Real-time Position Management Research Articles

Adaptive respiratory signal prediction using dual multi-layer perceptron neural networks

Purpose. To improve the prediction accuracy of respiratory signals by adapting the multi-layer perceptron neural network (MLP-NN) model to changing respiratory signals. We have previously developed an MLP-NN to predict respiratory signals obtained from a real-time position management (RPM) device. Preliminary testing results indicated that poor prediction accuracy may be observed after several seconds for irregular breathing patterns as only a set of fixed data was used in one-time training. To improve the prediction accuracy, we introduced a continuous learning technique using the updated training data to replace one-time learning using the fixed training data. We carried on this new prediction using an adaptation approach with dual MLP-NNs rather than single MLP-NN. When one MLP-NN was performing prediction of the respiratory signals, another one was being trained using the updated data and vice versa. The predicted performance was evaluated by root-mean-square-error (RMSE) between the predicted and true signals from 202 patients’ respiratory patterns each with 1 min recording length. The effects of adding an additional network, training parameter, and respiratory signal irregularity on the performance of the new predictor were investigated based on four different network configurations: a single MLP-NN, high-computation dual MLP-NNs (U1), two different combinations of high- and low-computation dual MLP-NNs (U2 and U3). The RMSEs using U1 method were reduced by 34%, 19%, and 10% compared to those using MLP-NN, U2 and U3 methods, respectively. Continuous training of an MLP-NN based on a dual-network configuration using updated respiratory signals improved prediction accuracy compared to one-time training of an MLP-NN using fixed signals.

Evaluation of patient specific quality assurance of gated field in field radiation therapy technique using two-dimensional detector array

Introduction: Gated tangential field-in-field (FIF) technique is used to lower the dose to organs at risk for breast cancer radiotherapy (RT). In this study, the authors investigated the accuracy of the delivered treatment plan with and without gating using a two-dimensional detector array for patient-specific verification purposes.Methods: In this study, a 6MV beams were used for the merged FIF RT (forward Intensity Modulated Radiation Therapy). The respiration signals for gated FIF delivery were obtained from the one-dimensional moving phantom using the real-time position management (RPM) system (Varian Medical Systems, Palo Alto, CA). RPM system used for four-dimensional computed tomography scanner light-speed, GE is based on an infrared camera to detect motion of external 6-point marker. The beams were delivered using a Clinac iX (Varian Medical Systems, Palo Alto, CA) with the multileaf collimator Millennium 120. The MapCheck2 (SunNuclear, Florida) was used for the evaluation of treatment plans. MapCheck2 was validated through a comparison with measurements from a farmer-type ion chamber. Gated beams were delivered using a maximum dose rate with varying duty cycles and analyzed the MapCheck2 data to evaluate treatment plan delivery accuracy.Results: Results of the gamma passing rate for relative and absolute dose differences for all ungated and gated beams were between 95.1% and 100%.Conclusion: Gated FIF technique can deliver an accurate dose to a detector during gated breast cancer RT. There is no significance between gated and ungated patient-specific quality assurance (PSQA); one can use ungated PSQA for verification of treatment plan delivery

Time-Driven Activity-Based Costing Comparison of CT-Guided Versus MR-Guided SBRT.

Magnetic resonance-guided radiation therapy (MRgRT) has recently become commercially available, offering the opportunity to accurately image and target moving tumors as compared with computed tomography-guided radiation therapy (CTgRT) systems. However, the costs of delivering care with these 2 modalities remain poorly described. With localized unresectable hepatocellular carcinoma as an example, we were able to use time-driven activity-based costing to determine the cost of treatment on linear accelerators with CTgRT compared with MRgRT. Process maps, informed via interviews with departmental personnel, were created for each phase of the care cycle. Stereotactic body radiation therapy was delivered at 50 Gy in 5 fractions, either with CTgRT using fiducial placement, deep inspiration breath-hold (DIBH) with real-time position management, and volumetric-modulated arc therapy, or with MRgRT using real-time tumor gating, DIBH, and static-gantry intensity-modulated radiation therapy. Direct clinical costs were $7,306 for CTgRT and $8,622 for MRgRT comprising personnel costs ($3,752 v $3,603), space and equipment costs ($2,912 v $4,769), and materials costs ($642 v $250). Increased MRgRT costs may be mitigated by forgoing CT simulation ($322 saved) or shortening treatment to 3 fractions ($1,815 saved). Conversely, adaptive treatment with MRgRT would result in an increase in cost of $529 per adaptive treatment. MRgRT offers real-time image guidance, avoidance of fiducial placement, and ability to use adaptive treatments; however, it is 18% more expensive than CTgRT under baseline assumptions. Future studies that elucidate the magnitude of potential clinical benefits of MRgRT are warranted to clarify the value of using this technology.

Data-Driven Respiratory Gating Outperforms Device-Based Gating for Clinical 18F-FDG PET/CT.

A data-driven method for respiratory gating in PET has recently been commercially developed. We sought to compare the performance of the algorithm with an external, device-based system for oncologic 18F-FDG PET/CT imaging. Methods: In total, 144 whole-body 18F-FDG PET/CT examinations were acquired, with a respiratory gating waveform recorded by an external, device-based respiratory gating system. In each examination, 2 of the bed positions covering the liver and lung bases were acquired with a duration of 6 min. Quiescent-period gating retaining approximately 50% of coincidences was then able to produce images with an effective duration of 3 min for these 2 bed positions, matching the other bed positions. For each examination, 4 reconstructions were performed and compared: data-driven gating (DDG) (we use the term DDG-retro to distinguish that we did not use the real-time R-threshold-based application of DDG that is available within the manufacturer's product), external device-based gating (real-time position management (RPM)-gated), no gating but using only the first 3 min of data (ungated-matched), and no gating retaining all coincidences (ungated-full). Lesions in the images were quantified and image quality scored by a radiologist who was masked to the method of data processing. Results: Compared with the other reconstruction options, DDG-retro increased the SUVmax and decreased the threshold-defined lesion volume. Compared with RPM-gated, DDG-retro gave an average increase in SUVmax of 0.66 ± 0.1 g/mL (n = 87, P < 0.0005). Although the results from the masked image evaluation were most commonly equivalent, DDG-retro was preferred over RPM-gated in 13% of examinations, whereas the opposite occurred in just 2% of examinations. This was a significant preference for DDG-retro (P = 0.008, n = 121). Liver lesions were identified in 23 examinations. Considering this subset of data, DDG-retro was ranked superior to ungated-full in 6 of 23 (26%) cases. Gated reconstruction using the external device failed in 16% of examinations, whereas DDG-retro always provided a clinically acceptable image. Conclusion: In this clinical evaluation, DDG-retro provided performance superior to that of the external device-based system. For most examinations the performance was equivalent, but DDG-retro had superior performance in 13% of examinations, leading to a significant preference overall.

Calibration of mechanical systems of in-house dynamic thorax phantom for radiotherapy dosimetry

The purpose of this study is to calibration a mechanical system of In-house Dynamic Thorax Phantom so that it can be used as a dosimetry on the Radiotherapy Real-Time Position Management (RPM) technique. In order to ensure the RPM technique had properly function, we need to employ a quality control tool that represents moving objects on the human body. Inspired by the CIRS Dynamic Thorax Phantom model 008A, we developed an In-house Dynamic Thorax Phantom using three Nema-17 stepper motors with 200 steps in a cycle as a mechanical system. In addition, the first stepper motor was used to move objects in superior-inferior direction; the second one was used to move the RPM marker in anterior-posterior direction; while the third one was used to move objects in rotation. Calibration of mechanical system was performed by finding out the number of counts needed each direction. Especially for rotational system, they were calibrated with Optical Interrupt Device. The result of calibration of the mechanical system between the actual readings compared with the system specification is 0.0% and < 1% for the measurement testing.

A new respiratory monitor system for four-dimensional computed tomography by measuring the pressure change on the back of body.

A novel respiratory monitoring method based on the periodical pressure change on the patient's back was proposed and assessed by applying to four-dimensional CT (4DCT) scanning. A pressure-based respiratory monitoring system is developed and validated by comparing to real-time position management (RPM) system. The pressure change and the RPM signal are compared with phase differences and correlations calculated. The 4DCT images are reconstructed by these two signals. Internal and skin artifacts due to mismatch between CT slices and respiratory phases are evaluated. The pressure and RPM signals shows strong consistency (R = 0.68±0.19 (1SD)). The time shift is 0.26 ± 0.51 (1SD) s and the difference of breath cycle is 0.02 ± 0.17 (1SD) s. The quality of 4DCT images reconstructed by two signals is similar. For both methods, the number of patients with artifacts is eight and the maximum magnitudes of artifacts are 20 mm (internal) and 10 mm (skin). The average magnitudes are 8.8 mm (pressure) and 8.2 mm (RPM) for internal artifacts, and 5.2 mm (pressure) and 4.6 mm (RPM) for skin artifacts. The mean square gray value difference shows no significant difference (p = 0.52). The pressure signal provides qualified results for respiratory monitoring in 4DCT scanning, demonstrating its potential application for respiration monitoring in radiotherapy. Pressure change on the back of body is a novel and promising method to monitor respiration in radiotherapy, which may improve treatment comfort and provide more information about respiration and body movement.

A Qualitative Analysis of Human Error During the DIBH Procedure

IntroductionThis quality assurance study analyzed human errors that occurred during the radiation treatment delivery of the deep-inspiration breath hold (DIBH) technique at a tertiary cancer centre. The intention is to recommend solutions and system changes that have the potential to decrease the frequency of errors based on human factors principles. MethodsEighty-two incident reports from January 2012 to July 2017 were retrieved and analysed to determine theme bins of performance-influencing factors contributing to the error. Performance-influencing factors were generated from the incident reports and from focus group discussions with volunteer radiation therapists in the department. Potential solutions to mitigate the error were sought from incident reports, focus groups, literature search, and an interview with a human factors specialist. The solutions were ranked based on the hierarchy of effectiveness, and recommendations were classified using a priority matrix. ResultsEighty-nine percent of the errors captured in the incident reports were defined as a slip or lapse error type, and 11% of the remaining errors were defined as a mistake error type. Treatment-related problem solving and distractions/interruptions were the highest frequency causative factors that contributed to the observed error. Potential solutions that were suggested across sources included implementing a forcing function, such as the real-time position management system, adding reminders, such as a console sign-off, and updating the current task checklist. DiscussionThe potential solutions generated were summarized into four recommendations that have varying degrees of association with known causative factors. The four recommendations include investing in (1) a forcing function, (2) updating/reinforcing the procedure, (3) managing workload, and (4) updating the checklist. A priority matrix was used to assess both potential effectiveness and cost/effort of each recommendation. Ideally, recommendation 1 would be implemented; however, it is understood that there would be an associated cost. It is therefore suggested that recommendations 2, 3, and 4 are implemented together to increase the effectiveness of the intervention until recommendation 1 can be achieved. ConclusionThis qualitative study introduced a method that analyzed human factors in a specialized procedure used in the treatment of a specific population of patients with cancer. Recommendations were formulated and proposed to the radiation therapy department in hopes of potentially decreasing the frequency of this specific error in the future.

Gated 18F-FDG PET/CT of the Lung Using a Respiratory Spirometric Gating Device: A Feasibility Study.

Spirometric gating devices (SGDs) can measure the respiratory signal with high temporal resolution and accuracy. The primary objective of this study was to assess the feasibility and tolerance of a gated lung PET/CT acquisition using an SGD. The secondary objective was to compare the technical quality, accuracy, and interoperability of the SGD with that of a standard respiratory gating device, Real-Time Position Management (RPM), based on measurement of vertical thoracoabdominal displacement. Methods: A prospective phase I monocentric clinical study was performed on patients undergoing 18F-FDG PET/CT for assessment of a solitary lung nodule, staging of lung malignancy, or planning of radiotherapy. After whole-body PET/CT, a centered gated acquisition of both PET and CT was simultaneously obtained with the SGD and RPM during normal breathing. Results: Of the 46 patients who were included, 6 were prematurely excluded (1 because of hyperglycemia and 5 because of distant metastases revealed by whole-body PET/CT, leading to an unjustified extra gated acquisition). No serious adverse events were observed. Of the 40 remaining patients, the gated acquisition was prematurely stopped in 1 patient because of mask discomfort (2.5%; confidence interval [CI], 0.1%-13.2%). This event was considered patient tolerance failure. The SGD generated accurately gated PET/CT images, with more than 95% of the breathing cycle detected and high temporal resolution, in 34 of the 39 patients (87.2%; 95% CI, 60.0%-100.0%) and failed to generate a biologic tumor volume in 1 of 21 patients with increased 18F-FDG uptake (4.8%; 95% CI, 0.1%-26.5%). The quality and accuracy of respiratory signal detection and synchronization were significantly better than those obtained with RPM (P < 0.05). Conclusion: This trial supports the use of an SGD for gated lung PET/CT because of its high patient tolerance and accuracy. Although this technique seems to technically outperform RPM for gated PET/CT, further assessment of its superiority and the clinical benefit is warranted. We believe that this technique could be used as a gold standard to develop innovative approaches to eliminate respiration-induced blurring artifacts.

Clinical implementation of respiratory-gated spot-scanning proton therapy: An efficiency analysis of active motion management.

PurposeThe aim of this work is to describe the clinical implementation of respiratory‐gated spot‐scanning proton therapy (SSPT) for the treatment of thoracic and abdominal moving targets. The experience of our institution is summarized, from initial acceptance and commissioning tests to the development of standard clinical operating procedures for simulation, motion assessment, motion mitigation, treatment planning, and gated SSPT treatment delivery.Materials and methodsA custom respiratory gating interface incorporating the Real‐Time Position Management System (RPM, Varian Medical Systems, Inc., Palo Alto, CA, USA) was developed in‐house for our synchrotron‐based delivery system. To assess gating performance, a motion phantom and radiochromic films were used to compare gated vs nongated delivery. Site‐specific treatment planning protocols and conservative motion cutoffs were developed, allowing for free‐breathing (FB), breath‐holding (BH), or phase‐gating (Ph‐G). Room usage efficiency of BH and Ph‐G treatments was retrospectively evaluated using beam delivery data retrieved from our record and verify system and DICOM files from patient‐specific quality assurance (QA) procedures.ResultsMore than 70 patients were treated using active motion management between the launch of our motion mitigation program in October 2015 and the end date of data collection of this study in January 2018. During acceptance procedures, we found that overall system latency is clinically‐suitable for Ph‐G. Regarding room usage efficiency, the average number of energy layers delivered per minute was <10 for Ph‐G, 10‐15 for BH and ≥15 for FB, making Ph‐G the slowest treatment modality. When comparing to continuous delivery measured during pretreatment QA procedures, the median values of BH treatment time were extended from 6.6 to 9.3 min (+48%). Ph‐G treatments were extended from 7.3 to 13.0 min (+82%).ConclusionsActive motion management has been crucial to the overall success of our SSPT program. Nevertheless, our conservative approach has come with an efficiency cost that is more noticeable in Ph‐G treatments and should be considered in decision‐making.

Intrafraction tumor motion during deep inspiration breath hold pancreatic cancer treatment.

PurposeBeam gating with deep inspiration breath hold (DIBH) has been widely used for motion management in radiotherapy. Normally it relies on some external surrogate for estimating the internal target motion, while the exact internal motion is unknown. In this study, we used the intrafraction motion review (IMR) application to directly track an internal target and characterized the residual motion during DIBH treatment for pancreatic cancer patients through their full treatment courses.Methods and MaterialsEight patients with pancreatic cancer treated with DIBH volumetric modulated arc therapy in 2017 and 2018 were selected for this study, each with some radiopaque markers (fiducial or surgical clips) implanted near or inside the target. The Varian Real‐time Position Management (RPM) system was used to monitor the breath hold, represented by the anterior‐posterior displacement of an external surrogate, namely reflective markers mounted on a plastic block placed on the patient's abdomen. Before each treatment, a cone beam computed tomography (CBCT) scan under DIBH was acquired for patient setup. For scan and treatment, the breath hold reported by RPM had to lie within a 3 mm window. IMR kV images were taken every 20° or 40° gantry rotation during dose delivery, resulting in over 5000 images for the cohort. The internal markers were manually identified in the IMR images. The residual motion amplitudes of the markers as well as the displacement from their initial positions located in the setup CBCT images were analyzed.ResultsEven though the external markers indicated that the respiratory motion was within 3 mm in DIBH treatment, significant residual internal target motion was observed for some patients. The range of average motion was from 3.4 to 7.9 mm, with standard deviation ranging from 1.2 to 3.5 mm. For all patients, the target residual motions seemed to be random with mean positions around their initial setup positions. Therefore, the absolute target displacement relative to the initial position was small during DIBH treatment, with the mean and the standard deviation 0.6 and 2.9 mm, respectively.ConclusionsInternal target motion may differ from external surrogate motion in DIBH treatment. Radiographic verification of target position at the beginning and during each fraction is necessary for precise RT delivery. IMR can serve as a useful tool to directly monitor the internal target motion.

Towards real-time respiratory motion prediction based on long short-term memory neural networks

Radiation therapy of thoracic and abdominal tumors requires incorporating the respiratory motion into treatments. To precisely account for the patient’s respiratory motions and predict the respiratory signals, a generalized model for predictions of different types of patients’ respiratory motions is desired. The aim of this study is to explore the feasibility of developing a long short-term memory (LSTM)-based generalized model for the respiratory signal prediction. To achieve that, 1703 sets of real-time position management (RPM) data were collected from retrospective studies across three clinical institutions. These datasets were separated as the training, internal validity and external validity groups. Among all the datasets, 1187 datasets were used for model development and the remaining 516 datasets were used to test the model’s generality power. Furthermore, an exhaustive grid search was implemented to find the optimal hyper-parameters of the LSTM model. The hyper-parameters are the number of LSTM layers, the number of hidden units, the optimizer, the learning rate, the number of epochs, and the length of time lags. The obtained model achieved superior accuracy over conventional artificial neural network (ANN) models: with the prediction window equaling to 500 ms, the LSTM model achieved an average relative mean absolute error (MAE) of 0.037, an average root mean square error (RMSE) of 0.048, and a maximum error (ME) of 1.687 in the internal validity data, and an average relative MAE of 0.112, an average RMSE of 0.139 and an ME of 1.811 in the external validity data. Compared to the LSTM model trained with default hyper-parameters, the MAE of the optimized model results decreased by 20%, indicating the importance of tuning the hyper-parameters of LSTM models to obtain superior accuracies. This study demonstrates the potential of deep LSTM models for the respiratory signal prediction and illustrates the impacts of major hyper-parameters in LSTM models.

Optimal Gating Window for Respiratory-Gated Radiotherapy with Real-Time Position Management and Respiration Guiding System for Liver Cancer Treatment

Respiratory-gated radiotherapy is one of the most effective approaches to minimise radiation dose delivery to normal tissue and maximise delivery to tumours under patient’s motion caused by respiration. We propose a respiration guiding system based on real-time position management with suitable gating window for respiratory-gated radiotherapy applied to liver cancer. Between August 2016 and February 2018, 52 patients with liver cancer received training in real-time position management and respiration guiding. Respiration signals were statistically analysed during unguided respiration and when applying the respiration guiding system. Phases of 30–60% and 30–70% retrieved the lowest respiration variability among patients, and 47 patients exhibited significant differences in terms of respiration reproducibility between unguided and guided respiration. The results suggest that either of these phases can establish suitable windows for gated radiotherapy applied to liver cancer, especially regarding respiration reproducibility.

Performance Validation of In-House Developed Four-dimensional Dynamic Phantom.

Objective:The objective of this study was to validate the performance characteristics of in-house developed four-dimensional (4D) dynamic phantom (FDDP).Materials and Methods:There are three target inserts of 1.0, 1.5 and 2.0 cm diameter. The targets were driven in sinusoidal pattern in the longitudinal direction, using the combinations of amplitudes of 0.5, 1.0, and 1.5 cm with frequencies of 0.2 and 0.25 Hz. The amplitude and frequency of motion were measured manually, and by using Real-Time Position Management (RPM) system also. The static, free-breathing, and 4D computed tomography (CT) scans of the phantom were acquired with 1.0 mm slice thickness. The 4DCT scans were sorted into 0%–90% phase, and the maximum intensity projection (MIP) images were also generated. The static, free-breathing, and 4DCT data sets and MIP images were contoured to get VStatic, VFB, V00......V90, and internal target volume ITV MIP, respectively. The individual phase volumes were summed to obtain V4D. The length of the target in the motion was measured using MIP image and compared with theoretical length (TL). The variation of 3D displacement vector of individual phase volume with respect to V00 with the phase of motion was studied at amplitude and frequency of 1.0 cm and 0.25 Hz, respectively. The degree of similarity between VFB and V4D and VFB and ITVMIP was also studied for all the target sizes at amplitude and frequency of 1.0 cm and 0.2 Hz and 1.0 cm and 0.25 Hz, respectively.Results:The amplitude and frequency of motion agreed within the limits of uncertainty with the manually and RPM measured values. The length of target in the motion matched within 1.0 mm with TL. The 3D displacement of individual phase volume showed no target size dependence, and the degree of similarity between VFB and V4D and VFB and ITVMIP decreases with increase in the displacement between the two volumes.Conclusions:The mechanical and imaging performances of FDDP were found within the acceptable limits. Therefore, this phantom can be used for quality assurance of 4D imaging process in radiotherapy.

A Novel 4D-CT Sorting Method Based on Combined Mutual Information and Edge Gradient

Although mutual information is a general method usually being used to measure the similarity of two images, the robustness is questionable due to the absence of spatial information. The purpose of this study is to develop a feasible sorting technique for 4D-CT. A novel sorting algorithm named mutual information and edge gradient (MIEG), which includes spatial information by combining mutual information with a term based on the edge gradient of the image, was proposed to sort sequential CT images. The edge of image was extracted by calculating the wavelet transform modulus maxima, and the gradient similarity coefficient of the edge image was calculated and used to multiply by mutual information to form the final similarity metric. This sorting technique was validated by comparing the 4D-CTs reconstructed using MIEG and Real-time Position Management system (Varian Medical Systems, Inc., Palo Alto, CA). Tumor motion trajectories derived from 4D-CTs were analyzed in three orthogonal directions. Their correlation coefficients (CC) and differences in tumor motion magnitude (Ds) were determined. In addition, Dice similarity coefficient (DSC) was used to measure how well the tumor volumes segmented from the two 4D datasets overlapped with each other. For all patients, the mean CC values were >0.95 in all directions. The mean Ds were <0.64 mm in all directions. In addition, good overlapping was achieved between the segmented volumes with the mean DSC = 0.97 at end- and median-inspiration phases, respectively. This study proposed a feasible sorting method for 4D-CT, and its usefulness in imaging accurate tumor motion has been demonstrated.

Impact of Respiratory Waveform on Gate Signal Generation on Respiratory Gated Irradiation and Evaluation Method of Respiratory Waveform to Be Used

The respiratory gated irradiation using the real-time position management system (RPM) was used to clarify the generation of the gated signal when the respiration waveform changed, and also the evaluation method of the respiration waveform was also examined. The respiratory waveform was changed using a moving phantom. Respiratory waveform was analyzed from the data recorded in RPM, and the out-of-phase gated rate was examined. Analysis was made by focusing on the coefficient of variation of the respiratory wavelength in the evaluation of respiratory waveform. Immediately after the change of respiratory wavelength from the short cycle to the long cycle, a gated signal was generated at a phase before the set gated phase, and a maximum advance of 1.259 ± 0.212 s occurred. Immediately after the change of respiratory wavelength from the long cycle to the short cycle, the gated signal was generated at the phase exceeding the set gated phase, and a delay of 0.997 ± 0.180 s occurred at the maximum. As the value of the coefficient of variation increased, the gated rate which was out of setting also increased. In respiratory gated irradiation using RPM, it became clear that the gated signal is generated out of the phase set by the respiratory waveform change. Coefficient of variation of the respiratory wavelength is considered to be an indicator for evaluating the respiratory waveform to be used in the respiratory gated irradiation.

Intrafraction esophageal motion in patients with clinical T1N0 esophageal cancer.

To investigate the intrafraction movement of the esophagus using fiducial markers. Studies on intrafraction esophageal motion using the fiducial markers are scarce. We retrospectively analyzed patients with clinical T1N0 esophageal cancer who had received fiducial markers at our hospital between July 2007 and December 2013. Real-Time Position Management System to track the patient's respiration was used, and each patient underwent three-dimensional computed tomography of the resting expiratory and inspiratory level. We used the center of the marker to calculate the distance between the expiratory and inspiratory breath-holds, which were measured with the radiotherapy treatment planning system in three directions: left-right (LR), superior-inferior (SI), and anterior-posterior (AP). The movements at each site were compared with the Kruskal-Wallis analysis and Wilcoxon rank sum test with a Bonferroni correction. A total of 101 patients with 201 fiducial markers were included. The upper, middle and lower thoracic positions had 40, 77, and 84 markers, respectively. The mean absolute magnitudes of the shifts (standard deviation) were 0.18 (0.19) cm, 0.68 (0.46) cm, and 0.24 (0.24) cm in the LR, SI, and AP directions, respectively. From the cumulative frequency distribution, we assumed that 0.35 cm LR, 0.8 cm SI, and 0.3 cm AP in the upper; 0.5 cm LR, 1.55 cm SI, and 0.55 cm AP in the middle; and 0.75 cm LR, 1.9 cm SI, and 0.95 cm AP in the lower thoracic esophagus covered 95% of the cases. The internal margin based on the site of esophagus was estimated.

P256] The effect of different breathing stages on treatment plans prepared using VMAT technique in early stage left breast irradiation

Purpose Volumetric Arc Therapy (VMAT) has become one of the most frequently used techniques in the treatment of left breast cancer radiotherapy due to its ability to reduce critical organ doses.In patients with left breast cancer, it is very important to reduce the amount of irradiated heart dose-volume in order to prevent possible cardiovascular morbidity associated with radiotherapy. Deep inspiration breath-hold (DIBH) is a technique that delivered the radiotherapy to the left breast patients in a deep breath hold position.In this study, VMAT plans on computed tomography (CT) images taken at different breath phases for left sided breast cancer patients were compared in terms of plan quality and risky organ doses. Methods In the study, VMAT plans were prepared on the CT images taken both free-breathing (FB) and DIBH modes. Eclipse treatment planning system (TPS) was used in order to prepare radiotherapy plans. The Varian Real Time Position Management System (RPM) was used for the DIBH technique.The dose was defined as 50 Gy/25 fractions. For all plans, 95% of the CTV breast volume was targeted to receive 46 Gy. In both techniques, target volume homogeneity index (HI) and conformity index (CI), critical organ doses and MU values were compared. Result CI values of VMAT plans prepared for both FB and DIBH CT images was found as 1.02 ± 0.04 and 1.014 ± 0.03; HI was found as 0.101 ± 0.018 and 0.113 ± 0.016, respectively. In terms of dose homogeneity, DIBH-VMAT plans were found to be better than FB-VMAT plans; the difference is statistically significant. The LAD mean doses were found to be lower in the DIBH-VMAT technique (p Conclusion DIBH-VMAT technique should be preferred instead of FB-VMAT technique for early stage left breast cancer irradiation. This work was supported by Scientific Research Projects Coordination Unit of Istanbul University (Project Number: I.U.BAP-23057).

P136] The correlation of infrared reflecting marker movement and lung expansion with deep inspiration breath-hold technique for left-sided breast cancer radiotherapy

Purpose This study was to quantify the correlation of the infrared reflecting marker movement and deep inspiration breath-hold (DIBH) lung expansions by setting the infrared reflecting marker below the xiphoid process on the abdominal wall. Methods Ten patients with left breast cancer post operation underwent free-breathing (FB) and DIBH computed tomography (CT) using infrared reflecting marker on the abdominal wall. Therapists coached patient breathing maneuvers before CT scans. The breath-hold level (BHL) was recorded by infrared reflecting marker in the anteroposterior (AP) direction by Real-time Position Management system (RPM) system during CT procedure. Lung diameter in AP direction on both free-breathing (FB) and DIBH images were measured at 4 levels: half upper, upper border of heart, half lower and diaphragm levels of lung. The first CT slice with visible lung tissue and diaphragm level were identified for lung diameter in superioinferior (SI) direction corresponding Z values (slice position). Pearson correlation test was used to examine the correlation significance among BHL, lung AP diameter, lung SI diameter, heart dose, and left lung dose. Results The BHL had strongly positive correlation with lung expansion in SI direction (Pearson correlation r = 0.770). When BHL increased 1.0 cm, the lung expansion in SI direction increased 2.6 cm. Also, moderately positive correlation was revealed between BHL and total lung volume (Pearson correlation r = 0.596). When BHL increased 1.0 cm, the total lung volume increased 414.1c.c. Heart mean dose and maximum dose were strongly negative correlated to BHL according to inverse square law. Weak or no correlation was noted between BHL and lung expansion in AP direction at 4 levels (Pearson correlation r = 0.318, 0.009, −0.105 and −0.238). DIBH plans had lower heart dose (2.64 vs. 1.32 Gy; P = 0.005) and ipsilateral lung V20 (%) relative volume (13.6 vs. 10.9 %; P = 0.007) than FB plans as previous studies. Conclusions Setting infrared reflecting marker below the xiphoid process on the abdominal wall is sufficient for DIBH technique to detect lung expansion in SI direction and decrease the heart dose significantly. Patient couching before CT scans is recommended.

Target position reproducibility in left-breast irradiation with deep inspiration breath-hold using multiple optical surface control points.

The aim of this study was to investigate the use of 3D optical localization of multiple surface control points for deep inspiration breath‐hold (DIBH) guidance in left‐breast radiotherapy treatments. Ten left‐breast cancer patients underwent whole‐breast DIBH radiotherapy controlled by the Real‐time Position Management (RPM) system. The reproducibility of the tumor bed (i.e., target) was assessed by the position of implanted clips, acquired through in‐room kV imaging. Six to eight passive fiducials were positioned on the patients' thoraco‐abdominal surface and localized intrafractionally by means of an infrared 3D optical tracking system. The point‐based registration between treatment and planning fiducials coordinates was applied to estimate the interfraction variations in patients' breathing baseline and to improve target reproducibility. The RPM‐based DIBH control resulted in a 3D error in target reproducibility of 5.8 ± 3.4 mm (median value ± interquartile range) across all patients. The reproducibility errors proved correlated with the interfraction baseline variations, which reached 7.7 mm for the single patient. The contribution of surface fiducials registration allowed a statistically significant reduction (p < 0.05) in target localization errors, measuring 3.4 ± 1.7 mm in 3D. The 3D optical monitoring of multiple surface control points may help to optimize the use of the RPM system for improving target reproducibility in left‐breast DIBH irradiation, providing insights on breathing baseline variations and increasing the robustness of external surrogates for DIBH guidance.

Predictors of pneumonitis-free survival following lung stereotactic body radiation therapy.

Radiation pneumonitis is a common toxicity following lung stereotactic body radiation therapy (SBRT). We explored whether motion management technique, in conjunction with patient and treatment characteristics, is a predictor of radiation pneumonitis-free survival (PNFS). A single institution multi-center lung SBRT database was retrospectively reviewed. PNFS was defined as time to earliest onset of radiation pneumonitis or last clinical follow-up. Patients were simulated using a 4-dimensional approach, and those with 1 cm or greater tumor motion were selected for respiratory-gated treatment. Real-time Position Management and phase-based gating were employed. Univariate and multivariable Cox proportional hazard models were fit for relevant covariates to determine the impact of free-breathing versus respiratory-gated treatment on PNFS. The initial treatment courses of 208 patients were included, with a median follow-up length of 23 months. The median age at treatment was 71 years. About 91.8% of patient had early stage (T1-2) non-small cell lung cancer and were treated with common regimens including 10 Gy ×5, 12 Gy ×4 and 18 Gy ×3; 26.4% underwent respiratory-gated SBRT. The overall rate of grade 3 or higher radiation pneumonitis was 10.1%. PNFS was not significantly different between patients treated with respiratory-gated versus free-breathing SBRT (HR =0.88; P=0.707); tumor location and fractionation were predictors of PNFS in the multivariate setting. The method of motion management does not appear to impact PNFS when the tolerance for tumor displacement is 1 cm or less for free-breathing treatment planning and delivery. This approach may be appropriate when selecting patients for respiratory gating.