Real-time Position Management Research Articles

Multileaf Collimator Position Accuracy of Respiratory Gated VMAT

Respiratory gated VMAT (volumetric modulated arc therapy) repeats rapid stop and go operations of a MLC (multileaf collimator) by turning the beam on and off by respiratory gating. The rapid stop and go operations of the MLC during respiratory gated irradiation may induce position error of the MLC and may affect output error and dose distribution. The purpose of this study was to clarify the relationship between the MLC position accuracy of the respiratory gated VMAT and the VMAT parameters. In the method, 1 arc, 2 arcs, and 4 arcs plan were created for the virtual target and irradiation was performed without the gated respiration and with the gated respiration. The respiratory gated system used a RPM (real-time position management system). The MLC position error, gap size error, and the MLC leaf speed were calculated from a log-file. In the histogram of the gap size error, the frequency of falling within the error range up to 0.2 mm was about 12 percentage points higher for the gated respiration. The MLC position error increased with increasing the MLC leaf speed. The correlation coefficient between the MLC leaf speed and the MLC position error exceeded 0.96, showing a strong correlation. Dose rate of VMAT parameters decreased with increasing arc number with the gated respiration and without the gated respiration. Gated irradiation was temporarily stopped, and it decreased by about 27% with respect to the dose rate without the gated respiration. The gantry rotation speed repeated the stop and re-rotation operations when gated irradiation was performed. For all arcs, the rotation speed decreased by about 30% compared with the rotation speed without the gated respiration. The pass rate of gamma analysis for each arc plan was about 95%. No effect on gated irradiation dose distribution was observed. Respiratory gated irradiation reduced dose rate change and gantry rotation speed of the VMAT. Reduction of the MLC leaf speed occurred, and the MLC position error and gap size error decreased. The MLC positional accuracy was secured, and it was confirmed that there was no effect on dose distribution by the respiratory gated VMAT.

Complicated respiratory motion analysis and new CT-based instruction model to enhance deep inspiration breath hold technique

Background: Adjuvant radiotherapy reduces local recurrence and improves overall survival of breast cancer. But adjuvant radiotherapy also leads to excess mortality caused by heart and lung disease. Deep inspiration breath hold (DIBH) technique has been proven to reduce radiation exposure to heart and lung. However, effectiveness of DIBH variates from each patient. This study collected one institutional data and further analyzed associated physical factors. Methods: Left breast cancer patients received left partial mastectomy followed by adjuvant radiotherapy with DIBH technique between Sep 1, 2013 and Sep 1, 2014 were included. Treatment plans based on both free breathing (FB) and DIBH computed tomography scan image for each patient were constructed. Physical factors including chest wall expansion at different levels were measured. Dose of ipsilateral, contralateral, and total lung, and heart were compared between FB and DIBH. Correlation between normal organ irradiated dose and physical factors change were calculated by Pearson correlation test. Results: Anterior-to-posterior diameter (APd) increase of lung at half upper chest wall level, upper heart border level, and half lower chest wall level, and extra lung length of DIBH had positive correlation with the reduction of irradiated lung dose. The increase of APd of lung at upper heart border level, half lower chest wall level, and right-to-left diameter (RLd) at diaphragm level also had positive correlation with the reduction of Dmax of heart (Pearson correlation r=0.795, 0.835, 0.655; P=0.006, 0.003, and 0.04, respectively) and Dmax of left anterior descending artery (LAD) (r=0.788, 0.795, 0.673; P=0.007, 0.006, and 0.033, respectively). Increase of free-contour heart height had negative correlation with V30, Dmax of heart, and Dmax of LAD. Conclusions: DIBH can reduce dose to critical organs. One-point chest wall motion monitored by Realtime Position Management TM (RPM) system can only represent the change of APd at diaphragm level and part of the change of half of lower chest wall level. Respiration is a complicated motion and multiple level monitoring system was suggested. Patient education to explain the idealist chest wall expansion condition with DIBH is important. Our CT-based instruction model provides a clearer instruction. This study suggested a new direction to evaluate and enhance DIBH performance.

Issues in quantification of registered respiratory gated PET/CT in the lung

PET/CT quantification of lung tissue is limited by several difficulties: the lung density and local volume changes during respiration, the anatomical mismatch between PET and CT and the relative contributions of tissue, air and blood to the PET signal (the tissue fraction effect). Air fraction correction (AFC) has been shown to improve PET image quantification in the lungs. Methods to correct for the movement and anatomical mismatch involve respiratory gating and image registration techniques. While conventional registration methods only account for spatial mismatch, the Jacobian determinant of the deformable registration transformation field can be used to estimate local volume changes and could therefore potentially be used to correct (i.e. Jacobian Correction, JC) the PET signal for changes in concentration due to local volume changes. This work aims to investigate the relationship between variations in the lung due to respiration, specifically density, tracer concentration and local volume changes. In particular, we study the effect of AFC and JC on PET quantitation after registration of respiratory gated PET/CT patient data. Six patients suffering from lung cancer with solitary pulmonary nodules underwent F-FDG PET/cine-CT. The PET data were gated into six respiratory gates using displacement gating based on a real-time position management (RPM) signal and reconstructed with matched gated CT. The PET tracer concentration and tissue density were extracted from registered gated PET and CT images before and after corrections (AFC or JC) and compared to the values from the reference images. Before correction, we observed a linear correlation between the PET tracer concentration values and density. Across all gates and patients, the maximum relative change in PET tracer concentration before (after) AFC was found to be 16.2% (4.1%) and the maximum relative change in tissue density and PET tracer concentration before (after) JC was found to be 17.1% (5.5%) and 16.2% (6.8%) respectively. Overall our results show that both AFC or JC largely explain the observed changes in PET tracer activity over the respiratory cycle. We also speculate that a second order effect is related to change in fluid content but this needs further investigation. Consequently, either AFC or JC is recommended when combining lung PET images from different gates to reduce noise.

Evaluation of the new respiratory gating system.

ObjectiveThe newly released Respiratory Gating for Scanners (RGSC; Varian Medical Systems, Palo Alto, CA, USA) system has limited existing quality assurance (QA) protocols and pertinent publications. Herein, we report our experiences of the RGSC system acceptance and QA.MethodsThe RGSC system integration was tested with peripheral equipment, spatial reproducibility, and dynamic localization accuracy for regular and irregular breathing patterns, respectively. A QUASAR Respiratory Motion Phantom and a mathematical fitting method were used for data acquisition and analysis.ResultsThe results showed that the RGSC system could accurately measure regular motion periods of 3–10 s. For irregular breathing patterns, differences from the existing Real-time Position Management (RPM; Varian Medical Systems, Palo Alto, CA) system were observed. For dynamic localization measurements, the RGSC system showed 76% agreement with the programmed test data within ±5% tolerance in terms of fitting period. As s comparison, the RPM system showed 66% agreement within ±5% tolerance, and 65% for the RGSC versus RPM measurements.ConclusionsNew functions and positioning accuracy improve the RGSC system’s ability to achieve higher dynamic treatment precision. A 4D phantom is helpful for the QA tests. Further investigation is required for the whole RGSC system performance QA.

Deep inspiration breath hold level variability and deformation in locoregional breast irradiation

PurposeThe purpose of this study was to evaluate the dosimetric effect of breath hold level variability and deformation on breast, chest wall, internal mammary chain (IMC) nodes, and heart. Methods and materialsLeft-sided post-lumpectomy (n = 12) and postmastectomy (n = 3) patients underwent deep inspiration breath hold (DIBH) and exhale breath hold (EBH) computed tomography (CT) scans. Forward-planned locoregional breast plans were created on the DIBH scan. Two effects were modeled assuming no setup uncertainties: residual motion within the gating window and systematically shallow breath hold levels (BHLs). Real-time position management (RPM) was used to monitor BHL at simulation and during treatment. The RPM data were scaled to simulate BHL variation within symmetric gating window widths of ±1, 3, 5, and 7 mm; the dosimetric impact of this motion was simulated in the treatment planning system. Systematically “shallow” BHL errors were modeled using deformable image registration to map the patient trajectory from DIBH to EBH (n = 12). The deformable vector fields were scaled to produce synthetic CT scans modeling patient position during breath holds 1, 3, 5, and 7 mm shallower than simulator BHL. The original treatment plans were applied to the synthetic CTs and dose was recalculated. ResultsAcceptable plan quality was maintained for most patients with motion within gating windows up to ±7 mm. Patients with shallow median BHLs experienced loss of coverage at simulated gating windows ±5 mm or larger. At systematic 3 mm shallow BHL error, 4/12 patients had clinical target volume IMC V80% < 99%; this increased to 11/12 patients at 5 mm. Change in heart dose from systematic BHL errors was negligible. ConclusionsMotion within gating windows has minimal dosimetric impact for most BHL variability; however, loss of IMC coverage can occur even for small gating windows when BHLs are systematically shallow. This can be mitigated by restricting lower BHL tolerances or accounting for known uncertainties in planning.

A novel dual gating approach using joint inertial sensors: implications for cardiac PET imaging

Positron emission tomography (PET) is a non-invasive imaging technique which may be considered as the state of art for the examination of cardiac inflammation due to atherosclerosis. A fundamental limitation of PET is that cardiac and respiratory motions reduce the quality of the achieved images. Current approaches for motion compensation involve gating the PET data based on the timing of quiescent periods of cardiac and respiratory cycles. In this study, we present a novel gating method called microelectromechanical (MEMS) dual gating which relies on joint non-electrical sensors, i.e. tri-axial accelerometer and gyroscope. This approach can be used for optimized selection of quiescent phases of cardiac and respiratory cycles. Cardiomechanical activity according to echocardiography observations was investigated to confirm whether this dual sensor solution can provide accurate trigger timings for cardiac gating. Additionally, longitudinal chest motions originating from breathing were measured by accelerometric- and gyroscopic-derived respiratory (ADR and GDR) tracking. The ADR and GDR signals were evaluated against Varian real-time position management (RPM) signals in terms of amplitude and phase. Accordingly, high linear correlation and agreement were achieved between the reference electrocardiography, RPM, and measured MEMS signals. We also performed a Ge-68 phantom study to evaluate possible metal artifacts caused by the integrated read-out electronics including mechanical sensors and semiconductors. The reconstructed phantom images did not reveal any image artifacts. Thus, it was concluded that MEMS-driven dual gating can be used in PET studies without an effect on the quantitative or visual accuracy of the PET images. Finally, the applicability of MEMS dual gating for cardiac PET imaging was investigated with two atherosclerosis patients. Dual gated PET images were successfully reconstructed using only MEMS signals and both qualitative and quantitative assessments revealed encouraging results that warrant further investigation of this method.

Three-Year Outcome Evaluation of Deep Inspiration Breath Hold Technique in Post–Radiation Therapy Patients After Left-Sided Breast-Conserving Surgery by Echocardiography With Quantitative Tissue Velocity Imaging

This study aims to evaluate the benefits of deep inspiration breath hold (DIBH) technique over free breathing (FB) by quantitative tissue velocity imaging(QTVI) in patients with left-sided breast cancer. Between 2012 and 2014, 68 patients with T1–2N0 breast cancer (BC) received whole breast radiotherapy (RT) after breast-conserving surgery (BCS) in our hospital. Patients treated with adjuvant chemotherapy were excluded. 30 patients treated with FB（A group and 38 patients took the DIBH technique (B group) using Varian real-time position management (RPM) system. The prescribed dose was 46 Gy in 23 fractions for the whole breast and 14Gy in 7 fractions for resection cavity. Each patient received the QTVI in the initial day of RT and 3 years after RT. The following data are acquired: The diameter of left atrial and late diastolic left ventricle (LV), the thickness of interventricular septum and LV posterior wall, LV ejection fraction, the peak velocity of mitral inflow (E/A). Regional velocity profiles of 6 walls of LV. The peak velocity of systolic (Vs), early diastolic velocity (Ea), late diastolic velocity(Aa) and the ratio of Ea and Aa(Ea/Aa). Parameters of interest about RT plan, cardiac V5, mean LAD dose, maximum LAD dose, and mean heart dose were calculated and analyzed. All enrolled patients completed the study. The cardiac V5, mean LAD dose, maximum LAD dose, and mean heart dose in RT plan of A group were always significant lower than those of B group (P<0.05). The differences of all parameters about QTVI between A and B group were not significant before radiotherapy. Compared with A group after radiotherapy, E/A、Vs、Ea/Aa were significantly reduced in B group after radiotherapy (P<0.05). This study shows that there is markedly reduced cardiac exposure in patients with DIBH as compared to FB. At the end of 3rd year, radioactive heart damage was significantly less in patients using DIBH technique than FB. The DIBH is a clinically useful heart protection for radiation therapy of left-sided breast cancer after BCS. Keyword: DIBH; radioactive heart damage; echocardiography; QTVI.

A Tool for Characterizing Randomness and Irregularity in Patient Respiratory Motion

Patient random irregular breathing undermines the fundamental assumption for 4DCT and radiotherapy based on it, either ITV-based or gated treatments. During patient’s 4DCT simulation, Respiratory Gating Signal Waveform (RGSW) is usually collected over a duration much longer than one breathing cycle. Therefore, RGSW may contain valuable information on irregularities in the patient breathing cycles and help to improve imaging and delivery in 4D radiation therapy. In this study, we developed a RGSW based tool for quantitatively accessing the randomness and irregularity level in patient respiratory motion. Totally 242 clinical RGSWs obtained with Varian Real-time Position Management (RPM) system were included in this study. For each RGSW, firstly we constructed its unwrapped phase φ(t) using Hilbert transform. Secondly, we computed the phase differences, i.e., φ(t+τ)-φ(t), between any two points along the waveform with a fixed time interval τ. Thirdly, we estimated the standard deviation σ of the phase differences. Finally, we calculated volatility (i.e., σ ⁄√τ) of the RGSW's phase in the given time interval τ. Sixteen intervals (i.e., τ=1s, 2s, 3s, …, 16s) were selected for volatility’s computation to examine if it was stable over different time intervals. The relative standard deviation of the acquired volatility values at all 16 time-intervals was computed for each RGSW. Fittings were performed on the histograms formed by observed volatility over a fixed time interval (e.g., 4s) and the relative standard deviation in its measured values with all 16 intervals from all patient RPM RGSWs. A stochastic random walk phase model was utilized to interpret the results. The unwrapped phase of an ideal periodic RGSW always has zero volatility. For our clinical RGSWs, the volatility is non-zero but reasonably stable over the selected time intervals. The observed volatility at 4s interval from the 242 RGSWs (∈ [0.07, 3.7] (1⁄√s), median=0.78 (1⁄√s)) could be approximated by a Gamma distribution Γ(3.0, 0.3). The population histogram of the relative standard deviation in volatility for the investigated time intervals (∈[2%, 35%], median=11.5%) could be fitted with a Gamma distribution Γ(3.5, 0.04). The stochastic random walk phase model, which yields a fixed volatility value over any given time interval, might serve as a useful tool in explaining our observations on phase volatility in patient's RGSW. We proposed a new application of patient’s RGSW for measuring the randomness and irregularity in patient respiratory motion. The volatility in RPM RGSW phase has a potential correlation to the eligibility and treatment response of a given patient to a given clinical procedure. Further studies are needed to explore its potential clinical role in patient 4D image acquisition and patient radiotherapy delivery.

A software platform for statistical evaluation of patient respiratory patterns in radiation therapy

AimThe aim of this work was to design and evaluate a software tool for analysis of a patient’s respiration, with the goal of optimizing the effectiveness of motion management techniques during radiotherapy imaging and treatment. Materials and methodsA software tool which analyses patient respiratory data files (.vxp files) created by the Varian Real-Time Position Management System (RPM) was developed to analyse patient respiratory data. The software, called RespAnalysis, was created in MATLAB and provides four modules, one each for determining respiration characteristics, providing breathing coaching (biofeedback training), comparing pre and post-training characteristics and performing a fraction-by-fraction assessment. The modules analyse respiratory traces to determine signal characteristics and specifically use a Sample Entropy algorithm as the key means to quantify breathing irregularity. Simulated respiratory signals, as well as 91 patient RPM traces were analysed with RespAnalysis to test the viability of using the Sample Entropy for predicting breathing regularity. ResultsRetrospective assessment of patient data demonstrated that the Sample Entropy metric was a predictor of periodic irregularity in respiration data, however, it was found to be insensitive to amplitude variation. Additional waveform statistics assessing the distribution of signal amplitudes over time coupled with Sample Entropy method were found to be useful in assessing breathing regularity. ConclusionsThe RespAnalysis software tool presented in this work uses the Sample Entropy method to analyse patient respiratory data recorded for motion management purposes in radiation therapy. This is applicable during treatment simulation and during subsequent treatment fractions, providing a way to quantify breathing irregularity, as well as assess the need for breathing coaching. It was demonstrated that the Sample Entropy metric was correlated to the irregularity of the patient’s respiratory motion in terms of periodicity, whilst other metrics, such as percentage deviation of inhale/exhale peak positions provided insight into respiratory amplitude regularity.

Respiratory signal prediction based on adaptive boosting and multi-layer perceptron neural network

To improve the prediction accuracy of respiratory signals using adaptive boosting and multi-layer perceptron neural network (ADMLP-NN) for gated treatment of moving target in radiation therapy. The respiratory signals acquired using a real-time position management (RPM) device from 138 previous 4DCT scans were retrospectively used in this study. The ADMLP-NN was composed of several artificial neural networks (ANNs) which were used as weaker predictors to compose a stronger predictor. The respiratory signal was initially smoothed using a Savitzky–Golay finite impulse response smoothing filter (S–G filter). Then, several similar multi-layer perceptron neural networks (MLP-NNs) were configured to estimate future respiratory signal position from its previous positions. Finally, an adaptive boosting (Adaboost) decision algorithm was used to set weights for each MLP-NN based on the sample prediction error of each MLP-NN. Two prediction methods, MLP-NN and ADMLP-NN (MLP-NN plus adaptive boosting), were evaluated by calculating correlation coefficient and root-mean-square-error between true and predicted signals. For predicting 500 ms ahead of prediction, average correlation coefficients were improved from 0.83 (MLP-NN method) to 0.89 (ADMLP-NN method). The average of root-mean-square-error (relative unit) for 500 ms ahead of prediction using ADMLP-NN were reduced by 27.9%, compared to those using MLP-NN. The preliminary results demonstrate that the ADMLP-NN respiratory prediction method is more accurate than the MLP-NN method and can improve the respiration prediction accuracy.

Technical Note: Issues related to external marker block placement for deep inspiration breath hold breast radiotherapy

It has been suggested that the Real-time Position Management (RPM) marker block should be placed directly on the breast or sternum to verify deep inspiration breath hold (DIBH) level for breast radiotherapy. We explore three potential issues with this practice: (a) surface dose effect of placing the marker block in the primary beam; (b) effect of marker block tilt on the accuracy of the RPM system; and (c) correlation between marker block positions on the patient surface and internal chest wall position. (a) The surface dose under the two-, four-, and six-dot marker blocks was measured at incident angles of 0° and 30°; (b) the motion amplitude detected when using the two- and six-dot marker blocks was recorded for block tilts from 0° to 60° about the RPM camera line of sight; (c) the correlation between median displacement of the chest wall and median displacement of the surface contour between breath holds was investigated for superior, middle, and inferior block positions using contours extracted from portal images of eight left-sided breast cancer patients. (a) The marker blocks increased the surface dose for a 6 MV direct field by 48.2-52.2% of Dmax ; (b) at lateral tilts greater than 10°, the two-dot marker block overestimated the motion amplitude; however, the six-dot marker block amplitude remained accurate up to 60°; (c) the whole, superior, and middle surface positions were strongly correlated with chest wall displacement (R2 = 0.83; R2 = 0.90; R2 = 0.83), whereas the inferior position was moderately correlated (R2 = 0.36). The RPM marker block can be placed on the breast for DIBH treatments; however, caution should be used regarding surface dose effects. The two-dot marker block should not be used for block tilts beyond 20°. Marker block placement at a middle or superior position on the breast results in the strongest correlation with chest wall position.

RADIOTHERAPY AFTER ORGAN-SPARING SURGERY FOR I-IIA STAGES OF LEFT-SIDED BREAST CANCER

Breast preserving surgery became a preferable method of treatment for breast cancer now. Adjuvant radiotherapy in breast cancer patients is a standard component of breast-preserving treatment that reduces the recurrence rate and mortality. The risk of cardiotoxicity after radiation therapy for left-sided breast cancer is high. The deep inspiration breath hold technique is used to reduce radiation dose to the heart. The Real-time Position Management, Respiratory Gating (RPM; Varian) system is used to monitor respiratory motion. Audio supporting system as a part of RPM has some technical lacks. The purpose of our research consists in developing video engineering system to control respiratory movements. Use of this system allowed patients to observe graphical sinusoid of amplitudes inspiration image in time interval. Also such system allowed to achieve shortening of treatment time and decrease psychological and emotional stress of the patients.

Evaluation of Accuracy About 2D vs 3D Real-Time Position Management System Based on Couch Rotation when non-Coplanar Respiratory Gated Radiation Therapy

Evaluation of Accuracy About 2D vs 3D Real-Time Position Management System Based on Couch Rotation when non-Coplanar Respiratory Gated Radiation Therapy

Four-Dimensional Dose Reconstruction for Lung Cancer in Volumetric Modulated Arc Therapy

The purpose of this study is to evaluate actual dose distribution for the cases with different respiratory motion using four-dimensional (4D) dose reconstruction system. In the thorax region, respiratory motion of tumor may lead changes in dose distribution due to interplay effects caused by motions of both multi-leaf collimators (MLCs) and tumors. It is important to assess actual dose distribution in each treatment. The authors have developed a 4D dose reconstruction system based on Linac log data and respiratory wave form during VMAT treatment. The 4D dose reconstruction was performed by reproducing the actual irradiation conditions on the treatment planning system using 4D-CT image sets and various field groups that were divided into multiple fields DICOM RT-Plan files created using log data according to each tumor position based on respiratory wave form and EPID cine images. The authors reported in the previous ASTRO annual meeting about results of the phantom experiment that this system could reconstruct the actual dose distribution within the error of 2%. In this study, the case of one lung cancer patient was evaluated. The patient received the 2-arc VMAT irradiation under free breathing (2Gy / fr). The reconstructed dose distributions with different respiratory motion were compared with each other, as well as the planned dose distribution. The patient was treated using Clinac21EX linear accelerator equipped with aS1000 EPID (Varian) and real time position management system (Varian) for acquisition the respiratory wave form. Dose calculation was performed using Eclipse 10.0.28 (Varian) on 4D-CT images (10 phases), and deformable registration was used to accumulate dose distributions of each respiratory phase. Dose distribution of each respiratory phase varied between different treatment sessions, i.e. different respiratory motion, while the end-of-exhale phase contained the highest point dose, and the phase included 30 to 40% of the total dose. The dose distributions and DVHs after accumulation of all respiratory phases were quite similar through different treatment sessions. Any effects of changes in respiratory conditions could not be observed. Additionally the reconstructed dose distributions and planned dose distribution showed similar shape, and the reconstructed CTV center doses were agreed with the planned dose within 0.5%. The change of the dose distribution was quite small due to small tumor motion. On the other hand, it was confirmed that the irradiation of the treatment plan has been correctly performed using (calculated on) this dose reconstruction system. Using the developed system, 4D dose distribution in the clinical case was successfully reconstructed, and the distribution difference between different respiratory motions was evaluated.

Poster – 41: External marker block placement on the breast or chest wall for left‐sided deep inspiration breath‐hold radiotherapy

Purpose:We investigate DIBH breast radiotherapy using the Real‐time Position Management (RPM) system with the marker‐block placed on the target breast or chest wall.Methods:We measured surface dose for three different RPM marker‐blocks using EBT3 Gafchromic film at 0° and 30° incidence. A registration study was performed to determine the breast surface position that best correlates with overall internal chest wall position. Surface and chest wall contours from MV images of the medial tangent field were extracted for 15 patients. Surface contours were divided into three potential marker‐block positions on the breast: Superior, Middle, and Inferior. Translational registration was used to align the partial contours to the first‐fraction contour. Each resultant transformation matrix was applied to the chest wall contour, and the minimum distance between the reference chest wall contour and the transformed chest wall contour was evaluated for each pixel.Results:The measured surface dose for the 2‐dot, 6‐dot, and 4‐dot marker‐blocks at 0° incidence were 74%, 71%, and 77% of dose to dmax respectively. At 30° beam incidence this increased to 76%, 72%, and 81%. The best external surface position was patient and fraction dependent, with no consistent best choice.Conclusions:The increase in surface dose directly under the RPM block is approximately equivalent to 3 mm of bolus. No marker‐block position on the breast surface was found to be more representative of overall chest wall motion; therefore block positional stability and reproducibility can be used to determine optimal placement on the breast or chest wall.

Time-resolved dosimetric verification of respiratory-gated radiotherapy exposures using a high-resolution 2D ionisation chamber array

The aim of this work was to track and verify the delivery of respiratory-gated irradiations, performed with three versions of TrueBeam linac, using a novel phantom arrangement that combined the OCTAVIUS® SRS 1000 array with a moving platform. The platform was programmed to generate sinusoidal motion of the array. This motion was tracked using the real-time position management (RPM) system and four amplitude gating options were employed to interrupt MV beam delivery when the platform was not located within set limits. Time-resolved spatial information extracted from analysis of x-ray fluences measured by the array was compared to the programmed motion of the platform and to the trace recorded by the RPM system during the delivery of the x-ray field. Temporal data recorded by the phantom and the RPM system were validated against trajectory log files, recorded by the linac during the irradiation, as well as oscilloscope waveforms recorded from the linac target signal. Gamma analysis was employed to compare time-integrated 2D x-ray dose fluences with theoretical fluences derived from the probability density function for each of the gating settings applied, where gamma criteria of 2%/2 mm, 1%/1 mm and 0.5%/0.5 mm were used to evaluate the limitations of the RPM system. Excellent agreement was observed in the analysis of spatial information extracted from the SRS 1000 array measurements. Comparisons of the average platform position with the expected position indicated absolute deviations of <0.5 mm for all four gating settings. Differences were observed when comparing time-resolved beam-on data stored in the RPM files and trajectory logs to the true target signal waveforms. Trajectory log files underestimated the cycle time between consecutive beam-on windows by 10.0 ± 0.8 ms. All measured fluences achieved 100% pass-rates using gamma criteria of 2%/2 mm and 50% of the fluences achieved pass-rates >90% when criteria of 0.5%/0.5 mm were used. Results using this novel phantom arrangement indicate that the RPM system is capable of accurately gating x-ray exposure during the delivery of a fixed-field treatment beam.

SU‐F‐J‐141: Real‐Time Position Management (RPM) System as a Valuable Tool to Predict Tumor Position Deviation in SBRT Lung and Liver Patients with Breath Hold Using Active Breathing Coordinator (ABC)

Purpose:ABC is a breath‐hold technique for SBRT treatments. At our institution, three CTs with ABC are acquired at simulation to check the breath‐hold consistency because tumor position may deviate among three scans. The purpose of this study was to determine whether RPM can predict tumor position deviation in SBRT breath‐hold treatments with ABC.Methods:Five SBRT lung and five SBRT liver patients were selected. An RPM block was placed on the patients between the xiphoid and the umbilicus. RPM recorded patients’ anterior‐posterior skin surface positions during breath holds with ABC. For each patient, three CTs (ABC1, ABC2, ABC3) were acquired at the same breath‐hold level. ABC2 and ABC3 CTs were rigidly registered to ABC1 CT. Tumor position deviations were measured on the fused CTs (ABC2 with ABC1, ABC3 with ABC1) and compared to the corresponding RPM position shifts. Linear regression, k‐means clustering, and decision tree classifier were used to determine the relationship between RPM position shifts and tumor position deviations.Results:Tumor position deviations were 0.4–8.7 mm for lung patients and 1.6–9.1 mm for liver patients. There were no significant linear correlations between RPM position shifts and tumor position deviations for both lung (R2=0.20) and liver (R2=0.45) patients. In contrast, decision tree classification showed that the RPM position shift of 3.45 mm or more indicated tumor position deviation &gt;5 mm for lower lobe lung tumors and liver tumors that are close to the diaphragm.Conclusion:RPM position shifts are not linearly correlated with tumor position deviations. However, RPM can be used to predict large tumor position deviations (&gt;5 mm) among three ABC CTs in SBRT for lower lobe lung tumors and liver tumors and to validate the breath‐hold consistency with the ABC technique. Therefore, RPM has the potential to replace the additional verification CT scans.

SU‐F‐E‐09: Respiratory Signal Prediction Based On Multi‐Layer Perceptron Neural Network Using Adjustable Training Samples

Purpose:Dynamic tracking of moving organs, such as lung and liver tumors, under radiation therapy requires prediction of organ motions prior to delivery. The shift of moving organ may change a lot due to huge transform of respiration at different periods. This study aims to reduce the influence of that changes using adjustable training signals and multi‐layer perceptron neural network (ASMLP).Methods:Respiratory signals obtained using a Real‐time Position Management(RPM) device were used for this study. The ASMLP uses two multi‐layer perceptron neural networks(MLPs) to infer respiration position alternately and the training sample will be updated with time. Firstly, a Savitzky‐Golay finite impulse response smoothing filter was established to smooth the respiratory signal. Secondly, two same MLPs were developed to estimate respiratory position from its previous positions separately. Weights and thresholds were updated to minimize network errors according to Leverberg‐Marquart optimization algorithm through backward propagation method. Finally, MLP 1 was used to predict 120∼150s respiration position using 0∼120s training signals. At the same time, MLP 2 was trained using 30∼150s training signals. Then MLP is used to predict 150∼180s training signals according to 30∼150s training signals. The respiration position is predicted as this way until it was finished.Results:In this experiment, the two methods were used to predict 2.5 minute respiratory signals. For predicting 1s ahead of response time, correlation coefficient was improved from 0.8250(MLP method) to 0.8856(ASMLP method). Besides, a 30% improvement of mean absolute error between MLP(0.1798 on average) and ASMLP(0.1267 on average) was achieved. For predicting 2s ahead of response time, correlation coefficient was improved from 0.61415 to 0.7098.Mean absolute error of MLP method(0.3111 on average) was reduced by 35% using ASMLP method(0.2020 on average).Conclusion:The preliminary results demonstrate that the ASMLP respiratory prediction method is more accurate than MLP method and can improve the respiration forecast accuracy.

TH-CD-207A-04: Optimized Respiratory Gating for Abnormal Breathers in Pancreatic SBRT

Purpose: Pancreatic SBRT is uniquely challenging due to both the erratic/unstable motion of the pancreas and the close proximity of the radiosensitive small bowel. Respiratory gating can mitigate this effect, but the irregularity of motion severely affects traditional phase-based gating. The purpose of this study was to analyze real-time motion data of pancreatic tumors to optimize the efficacy and accuracy of respiratory gating, with the overall goal of enabling dose escalated pancreatic SBRT. Methods: Fifteen pancreatic SBRT patients received 30–33 Gy in 5 fractions on a Varian TrueBeam STx unit. Abdominal compression was used to reduce the amplitude of tumor motion, and daily cone-beam computed tomography (CBCT) scans were acquired prior to each treatment for target localization purposes. For this study, breathing data (phase and amplitude) were collected during each CBCT scan using Varian's Real-Time Position Management system. An in-house template matching technique was used to track the superior-inferior motion of implanted fiducial markers in CBCT projection images. Using tumor motion and breathing data, phase-based or amplitude-based respiratory gating was simulated for all 75 fractions, targeting either end-exhalation or end-inhalation phases of breathing. Results: For the average patient, gating at end-exhalation offered the best reductions in effective motion for equal duty cycles. However, optimal central phase angle varied widely (range: 0–92%, mean±SD: 49±12%), and phase-based gating windows typically associated with end-exhalation (i.e., “30–70%”) were rarely ideal. Amplitude-based gating significantly outperformed phase-based gating, with average effective ranges for amplitude-based gating 25% lower than phase-based gating ranges (as much as 73% lower). Amplitude-based gating was consistently better suited to accommodate abnormal breathing patterns. For both phase-based and amplitude-based gating, end-exhalation provided significantly better results than end-inhalation. Conclusion: Amplitude-based gating reliably outperformed phase-based gating, and end-exhalation was more suitable than end-inhalation. These results will be used to guide future dose-escalation trials. Research funding provided by Varian Medical Systems to Miften and Jones.

Technical Note: High temporal resolution characterization of gating response time.

Low temporal latency between a gating ON/OFF signal and the LINAC beam ON/OFF during respiratory gating is critical for patient safety. Here the authors describe a novel method to precisely measure gating lag times at high temporal resolutions. A respiratory gating simulator with an oscillating platform was modified to include a linear potentiometer for position measurement. A photon diode was placed at linear accelerator isocenter for beam output measurement. The output signals of the potentiometer and diode were recorded simultaneously at 2500 Hz with an analog to digital converter for four different commercial respiratory gating systems. The ON and OFF of the beam signal were located and compared to the expected gating window for both phase and position based gating and the temporal lag times extracted. For phase based gating, a real-time position management (RPM) infrared marker tracking system with a single camera and a RPM system with a stereoscopic camera were measured to have mean gate ON/OFF lag times of 98/90 and 86/44 ms, respectively. For position based gating, an AlignRT 3D surface system and a Calypso magnetic fiducial tracking system were measured to have mean gate ON/OFF lag times of 356/529 and 209/60 ms, respectively. Temporal resolution of the method was high enough to allow characterization of individual gate cycles and was primary limited by the sampling speed of the data recording device. Significant variation of mean gate ON/OFF lag time was found between different gating systems. For certain gating devices, individual gating cycle lag times can vary significantly.