Cine EPID Images Research Articles

Prospective Randomized Trial Comparing 2 Devices for Deep Inspiration Breath Hold Management in Breast Radiation Therapy: Results of the BRAVEHeart Trial

Aims: The BRAVEHeart trial prospectively randomized left-sided breast cancer patients to one of two deep inspiration breath hold (DIBH) biofeedback devices: a novel chest surface tracking system and an abdominal block tracking system. The primary hypothesis was that the accuracy of chest tracking would be higher than that of abdominal tracking as the chest is a more direct surrogate of the breast target.Methods: Left-sided breast cancer patients were treated in DIBH with IMRT delivery. Patients were randomized to either the novel chest surface system or abdominal block system for active management of breath hold with visual feedback. On both trial arms the unallocated system monitored passively.239,296 cine EPID images were analyzed retrospectively to extract the chest wall position. Treatment accuracy was quantified as the deviation of the internal chest wall during treatment relative to the planned position from the digitally reconstructed radiograph. The correlation between motion of the external surrogate and internal chest wall was calculated per breath hold. Ease of use was assessed with questionnaires for both radiation therapists and patients and appointment length recorded.Results: Data from 26 participants were available for analysis. No difference was found in delivered treatment accuracy between arms. Across all patients and fractions, the median correlation between internal chest wall movement and external surrogate was 0.69 for the chest surface and 0.17 for the abdominal block. Patients found it easy to follow visual feedback from both systems. No difference was found in appointment length between arms.Conclusions: No statistical evidence was found for superior treatment accuracy, satisfaction or appointment length for the novel chest surface tracking device compared to the abdominal block system. During DIBH, the median per-breath hold correlation of internal chest wall movement to the motion of the chest surface was higher than the median correlation of the abdominal block to the chest surface.

Automated review of patient position in DIBH breast hybrid IMRT with EPID images.

Deep Inspiration Breath Hold (DIBH) is a respiratory-gating technique adopted in radiation therapy to lower cardiac irradiation. When performing DIBH treatments, it is important to have a monitoring system to ensure the patient's breath hold level is stable and reproducible at each fraction. In this retrospective study, we developed a system capable of monitoring DIBH breast treatments by utilizing cine EPID images taken during treatment. Setup error and intrafraction motion were measured for all fractions of 20 left-sided breast patients. All patients were treated with a hybrid static-IMRT technique, with EPID images from the static fields analyzed. Ten patients had open static fields and the other ten patients had static fields partially blocked with the multileaf collimator (MLC). Three image-processing algorithms were evaluated on their ability to accurately measure the chest wall position (CWP) in EPID images. CWP measurements were recorded along a 61-pixel region of interest centered along the midline of the image. The median and standard deviation of the CWP were recorded for each image. The algorithm showing the highest agreement with manual measurements was then used to calculate intrafraction motion and setup error. To measure intrafraction motion, the median CWP of the first EPID frame was compared with that of the subsequent EPID images of the treatment. The maximum difference was recorded as the intrafraction motion. The setup error was calculated as the difference in median CWP between the MV DRR and the first EPID image of the lateral tangential field. The results showed that the most accurate image-processing algorithm can identify the chest wall within 1.2mm on both EPID and MV DRR images, and measures intrafraction motion and setup errors within 1.4mm.

Appropriate margin for planning target volume for breast radiotherapy during deep inspiration breath-hold by variance component analysis

BackgroundThis study aimed to quantify errors by using a cine electronic portal imaging device (cine EPID) during deep inspiration breath-hold (DIBH) for left-sided breast cancer and to estimate the planning target volume (PTV) by variance component analysis.MethodsThis study included 25 consecutive left-sided breast cancer patients treated with whole-breast irradiation (WBI) using DIBH. Breath-holding was performed while monitoring abdominal anterior–posterior (AP) motion using the Real-time Position Management (RPM) system. Cine EPID was used to evaluate the chest wall displacements in patients. Cine EPID images of the patients (309,609 frames) were analyzed to detect the edges of the chest wall using a Canny filter. The errors that occurred during DIBH included differences between the chest wall position detected by digitally reconstructed radiographs and that of all cine EPID images. The inter-patient, inter-fraction, and intra-fractional standard deviations (SDs) in the DIBH were calculated, and the PTV margin was estimated by variance component analysis.ResultsThe median patient age was 55 (35–79) years, and the mean irradiation time was 20.4 ± 1.7 s. The abdominal AP motion was 1.36 ± 0.94 (0.14–5.28) mm. The overall mean of the errors was 0.30 mm (95% confidence interval: − 0.05–0.65). The inter-patient, inter-fraction, and intra-fractional SDs in the DIBH were 0.82 mm, 1.19 mm, and 1.63 mm, respectively, and the PTV margin was calculated as 3.59 mm.ConclusionsErrors during DIBH for breast radiotherapy were monitored using EPID images and appropriate PTV margins were estimated by variance component analysis.

An error detection method for real-time EPID-based treatment delivery quality assurance.

To quantify the error detection power of a new treatment delivery error detection method. The method validates monitor unit (MU) resolved beam apertures using real-time EPID images. The on-board EPID imager was used to measure cine-EPID (~10Hz) images for 27 beams from 15 VMAT/SBRT clinical treatment plans and five nonclinical plans. For each frame acquisition, planned apertures were interpolated from the treatment plan multileaf collimator (MLC) positions expected during the frame acquisition interval. Inaccurate deliveries were identified by monitoring in-aperture missed fluence and out-of-aperture excess fluence beyond a specified buffer. Delivery errors were simulated by perturbing the planned MLC positions before comparison with nonperturbed measured apertures. Systematic 1-5mm MLC leaf shifts were used to train a logistic regression model to determine the error detection threshold. Model accuracy was monitored using tenfold cross-validation. The model's error detection ability was tested with other error modes: plan control point (CP) weight perturbations, collimator rotations, random MLC leaf position errors, EPID imager shift, and stuck MLC leaf. The error detection accuracy was evaluated using the Matthews correlation coefficient (MCC) and the false positive rate (FPR). Per-beam error thresholds of >1, >5, and >10% errant frames were tested to label per-beam errors. The model also was tested for its ability to distinguish five cases with highly similar plans and compared with gamma analysis. Delivery errors were detected by monitoring intended per-frame images with a 2mm MLC buffer. Frame-by-frame aperture errors were identified with an optimal threshold of 0.3% of the expected aperture area. The per-frame FPR was 0.02%. The MCC was 1.00 (perfect classification) for detection based on 1% of frames for random CP weight shift, 3mm random MLC shifts, 90° and 180° collimator rotations, and an MLC leaf stuck after 10% of the beam delivery. The MCC for 2°, 4°, and 8° collimator rotation were 0.53, 0.76, and 0.96, respectively, for the 1% of beam delivery threshold. The 3mm EPID shift had poor detection, with a minimum MCC of 0.14. The highly similar plans were reliably detected by the aperture check but were not detectable with gamma analysis. The high error detection sensitivity and low FPR makes the aperture check error detection method well suited to pretreatment and during-treatment beam delivery quality assurance (QA). The aperture check detects subtle beam delivery errors, including those resulting from MLC leaf positioning deviations, CP MU shifts, and stuck MLC leaves. Furthermore, the method can distinguish between highly similar treatment plans. Since the aperture check method monitors for the aperture shapes over a given MU interval, it is also sensitive to errors in MU per CP, without requiring dosimetric calibration of the EPID. The aperture check is one part of a Swiss cheese error detection scheme, which provides redundant error testing of multiple error modes, including nonaperture related errors. The rapid error detection, at 1% of a beam's delivery, make the aperture check a potential candidate for QA of on-line adaptive radiotherapy, or other situations in which pretreatment delivery QA is impractical.

Quantifying the performance of two different types of commercial software programs for 3D patient dose reconstruction for prostate cancer patients: Machine log files vs. machine log files with EPID images

We clarified the reconstructed 3D dose difference between two different commercial software programs (Mobius3D v2.0 and PerFRACTION v1.6.4).Five prostate cancer patients treated with IMRT (74 Gy/37 Fr) were studied. Log files and cine EPID images were acquired for each fraction. 3D patient dose was reconstructed using log files (Mobius3D) or log files with EPID imaging (PerFRACTION). The treatment planning dose was re-calculated on homogeneous and heterogeneous phantoms, and log files and cine EPID images were acquired. Measured doses were compared with the reconstructed point doses in the phantom. Next, we compared dosimetric metrics (mean dose for PTV, rectum, and bladder) calculated by Mobius3D and PerFRACTION for all fractions from five patients.Dose difference at isocenter between measurement and reconstructed dose for two software programs was within 3.0% in both homogeneous and heterogeneous phantoms. Moreover, the dose difference was larger using skip arc plan than that using full arc plan, especially for PerFRACTION (e.g., dose difference at isocenter for PerFRACTION: 0.34% for full arc plan vs. −4.50% for skip arc plan in patient 1).For patients, differences in dosimetric parameters were within 1% for almost all fractions. PerFRACTION had wider range of dose difference between first fraction and the other fractions than Mobius3D (e.g., maximum difference: 0.50% for Mobius3D vs. 1.85% for PerFRACTION), possibly because EPID may detect some types of MLC positioning errors such as miscalibration errors or mechanical backlash which cannot be detected by log files, or that EPID data might include image acquisition failure and image noise.

Sci‐Fri PM: Radiation Therapy, Planning, Imaging, and Special Techniques ‐ 11: Quantification of chest wall motion during deep inspiration breast hold treatments using cine EPID images and a physics based algorithm

Purpose:This work presents an algorithm used to quantify intra‐fraction motion for patients treated using deep inspiration breath hold (DIBH). The algorithm quantifies the position of the chest wall in breast tangent fields using electronic portal images.Methods:The algorithm assumes that image profiles, taken along a direction perpendicular to the medial border of the field, follow a monotonically and smooth decreasing function. This assumption is invalid in the presence of lung and can be used to calculate chest wall position. The algorithm was validated by determining the position of the chest wall for varying field edge positions in portal images of a thoracic phantom. The algorithm was used to quantify intra‐fraction motion in cine images for 7 patients treated with DIBH.Results:Phantom results show that changes in the distance between chest wall and field edge were accurate within 0.1 mm on average. For a fixed field edge, the algorithm calculates the position of the chest wall with a 0.2 mm standard deviation. Intra‐fraction motion for DIBH patients was within 1 mm 91.4% of the time and within 1.5 mm 97.9% of the time. The maximum intra‐fraction motion was 3.0 mm.Conclusions:A physics based algorithm was developed and can be used to quantify the position of chest wall irradiated in tangent portal images with an accuracy of 0.1 mm and precision of 0.6 mm. Intra‐fraction motion for patients treated with DIBH at our clinic is less than 3 mm.

TH‐CD‐207A‐11: Sensitivity Analysis of Action Limits for Real‐Time EPID‐Based Delivery Verification System Using Artificial Clinical Relevant Error Simulations

Purpose:A real‐time patient treatment delivery verification system using EPID (Watchdog) has been developed as an advanced patient safety tool. In a pilot study data was acquired for 119 prostate and head and neck (HN) IMRT patient deliveries to generate body‐site specific action limits using statistical process control. The purpose of this study is to determine the sensitivity of Watchdog to detect clinically significant errors during treatment delivery.Methods:Watchdog utilizes a physics‐based model to generate a series of predicted transit cine EPID images as a reference data set, and compares these in real‐time to measured transit cine‐EPID images acquired during treatment using chi comparison (4%, 4mm criteria) after the initial 2s of treatment to allow for dose ramp‐up. Four study cases were used; dosimetric (monitor unit) errors in prostate (7 fields) and HN (9 fields) IMRT treatments of (5%, 7%, 10%) and positioning (systematic displacement) errors in the same treatments of (5mm, 7mm, 10mm). These errors were introduced by modifying the patient CT scan and re‐calculating the predicted EPID data set. The error embedded predicted EPID data sets were compared to the measured EPID data acquired during patient treatment. The treatment delivery percentage (measured from 2s) where Watchdog detected the error was determined.Results:Watchdog detected all simulated errors for all fields during delivery. The dosimetric errors were detected at average treatment delivery percentage of (4%, 0%, 0%) and (7%, 0%, 0%) for prostate and HN respectively. For patient positional errors, the average treatment delivery percentage was (52%, 43%, 25%) and (39%, 16%, 6%).Conclusion:These results suggest that Watchdog can detect significant dosimetric and positioning errors in prostate and HN IMRT treatments in real‐time allowing for treatment interruption. Displacements of the patient require longer to detect however incorrect body site or very large geographic misses will be detected rapidly.

SU‐E‐T‐428: Feasibility Study of 4D Image Reconstruction by Organ Motion Vector Extension Based On Portal Images

Purpose:To develop and to test a method to generate a new 4D CT images of the treatment day from the old 4D CT and the portal images of the day when the motion extent exceeded from that represented by plan CTs.Methods:A motion vector of a moving tumor in a patient may be extended to reconstruct the tumor position when the motion extent exceeded from that represented by plan CTs. To test this, 1. a phantom that consists of a polystyrene cylinder (tumor) embedded in cork (lung) was placed on a moving platform with 4 sec/cycle and amplitudes of 1 cm and 2 cm, and was 4D‐scanned. 2. A 6MV photon beam was irradiated on the moving phantoms and cineEPID images were obtained. 3. A motion vector of the tumor was acquired from 4D CT images of the phantom with 1 cm amplitude. 4. From cine EPID images of the phantom with the 2 cm amplitude, various motion extents (0.3 cm, 0.5 cm, etc) were acquired and programmed into the motion vector, producing CT images at each position. 5. The reconstructed CT images were then compared with pre‐acquired “reference” 4D CT images at each position (i.e. phase).Results:The CT image was reconstructed and compared with the reference image, showing a slight mismatch in the transition direction limited by voxel size (slice thickness) in CT image. Due to the rigid nature of the phantom studied, the modeling the displacement of the center of object was sufficient. When deformable tumors are to be modeled, more complex scheme is necessary, which utilize cine EPID and 4D CT images.Conclusion:The new idea of CT image reconstruction was demonstrated. Deformable tumor movements need to be considered in the future.

SU‐E‐J‐35: Using CBCT as the Alternative Method of Assessing ITV Volume

PurposeTo study the accuracy of Internal Target Volumes (ITVs) created on cone beam CT (CBCT) by comparing the visible target volume on CBCT to volumes (GTV, ITV, and PTV) outlined on free breathing (FB) CT and 4DCT.MethodsA Quasar Cylindrical Motion Phantom with a 3cm diameter ball (14.14 cc) embedded within a cork insert was set up to simulate respiratory motion with a period of 4 seconds and amplitude of 2cm superioinferiorly and 1cm anterioposteriorly. FBCT and 4DCT images were acquired. A PTV‐4D was created on the 4DCT by applying a uniform margin of 5mm to the ITV‐CT. PTV‐FB was created by applying a margin of the motion range plus 5mm, i.e. total of 1.5cm laterally and 2.5cm superioinferiorly to the GTV outlined on the FBCT. A dynamic conformal arc was planned to treat the PTV‐FB with 1mm margin. A CBCT was acquired before the treatment, on which the target was delineated. During the treatment, the position of the target was monitored using the EPID in cine mode.ResultsITV‐CBCT and ITV‐CT were measured to be 56.6 and 62.7cc, respectively, with a Dice Coefficient (DC) of 0.94 and disagreement in center of mass (COM) of 0.59 mm. On the other hand, GTV‐FB was 11.47cc, 19% less than the known volume of the ball. PTV‐FB and PTV‐4D were 149 and 116 cc, with a DC of 0.71. Part of the ITV‐CT was not enclosed by the PTV‐FB despite the large margin. The cine EPID images have confirmed geometrical misses of the target. Similar under‐coverage was observed in one clinical case and captured by the CBCT, where the implanted fiducials moved outside PTV‐FB.ConclusionITV‐CBCT is in good agreement with ITV‐CT. When 4DCT was not available, CBCT can be an effective alternative in determining and verifying the PTV margin.

WE-D-BRA-03: Four-Dimensional Dose Reconstruction Through Retrospective Phase Determination Using Cine Images of Electronic Portal Imaging Device

Purpose: To test a method to reconstruct a four-dimensional (4D) dose distribution using the correlation of pre-calculated 4D electronic portal imaging device (EPID) images and measured cine-EPID images. Methods: 1. A phantom designed to simulate a tumor in lung (a polystyrene block with 3.0 cm diameter embedded in cork) was placed on a sinusoidally moving platform with 2 cm amplitude and 4 sec/cycle. Ten-phase 4D CT images were acquired for treatment planning and dose reconstruction. A 6MV photon beam was irradiated on the phantom with static (field size=5×8.5 cm2) and dynamic fields (sliding windows, 10×10 cm2, X1 MLC closing in parallel with the tumor movement). 2. 4D and 3D doses were calculated forwardly on PTV (1 cm margin). 3. Dose images on EPID under the fields were calculated for 10 phases. 4. Cine EPID images were acquired during irradiation. 5. Their acquisition times were correlated to the phases of the phantom at which irradiation occurred by inter-comparing calculated “reference” EPID images with measured images (2D gamma comparison). For the dynamic beam, the tumor was hidden under MLCs during a portion of irradiation time; the correlation performed when the tumor was visible was extrapolated. 6. Dose for each phase was reconstructed on the 4D CT images and summed over all phases. The summation was compared with forwardly calculated 4D and 3D dose distributions. Monte Carlo methods were used for all calculations. Results: For the open and dynamic beams, the 4D reconstructed doses showed the pass rates of 92.7 % and 100 %, respectively, at the isocenter plane given 3% / 3 mm criteria. The better agreement of the dynamic beam was from its dose gradient which blurred the otherwise sharp difference between forward and reconstructed doses. This also contributed slightly better agreement in DVH of PTV. Conclusion: The feasibility of 4D reconstruction was demonstrated.

Quantifying intra- and inter-fractional motion in breast radiotherapy.

IntroductionThe magnitude of intra- and inter-fractional variation in the set up of breast cancer patients treated with tangential megavoltage photon beams was investigated using an electronic portal imaging device (EPID).MethodsDaily cine-EPID images were captured during delivery of the tangential fields for ten breast cancer patients treated in the supine position. Measurements collected from each image included the central lung distance (CLD), central flash distance (CFD), superior axial measurement (SAM) and the inferior axial measurement (IAM). The variation of motion within a fraction (intra-fraction) and the variation between fractions (inter-fraction) was analysed to quantify set up variation and motion due to respiration.ResultsAltogether 3775 EPID images were collected from 10 patients. The effect of respiratory motion during treatment was <0.1 cm standard deviation (SD) in the anterior–posterior (AP) direction. The inter-fraction movement caused by variations in daily set up was larger at 0.28 cm SD in the AP direction. Superior–inferior (SI) variation was more difficult to summarise and proved unreliable as the measurements were taken to an ambiguous point on the images. It was difficult to discern true SI movement from that implicated by AP movement.ConclusionThere is minimal intra-fractional chest wall motion due to respiration during treatment. Inter-fractional variation was larger, however, on average it remained within departmental tolerance (0.5 cm) for set up variations. This review of our current breast technique provides confidence in the feasibility of utilising advanced treatment techniques (field-in-field, intensity modulated radiotherapy or volumetric modulated arc therapy) following a review of the current imaging protocol.

SU‐E‐T‐02: 3D Dose Reconstruction of Volumetric Modulated Arc Therapy From Delivered EPID Dose Image

Purpose:To evaluate dose reconstruction of volumetric modulated arc therapy (VMAT) from EPID image prediction.Methods:A test VMAT plan with dynamic MLCs was developed on a water equivalent cubic phantom with dimension of 31.4×34×34 cm3. From the initial MLC opening of 5×5 cm2 at gantry 0°, they moved with a constant speed as the gantry rotates, until they reached X1 at −2.5 cm and X2 at = −2.237 cm at gantry 90°. The plan was delivered while cine EPID images were acquired. For the 84 gantry angle segments, doses to the phantom were calculated by XVMC and reconstructed from EPID based on fast density‐scaled model and non‐iterative dose reconstruction method we have previously developed. The reconstructed dose was compared with the forwardly calculated dose individually over 84 gantry angle segments and cumulatively in terms of gamma evaluation and dose volume histogram for PTV (3×3×3 cm3) positioned at the center of the phantom.Results:3D‐gamma pass rate was 97.0 % cumulatively with 3 %/3 mm criteria and maximum 100% individually. The DVHs of PTV were indistinguishable except in the region of high dose (&gt;95% of prescribed dose) where the difference of a few percent in volume was found.Conclusion:Showing good agreement with the forwardly calculated dose, we have extended our model of dose reconstruction successfully to the verification of VMAT. Although more clinical demonstration is to be followed, our study presents a promising tool for time‐resolved and cumulative VMAT validation.

WE‐C‐WAB‐10: Estimation of 3D Fluoroscopic Treatment Images Using a Single Cine EPID Image

Purpose: A novel algorithm for volumetric time‐varying image estimation and 3D tumor localization from a single cine EPID image. Methods: Generating a volumetric fluoroscopic (time‐varying) image from a single EPID image is a two‐step process. In the first step a patient‐specific motion model is constructed and in the second step the motion model is updated according to the information in the EPID image. The patient‐specific motion model is a compact representation of lung motion represented in displacement vector fields (DVFs). DVFs are calculated through deformable image registration of a reference 4DCT image (typically peak‐exhale) to the 4DCT images corresponding to different phases of a breathing cycle. The salient characteristics in the DVFs are captured in a compact representation through Principal Component Analysis (PCA). PCA decouples the spatial and temporal components of the DVFs. Spatial information is represented in eigenvectors and the temporal information is represented by eigen‐coefficients. To generate a new volumetric image, the eigen coefficients are updated via cost function optimization based on DRRs and projection images. The updated eigen‐coefficients are then used to obtain updated DVFs that in turn give the volumetric image corresponding to the EPID image. Results: The algorithm was tested on 1) XCAT digital phantom data based on 8 irregular patient breathing patterns; and 2) patient cine EPID images acquired during SBRT. The root‐mean‐squared vector tumor localization error is (0.73 ± 0.63) mm for the XCAT data and (1.9 ± 1.6) mm for the patient data. Conclusions: We introduced a novel method of estimating volumetric time‐varying images from a single cine EPID image. This is the first method to estimate volumetric time‐varying images from MV cine EPID, and has the potential to provide 3D information with no additional imaging dose to the patient. The project described was supported, in part, by an RSNA Research Scholar Grant and Award Number R21CA156068 from the National Cancer Institute. The content is solely the responsibility of the authors and does not necessarily represent the official views of the RSNA, National Cancer Institute or the National Institutes of Health.

Gantry angle determination during arc IMRT: evaluation of a simple EPID-based technique and two commercial inclinometers

The increasing popularity of intensity‐modulated arc therapy (IMAT) treatments requires specifically designed linac quality assurance (QA) programs. Gantry angle is one of the parameters that has a major effect on the outcome of IMAT treatments since dose reconstruction for patient‐specific QA relies on the gantry angle; therefore, it is essential to ensure its accuracy for correct delivery of the prescribed dose. In this study, a simple measurement method and algorithm are presented for QA of gantry angle measurements based on integrated EPID images acquired at distinct gantry angles and cine EPID images during an entire 360° arc. A comprehensive study was carried out to evaluate this method, as well as to evaluate two commercially available inclinometers (NG360 and IBA GAS supplied in conjunction with popular array dosimeters Delta4 and MatriXXEvolution, respectively) by comparing their simultaneous angle measurement results with the linac potentiometer readouts at five gantry speeds. In all tested measurement systems, the average differences with the reference angle data were less than 0.3° in static mode. In arc mode, at all tested gantry speeds the average difference was less than 0.1° for the IBA GAS and the proposed EPID‐based method, and 0.6° for the NG360 after correction for the inherent systematic time delay of the inclinometer. The gantry rotation speed measured by the three independent systems had an average deviation of about 0.01°/s from the nominal gantry speed.PACS numbers: 06.30.Bp; 87.56.Fc; 87.56.‐v

WE-G-213CD-03: A Dual Complementary Verification Method for Dynamic Tumor Tracking on Vero SBRT.

to use complementary cine EPID and gimbals log file analysis for in-vivo tracking accuracy monitoring. A clinical prototype of dynamic tracking (DT) was installed on the Vero SBRT system. This prototype version allowed tumor tracking by gimballed linac rotations using an internal-external correspondence model. The DT prototype software allowed the detailed logging of all applied gimbals rotations during tracking. The integration of an EPID on the vero system allowed the acquisition of cine EPID images during DT. We quantified the tracking error on cine EPID (E-EPID) by subtracting the target center (fiducial marker detection) and the field centroid. Dynamic gimbals log file information was combined with orthogonal x-ray verification images to calculate the in-vivo tracking error (E-kVLog). The correlation between E-kVLog and E-EPID was calculated for validation of the gimbals log file. Further, we investigated the sensitivity of the log file tracking error by introducing predefined systematic tracking errors. As an application we calculate gimbals log file tracking error for dynamic hidden target tests to investigate gravity effects and decoupled gimbals rotation from gantry rotation. Finally, calculating complementary cine EPID and log file tracking errors evaluated the clinical accuracy of dynamic tracking. A strong correlation was found between log file and cine EPID tracking error distribution during concurrent measurements (R=0.98). We found sensitivity in the gimbals log files to detect a systematic tracking error up to 0.5 mm. Dynamic hidden target tests showed no gravity influence on tracking performance and high degree of decoupled gimbals and gantry rotation during dynamic arc dynamic tracking. A submillimetric agreement between clinical complementary tracking error measurements was found. Redundancy of the internal gimbals log file with x-ray verification images with complementary independent cine EPID images was implemented to monitor the accuracy of gimballed tumor tracking on Vero SBRT. Research was financially supported by the Flemish government (FWO), Hercules Foundation and BrainLAB AG.

Investigation of the sag in linac secondary collimator and MLC carriage during arc deliveries

In modern radiotherapy, it is vitally important to monitor the performance of all linac components including the collimation system. In this study, a simple measurement method and accurate algorithm are introduced for investigation of the secondary and tertiary collimator sag during radiotherapy arc treatments. The method is based on cine EPID images of a ball bearing marker fixed to the gantry head and determines the jaw and MLC sag in all directions relative to the reference at zero gantry angle. Analysis was performed using different field sizes and collimator angles, different linacs and different gantry rotation directions. The accuracy of the method was tested and was less than 0.02 mm. The repeatability and reproducibility of the method was 0.005 and 0.09 mm, respectively. The setup is easy and quick and the algorithm is fast and fully automatic with sub-pixel accuracy. This method is suitable to be included in the routine quality assurance of linacs to monitor the collimator system performance.

Verification of the linac isocenter for stereotactic radiosurgery using cine‐EPID imaging and arc delivery

Verification of the mechanical isocenter position is required as part of comprehensive quality assurance programs for stereotactic radiosurgery/radiotherapy (SRS/SRT) treatments. Several techniques have been proposed for this purpose but each of them has certain drawbacks. In this paper, a new efficient and more comprehensive method using cine-EPID images has been introduced for automatic verification of the isocenter with sufficient accuracy for stereotactic applications. Using a circular collimator fixed to the gantry head to define the field, EPID images of a Winston-Lutz phantom were acquired in cine-imaging mode during 3600 gantry rotations. A robust MATLAB code was developed to analyze the data by finding the center of the field and the center of the ball bearing shadow in each image with sub-pixel accuracy. The distance between these two centers was determined for every image. The method was evaluated by comparison to results of a mechanical pointer and also by detection of a manual shift applied to the phantom position. The repeatability and reproducibility of the method were tested and it was also applied to detect couch and collimator wobble during rotation. The accuracy of the algorithm was 0.03 +/- 0.02 mm. The repeatability was less than 3 pm and the reproducibility was less than 86 microm. The time elapsed for the analysis of more than 100 cine images of Varian aS1000 and aS500 EPIDs were approximately 65 and 20 s, respectively. Processing of images taken in integrated mode took 0.1 s. The output of the analysis software is printable and shows the isocenter shifts as a function of angle in both in-plane and cross-plane directions. It gives warning messages where the shifts exceed the criteria for SRS/SRT and provides useful data for the necessary adjustments in the system including bearing system and/or room lasers. The comprehensive method introduced in this study uses cine-images, is highly accurate, fast, and independent of the observer. It tests all gantry angles and is suitable for pretreatment QA of the isocenter for stereotactic treatments.

SU-E-T-504: Development of An Offline Based Internal Organ Motion Verification System during Treatment Using Sequential Cine EPID Images

Purpose: We developed an offline based organ motion verification system using cine EPID images and evaluated its accuracy and availability through phantom study. Methods: The system was designed for import of cine EPID images, which are obtained sequentially during treatment through a DICOM format and reconstructed into a live image. For verification of organ motion, a pattern matching algorithm using an internal surrogate was employed in the self-developed analysis software. For the system performance test, we developed a linear motion phantom, which consists of a human body shaped phantom with a fake tumor in the lung and linear motion cart with control software. The phantom was operated with a motion of 2 cm at 4 sec per cycle and cine EPID images were obtained at a rate of 3.3 frames/sec with 1024 × 768 pixel counts in a linear accelerator (10 MVX). Organ motion of the target was tracked using self-developed analysis software and compared with data from the RPM system (Varian, USA). For quantitative analysis, we analyzed correlation between two data sets in terms of average cycle, amplitude, and pattern (root mean square, RSM) of motion. Results: Averages for the cycle of motion from cine EPID and RPM system were 3.95±0.02 and 3.98±0.11 sec, respectively, and showed good agreement on real value (4 sec). Average of the amplitude of motion tracked by our system (1.71 ±0.02 cm) showed a slightly different value, compared with the actual value (2 cm), due to time resolution for image acquisition. The value of the RMS from the cine EPID image (0.379) grew slightly, by 3.8%, compared with data from the RPM (0.365). Conclusions: Our system showed good representation of its motion in a preliminary phantom study. The system can be implemented for clinical purposes, which include organ motion verification and its feedback for accurate dose delivery to the moving target.

TH‐C‐BRC‐08: An Innovative Tracking Device to Improve Left Breast Treatment Accuracy during the Deep Inspiration Breath Hold Technique

Purpose: The goal of the Deep Inspiration Breath Hold (DIBH) technique is to avoid direct radiation to the heart while treating the left breast tissue. Accurate and reproducible treatment setup of the patient is very important to successfully achieve the benefits of the DIBH technique. The purpose of this work is to introduce a device named the Real Time Skin Surface Distance (RTSSD) that accurately tracks the patient position during therapy. Methods: Five left‐sided breast cancer patients were used for this study. The RTSSD device consists of a laser distance meter rigidly fixed to the treatment couch. At the time of simulation, the RTSSD device measures chest position at the sternum tattooed location during free‐breathing and DIBH CT scans. Prior to each radiation treatment, free breathing and DIBH distances are measured at the sternum tattoo location using the RTSSD device and compared to the ones measured at the time of the CT scans. If a difference larger than +/− 3mm is found, the patient is repositioned. KV orthogonal images are acquired during DIBH and compared to the reference digital radiographs for internal verification. During irradiation, the therapist monitors the RTSSD measurement and interrupts the treatment if it exceeds the 3 mm tolerance. Cine EPID images, acquired during irradiation, are used retrospectively to validate the localization. Results: The RTSSD device tracked the patient accurately during the DIBH treatment as confirmed by in‐treatment imaging. The cine EPID images showed agreement with reference DRRs within the +/− 3mm tolerance. Conclusions: The RTSSD device is non‐invasive, easy to operate and well tolerated by the patients. The device tracked accurately the distance from patientˈs sternum tattooed location during DIBH simulation and delivery. Imaging results confirmed that the device measurements were reliable to use for patient positioning during DIBH treatment.

SU-E-T-571: EPID Assisted Dosimetric Evaluation of Treatment Planning Using Helical or 4D CT in Stereotactic Radiotherapy of Lung Cancer

Purpose: To investigate potential dosimetric discrepancy due to the choice of CT scanning protocols (helical versus 4DCT) for treatment planning in stereotactic body radiotherapy of lungcancer. Method and Materials: For each lungcancer patient treated with SBRT, both free‐breathing helical CT and 4DCT images were obtained. A conventional PTV (PTVconv) was created by applying an anisotropic margin of 5mm in the axial plane and 10mm in the longitudinal (craniocaudal) plane to the GTV outlined on the helical CT. Similarly, a PTV4D was created by applying a uniform margin of 5 mm to the internal target volume (ITV) contoured on the average intensity image set derived from the 4DCT. Both approaches are accepted by RTOG protocols (0813 and 0915). In this study, two SBRT plans were created for each patient using identical beam portals except the aperture of each beam is adjusted according to the PTVconv or PTV4D, respectively. The volume difference of both PTVs and target coverage were compared between two plans. On a daily basis, the position of the GTV was monitored real‐time for each field using an EPID in cine mode. The dose distribution was recalculated based on geometrical information obtained from the cine images. Results: It was observed that the volume of PTVconv is 16% smaller than PTV4D. Subsequently the SBRT plan using PTVconv provides suboptimal dose coverage to the tumor than PTV4D. The V100 of PTV4D decreased from 95% to 82% and D95 of ITV decreased from 47Gy to 42Gy. The cine EPIDimages have confirmed tumor excursion with respiratory motion is within the predefined ITV boundary. However if the plan is based on apertures designed using PTVconv, there could be geometrical misses resulting in target under dosage. Conclusion: It is recommended to perform 4D CT and use PTV4D for treatment planning for SBRTlungcancer.