved to• effectively quantify the degradation in dose uniformity resulting from a moving target within a static planning target volume, and• integrate with a commercial respiratory gating system to ensure that the system is working effectively.Such a device is discussed as a potential tool to optimize the gating duty cycle to maximize delivery efficiency while minimizing dose variability.PACS numbers: 87.50.Gi, 87.53.Dq, 87.53.Kn, 87.53.Mr, 87.53.Tf, 87.56.Fc, 87.58.Sp

D7-01: 4-dimensional respiration-gated radiotherapy for concurrent chemo-radiotherapy in lung cancer

4-dimensional respiration-gated radiotherapy for concurrent chemo-radiotherapy in lung cancer Senan, Suresh1 Spoelstra, Femke O.1 Phernambucq, Erik C.2 van Sornsen de Koste, John R.1 Cuijpers, Johan C.1 Smit, Egbert F.2 Slotman, Ben J.1 Lagerwaard, Frank J.1 1 Department of Radiation Oncology, VU University Medical Center, Amsterdam, The Netherlands 2 Department of Pulmonology, VU University Medical Center, Amsterdam, The Netherlands Background: Concurrent chemo-radiotherapy (CT-RT) is the treatment of choice in locally advanced lung cancer, but it is associated with increased esophageal and lung toxicity. Toxicity can be reduced using respiration-gated radiotherapy (RGRT) in which treatment is delivered during pre-selected phases of the respiratory cycle, thereby allowing for smaller target volumes. The feasibility of such interventions for motion management during thoracic CT-RT is unknown. We studied the compliance and toxicity of RGRT in this setting. Methods: Details of gated concurrent CT-RT were analyzed in 31 consecutive patients with a minimum follow-up of 3 months. They included 28 with locally-advanced non-small cell lung cancer (LANSCLC) and 3 with limited-stage SCLC. A subgroup of LA-NSCLC patients underwent induction CT-RT, followed by mediastinal re-staging and surgery in downstaged patients. Concurrent chemotherapy consisted of either 2-3 cycles of cisplatin-etoposide or weekly cycles of cisplatin-docetaxel. Involved-field radiotherapy was delivered to a target volume defined using a 4-dimensional (4D) CT scan, FDG-PET and biopsy-proven nodal disease. The Varian Real-time Position Management respiratory gating system (Varian Medical Systems) was used for 4DCT acquisition, and for treatment delivery. A dose of 45-60 Gy was delivered in once-daily fractions of 1.8-2.0 Gy on a Varian 2100 C/D linear accelerator. RGRT was performed using either (i) audio-coached, phase-based gating in which radiation delivery is limited to the endinspiratory phase or (ii) amplitude-based gating in which radiation is delivered within the breathing threshold levels observed during 4DCT imaging. Audio-coached, end-inspiratory gating was the procedure of choice when tumour motion exceeded 7.5 mm and/or when a significant increase in total lung volume was expected. All other patients were treated with amplitude-based gating. Results: Only 1 patient, who died of pulmonary haemorrhage during treatment, failed to complete the planned treatment. Ten patients who were downstaged underwent surgical resection. The median follow-up was 5 months. The median planning target volume (PTV) was 649.9 cc (range 274-1616). The percentage of lung volume receiving a threshold dose of 20 Gy (V20) ranged from 12.4 44.6%. The median oesophageal circumference receiving 46 Gy was 6.6 (range 0-13.5) cm. CTC grade 3 esophagitis was observed in 23% (7 pts), and 52% (16 pts.) developed grade 2 esophagitis. The mean weight loss was 2.9 kg (range 0.5-10) and 13 patients treated with preoperative CT-RT received a feeding tube, which was a prophylactic measure in 6 patients. CTC grades 2 and 3 radiation pneumonitis developed in 2 patients at 4 and 3 months, respectively, post-treatment. At the last follow-up, 23 patients were free from disease progression. Five patients had extrathoracic metastases, with 4 having exclusively distant metastasis. Loco-regional failure was observed in 3 patients, with two being out-of-field recurrences. Conclusion: RGRT is well tolerated in patients undergoing concurrent CT-RT for lung cancer. The treatment drop-out rate of 3% in this unselected cohort compares favourably with the rates of 16-19% reported in phase III trials in LA-NSCLC [Fournel 2005; Albain 2005]. Updated results at a median follow-up of 11 months will be available.

A respiratory‐gated treatment system for proton therapy

Proton therapy offers the potential for excellent dose conformity and reduction in integral dose. The superior dose distribution is, however, much more sensitive to changes in radiological depths along the beam path than for photon fields. Respiratory motion can cause such changes for treatments sites like lung, liver, and mediastinum and thus affect the proton dose distribution significantly. We have implemented and commissioned a respiratory-gated system for range-modulated treatment fields. The gating system was designed to ensure that each gate always contains complete modulation cycles so that for any beam segment the delivered dose has the planned depth-dose distribution. Measurements have been made to estimate the time delays for the various components of the system. The total delay between the actual motion and the beam on/off control is in the range of 65-195 ms. Time-resolved dose measurements and film tests were also conducted to examine the overall gating effect.

SU‐FF‐T‐419: Time‐Delay Measurement of Phase Gated Treatment with a Varian Real‐Time Position Management System Using Computed Radiography

Purpose: Respiratory gating systems have been developed to reduce treatment uncertainty caused by organ motions related to respiration. The time‐delay between when the gating signal is triggered and when the beam is turned on may affect the actual radiation dose delivered. In this study, we investigated the time delay for a Varian real‐time position management(RPM) respiratory gating treatment system. Method and Materials: A motion phantom with infrared markers was used to simulate a respiration signal. An infrared camera was used to observe the motion of the phantom and to plot out a true position‐versus‐time curve. Computed Radiography(CR) images were taken with a Varian Clinac 21EX at 10% intervals of the simulated breathing cycle with a 2% window. On the CR images, the center of a metal disk attached to the phantom was used to determine the measured position of the phantom relative to a stationary disk. The RPM curve and measured curve were both normalized. The RPM curve was then shifted relative to the measured curve in steps of 0.1milliseconds, and correlation between the two curves was measured at each step using MATLAB. The time shift corresponding to the greatest correlation was defined as the time delay in the gating system. Results: Three measurements were taken and averaged (arithmetic mean) to determine the time delay. The time‐delay between the actual beam‐on time and the RPM beam‐on triggering signal was found to be 0.513 ± 0.077 seconds. Conclusion: This work shows that there is a significant time delay between the gating signal and the time when the radiation beam is turned on for the Varian RPM system. The effect of time‐delay on patient dosimetry needs to be further investigated.Partially supported by Varian research grant.

TU‐D‐M100F‐06: Temporal Precision of Gated Step‐And‐Shoot Intensity Modulated Radiation Therapy

Purpose: The use of gating in the delivery of radiation therapy (RT) has been shown as an effective means of treating tumors susceptible to motion. However, due to electronic and mechanical limitations of the radiation delivery equipment, the precision of gated IMRT is susceptible to equipment process time delays. The purpose of this study is to investigate the temporal accuracies of gated RT using a simple and inexpensive method that doesn't require complex or fast sampling equipment to measure response times. Method and Materials: Using a Varian Trilogy treatment delivery system together with a Varian RPM respiratory gating system, the response times of both normal RT and step‐and‐shoot (SS) IMRT gated deliveries are investigated. Radiation dosage profiles, recorded using radiographic films, are digitalized to allow extraction of temporal data. In addition the same response time technique is used with an electronic portal imagining device (EPIDs). Results: Static to dynamic comparison of specific MLC patterns is used to extract the gating response time. It is found that the time, from when an object enters into the specified phase to when the radiation beam is actually turned on, is approximately 0.15 s. This time error is found to be systematic in nature and is approximately the same for both normal and SS‐IMRT gated deliveries. Conclusion: A general and simple procedure has been developed that will allow the response time of a LINAC based gating systems to be made. The ∼150 ms systematic timing error found to occur in the Trilogy/RPM system can be corrected by modification of the gating software provided if the patients breathing is regular and periodic.

TU‐C‐M100J‐10: Respiration Motion Prediction Using Time‐Delay Kernel Regression Modeling

Purpose: The purpose of this study was to develop a novel technique for dynamically predicting respiration motion and uncertainty up to 1.5 seconds in the future in real‐time using Time‐Delay Kernel Regression (TDKR) modeling. Unlike neural network based prediction techniques, kernel regression models are continuously learning new respiration cycles for each patient without the need for computationally and time intensive re‐training. Method and Materials: Recorded respiration data from a real‐time respiratory gating system was used to develop a model to predict the amplitude of the marker block at a future time. An empirical TDKR model was designed to compensate for the latency that occurs between the acquisition of the image and the point at which the beam actually turns on/off in beam tracking systems. This non‐parametric model incorporates the temporal information present in the input data. The model was tested using respiration data from 4 patients. Results: The rootmean squared error (RMSE) between the model prediction's and the measured data was computed for each patient at different latencies, and then the average was taken over all the patients. For predictions 1.5 seconds into the future the average RMSE was 1.4%. For predictions 1 second into the future, the RMSE dropped to 1.2%, and for 0.5 seconds it was only 0.7%. The average uncertainty for the predictions at 0.5, 1, and 1.5 seconds into the future was 2.4%, 3.2%, and 3.4%, respectively. Conclusion: This study proves that a TDKR model can learn the relationships present in respiration data. The reported results showed that the TDKR model has the same, if not better predictive performance, as previously studied parametric models. However, because TDKR is non‐parametric, it has several distinct advantages over these models that make it more suited for respiratory gating applications.

TU‐D‐L100J‐04: Respiration Motion Correction in Helical Tomotherapy Imaging

Purpose: In helical tomotherapy, Megavoltage CT (MVCT) images are acquired at a rate of one transverse slice per five seconds. The slow acquisition of raw data results in CT images with varying degrees of motion‐induced artifacts. In addition, the high energy of the x‐ray beam (4–6 MV) presents unique challenges for image reconstruction. The purpose of this work was to correct for motion in the superior‐inferior (S/I) direction using retrospective manipulation of the unprocessed sinogram data. Method and Materials: A respiratory motion platform with a S/I motion range of 3‐cm, and periods of 2, 4 and 6 seconds was used to move an ACR CT accreditation phantom and a anthropomorphic thorax phantom in the superior‐inferior direction. The motion of the respiratory platform was measured using the Varian respiratory gating system. Sinogram data was acquired axially with the couch stationary for 5 complete rotations of the gantry. The respiration cycle was synchronized to the projections in the raw sinogram so that the position of the moving object was known in each projection. Motion correction was performed by identifying and extracting projections occurring within a specified respiration phase, correcting for missing projections, and then reconstructing the CT images. Results: Images acquired with the ACR CT phantom showed considerable improvement with motion correction using temporal re‐binning over the uncorrected images. Images acquired with the RANDO phantom suffered from greater image artifacts than the ACR phantom because of the larger variation in the phantom's anatomy over the 3‐cm range of S/I motion. Improvement was seen for each period, however only slight improvement was seen for the 2‐second period of motion. Conclusion: Temporal re‐binning can successfully reduce S/I motion artifacts in reconstructed MVCT images. Conflict of Interest: This research is supported by TomoTherapy, Inc.

SU-FF-J-78: 3-D Target Localization at the End of Expiration Prior to Respiration-Gated Radiotherapy Treatment

Objective: Many tumors in lung and liver can not be visualized with 2‐D on‐board kV imaging. For these patients receiving gated‐treatment, currently there is no tool to assess 3‐D target position at the end of expiration before gated‐treatment. The study explores the feasibility of using (a) free breathing CBCT, and (b) phase‐selective reconstruction of “gated” CBCT to determine 3‐D target position at the end of expiration. Methods: A motor controlled moving phantom was used to simulate longitudinal target motion of 2.0 cm at breathing rate of 2–4 sec/cycle. Images of static phantom as well as moving phantom were acquired on simulation CT and CBCT. Target position in gated CTimages was compared to that of free breathing CBCT. The start of CBCT beam‐on at initial gantry angle was synchronized with respiratory motion recorded using a commercial respiratory gating system. Results: Static target position in CBCT agrees with gated CT (except for 2 sec/cycle motion as found before). The moving target position in CBCT were similar for motion rates of 2–4 sec/cycle and their ITV volumes are dependent on image threshold. The target position at the end of expiration could be off ⩾1.0 cm if defined by full‐width‐half‐maximum (FWHM), and ∼ 0.15 cm off if defined by FWTM. Target position in “gated” CBCT agree with gated CT at end of expiration, but image quality is dependant on sample number of gantry angle, in this phantom test, a minimum of 3 minute acquisition (3 sec/cycle motion) was required to avoid substantial alias artifacts. Conclusion: Determination of 3‐D target position at the end of expiration before gated treatment needs to be addressed. While clinical images are more complicated, free breathing CBCT or phase‐selective reconstruction of CBCT can be helpful as a QA tool and deserve to be further explored.

Intensity modulation with respiratory gating for radiotherapy of the pleural space

We wanted to describe a technique for the implementation of intensity-modulated radiotherapy (IMRT) with a real-time position monitor (RPM) respiratory gating system for the treatment of pleural space with intact lung. The technique is illustrated by a case of pediatric osteosarcoma, metastatic to the pleura of the right lung. The patient was simulated in the supine position where a breathing tracer and computed tomography (CT) scans synchronized at end expiration were acquired using the RPM system. The gated CT images were used to define target volumes and critical structures. Right pleural gated IMRT delivered at end expiration was prescribed to a dose of 44 Gy, with 55 Gy delivered to areas of higher risk via simultaneous integrated boost (SIB) technique. IMRT was necessary to avoid exceeding the tolerance of intact lung. Although very good coverage of the target volume was achieved with a shell-shaped dose distribution, dose over the targets was relatively inhomogeneous. Portions of target volumes necessarily intruded into the right lung, the liver, and right kidney, limiting the degree of normal tissue sparing that could be achieved. The radiation doses to critical structures were acceptable and well tolerated. With intact lung, delivering a relatively high dose to the pleura with acceptable doses to surrounding normal tissues using respiratory gated pleural IMRT is feasible. Treatment delivery during a limited part of the respiratory cycle allows for reduced CT target volume motion errors, with reduction in the portion of the planning margin that accounts for respiratory motion, and subsequent increase in the therapeutic ratio.

Assessment of Respiratory Tumor Movement using 4D Computed Tomography for Stereotactic Radiosurgery in Lung Tumor

Purpose: To assess the respiratory tumor movement using 4D-CT (4-dimensional computed tomography) for minimizing setup and target volume uncertainty of body-frame based stereotactic radiosurgery (SRS) in lung tumor. Materials and Methods: Fifty-seven stereotactic radiation therapies with respiratory gating system in 44 patients (two targets in seven patients and three in three patients) were executed in Asan Medical Center from May 2005 to June 2006. We used respiratory gating system consisted of RPM (Real-time Positioning Management system, Varian, USA) and 4D-CT (GE healthcare, USA), if tumor movement was exceeding 5 mm by respiration on fluoroscopy. Accurate tumor movement on reconstructed 4D-CT image was determined for respiratory gated therapy. Respiratory gated therapy was done if tumor movement was exceeding 5 mm, and non-gated therapy was done if it was below 5 mm. Results: Forty-five tumors were treated with supine position, and the other twelve were with prone position. Median tumor movement (3-dimensional) by respiration was 8.78±5.30 mm, and it was mostly affected by superior-inferior movement (8.53±5.23 mm). Tumor movements were different by tumor location, whether upper (5.38±2.85 mm) or lower (10.12±5.08 mm) lobe (p=0.015). Tumor movement was exceeding 5 mm in 27 (47.3%) tumors, and below 5 mm in 30 tumors in 4D-CT evaluation. Tumor movements on adopted respiratory gated phase were wholly below 5 mm, and its median value was 3.70±1.13 mm. Conclusion: Assessment of respiratory tumor movement using 4D-CT and gating system was helpful for minimizing target volume uncertainty. As a result, image-guided radiation therapy could improve the treatment accuracy of high precision stereotactic radiosurgery. (J Lung Cancer 2007; 6(1):24 �� 28)

An algorithm for automatic determination of the respiratory phases in four-dimensional computed tomography

Recently, several techniques have been developed to improve the quality of computed tomography (CT) images of the thoracic and abdominal region that are degraded by the interference of the scanning process and respiration. Several devices for respiratory-correlated CT are available for clinical usage. They are based on the synchronization of the acquired CT image data with the respiratory motion using a signal from an external respiratory monitoring system. In this work, some practical aspects of clinical implementation of the multi-slice 4D CT scanner Somatom Sensation Open (Siemens Medical Solutions, Erlangen, Germany) equipped with a respiratory gating system (RGS) AZ-733V (Anzai Medical, Tokyo, Japan) are discussed. A new algorithm developed for automatic respiratory phase determination needed for the reconstruction of the 4D CT images is presented.

Integrating respiratory gating into a megavoltage cone‐beam CT system

We have previously described a low-dose megavoltage cone beam computed tomography (MV CBCT) system capable of producing projection image using one beam pulse. In this study, we report on its integration with respiratory gating for gated radiotherapy. The respiratory gating system tracks a reflective marker on the patient's abdomen midway between the xiphoid and umbilicus, and disables radiation delivery when the marker position is outside predefined thresholds. We investigate two strategies for acquiring gated scans. In the continuous rotation-gated acquisition, the linear accelerator (LINAC) is set to the fixed x-ray mode and the gantry makes a 5 min, 360 degree continuous rotation, during which the gating system turns the radiation beam on and off, resulting in projection images with an uneven distribution of projection angles (e.g., in 70 arcs each covering 2 degrees). In the gated rotation-continuous acquisition, the LINAC is set to the dynamic arc mode, which suspends the gantry rotation when the gating system inhibits the beam, leading to a slightly longer (6-7 min) scan time, but yielding projection images with more evenly distributed projection angles (e.g., approximately 0.8 degrees between two consecutive projection angles). We have tested both data acquisition schemes on stationary (a contrast detail and a thoracic) phantoms and protocol lung patients. For stationary phantoms, a separate motion phantom not visible in the images is used to trigger the RPM system. Frame rate is adjusted so that approximately 450 images (13 MU) are acquired for each scan and three-dimensional tomographic images reconstructed using a Feldkamp filtered backprojection algorithm. The gated rotation-continuous acquisition yield reconstructions free of breathing artifacts. The tumor in parenchymal lung and normal tissues are easily discernible and the boundary between the diaphragm and the lung sharply defined. Contrast-to-noise ratio (CNR) is not degraded relative to nongated scans of stationary phantoms. The continuous rotation-gated acquisition scan also yields tomographic images with discernible anatomic features; however, streak artifacts are observed and CNR is reduced by approximately a factor of 4. In conclusion, we have successfully developed a gated MV CBCT system to verify the patient positioning for gated radiotherapy.

SU‐FF‐T‐432: Time‐Delay Measurement of a Varian Real‐Time Position Management System

Purpose: Respiratory gating systems have been developed to reduce treatment uncertainty caused by organ motions related to respiration. The time‐delay between when the gating signal is triggered and when the beam is turned on may affect the actual radiation dose delivered. In this study, we investigated the time delay for a Varian real‐time position management (RPM) respiratory gating system. Method and Materials: A motion phantom with infrared markers was used to simulate a respiration signal. A small metal target ball was also attached to the phantom. An infrared camera was used to observe the motion of the phantom and to plot out a true motion curve. A narrow 2% phase gating window was set at various phases of 0%, 10%, 20%… and 90%. Electronic Portal Vision images were taken with a Varian Linear Accelerator at various phases of the simulated breathing cycle, and the location of the target ball was then plotted verses time to give a measured motion curve. The shift in time between the RPM gating signal and motion curve from portal imager was then measured to determine the time delay in the gating system. Results: The time‐delay in Varian's gating system was found to be 0.0173 seconds. Conclusion: This work shows that there was a short time delay between the gating signal and the time when the radiation beam is turned on. The effect of time‐delay on patient dosimetry needs to be further investigated.Partially supported by Varian research grant.

TH‐D‐ValB‐06: Four‐Dimensional MVCT Reconstruction Using Temporal Re‐Binning

Purpose: To reconstruct 4D‐CT images using temporal re‐binning of sinogram projections from a slow‐CT scan of an object simulating respiratory motion. Methods and Materials: A slow‐CT scan of a battery‐powered motion phantom was taken using a helical tomotherapy machine. The motion phantom consisted of an elliptical disk with maximum diameter of 8.5‐cm, mounted on a spindle offset from the center of the major axis of the disk. The rotating spindle created a wobble in the disk's motion that simulated respiration. Atop the motion phantom, a CT resolution plug rested on a platform that moved vertically 1.5‐cm with a rotational period of 7 seconds. The plug was made of water equivalent material with seven holes of with diameters ranging from 2.00‐mm to 0.5‐mm. The vertical motion of the resolution plug was measured using a real‐time respiratory gating system. The CT image sinogram obtained from the machine detector was reconstructed using a program from the manufacturer of the helical tomotherapy machine. A program was written to shuffle sinogram projections, producing reconstructed slices over a range of movement (bin). The sorted slices were compared to the original CT images of the resolution plug. Not every bin contained a full gantry rotation of projections, and missing projections were replaced. Three replacement methods were used to obtain the best possible slice reconstructions. Results: The reconstructed CT slices showed improvement in resolution over the CT image of the plug in motion over the entire range of bins examined. Smaller bins with fewer missing projections had similar resolution to the static CT image of the plug. It was shown that bin enlargement showed the finest resolution of the projection replacement methods examined. Conclusions: Temporal re‐binning of slow‐CT sinogram projections can reduce motion artifacts and improve image resolution, but methods must be devised to accommodate for missing data.