SI Direction Research Articles

SU‐GG‐I‐157: Phase Correlated Image Registration to 4D CT for Liver Stereotactic Radiosurgery with Image Guidance CT

Purpose: Stereotactic radiosurgery (SRS) is characterized by high doses delivered to small targets in a few fractions. Image guidance systems, cone beam computed tomography (CBCT), that yield volume imaging and sufficient soft‐tissue contrast have permitted daily imaging of bony anatomy and target permitting online correction of tumor position prior to treatment delivery, but CBCT is not perfectly support in phase correlated image guidance for targeting. The main objective was to overcome breathing induced uncertainties in the process of image guided RT using phase correlated image registration both the planning CT scan and CBCT for verification imaging scan. Methods and materials: For 20 patients with liver SRS, respiratory correlated 4D CT scan and CBCT for image guidance was acquired. Internal target volume (ITV) is determined utilizing 50% phase end exhalation reconstructed images by 4DCT, In order to check the accuracy of maximum intensity projection for various target motion, especially for target moving irregularly with three implanted gold seed. The data analyzed were respiratory‐induced peak‐to‐trough distance, 3D motion from LR, AP and SI directions and motion nonlinearity and hysteresis. Motion hysteresis occurs when three seed follows different paths between inhale and exhale phases during a respiratory cycle. Results: The overall mean respiratory‐induced peak‐to‐trough distance is 0.48 cm, with individual treatment fraction means ranging from 0.02 to 1.44 cm. The overall mean respiratory period is 3.8 s, with individual treatment fraction means ranging from 2.2 to 6.4 s. Conclusions: The motion nonlinearity and hysteresis are important characteristics of respiratory tumor motion. From 3D tumor trajectories, they showed that the hysteresis ranged from 0.1 to 0.6 cm for 20 tumors.

SU‐GG‐J‐110: The Use of 4D Dose Calculations to Evaluate Whether the ITV Can Be Created Using a Reduced Set of 4DCT Phases

Purpose: To investigate whether it is possible to create the ITV using a reduced set of 4DCT phases Methods: Target contours were drawn on the exhale phase of a 4DCT image set for 10 lung cancer patients. Each target volume was mapped to all 10 phases of the 4DCT using deformable registration. An ITV was created as the union of the target volumes for the N phases closest to exhale, where N≤10 (10 indicates the full envelope of motion). VMAT plans were created for each ITV using the exhale phase CT (3D dose calculation). Doses were calculated on each of the 10 phases, and the cumulative dose calculated by deformable mapping of the doses to the exhale phase (4D dose calculation). For each plan, the dose received by 95% of the target (D95) on the exhale phase from the 4D dose calculation was compared with that calculated for N=10. The 4D and 3D dose calculations were also compared. Results: Average tumor motion was 0.7cm (range 0.5 – 1.5cm), 0.9cm (0.5 – 1.5cm) and 0.8cm (0.5 – 1.0cm) in the AP, SI and RL directions, respectively. For all patients, reducing N from 10 to 8 reduced D95 by less than 5%, but the average reduction in ITV volume was only 5±3% (range: 1–10%). For N=7, there was up to a 13% reduction in D95. One patient was an exception: For this patient reducing N from 10 to 6 reduced D95 by just 4%, and reduced the ITV volume by 18% (10 c.c.) Conclusions: For the majority of patients, it is not possible to reduce the size of the ITV appreciably without reducing coverage. There may be a subset of patients for whom the ITV could be further reduced, but they cannot be identified without 4D dose calculations.

SU-GG-T-08: A Novel Treatment Couch for Real-Time Tracking of Respiration Induced Target Motion: Evaluating Its Geometric Accuracy

Purpose: To characterize the performance of a novel treatment couch designed and developed for respiration-induced real-time tumor motion tracking and to investigate its behavior with real tumor trajectories. Method and Materials: A new treatment couch has been developed to track targets in real-time. The system is capable of tracking targets with less than 0.5 mm accuracy. The max physical velocity for the couch is 13 cm/sec along SI and ML directions and 10 cm/sec along AP direction. It can reach a maximum acceleration of 100 cm/sec2 along all the three axes. To quantify the performance of the couch we used 25 tumor trajectories showing a peak-to-peak displacement in the range of 20 – 39 mm and sampled at 38 ms. These trajectories were fed to 3D time-varying trajectories derived from patient data. The motion of the phantom measured in real-time via a camera system was feed into the couch control system. Tracking errors were analyzed. Results: We found that the mean error in tracking for 20 tumor trajectories was 0.37 mm with a standard deviation of 0.29 mm. The system dead time was approximately 40ms. For individual trajectories, the median error varied from 0.15 mm to 0.70 mm whereas the individual mean values varied from 0.17 mm to 0.74 mm. Couch speed and acceleration were acceptable for real-time target tracking. Conclusion: We have developed a new couch for use with a linear accelerator that can track and correct for intrafraction (including respiration-induced) target motion. Mean and median tracking errors are less than 0.5 mm even for peak-to-peak displacements up to 4 cm. The couch shows a fairly stable and vibration free behavior over a wide range of tumor displacements studied. Conflict of Interest: NIH grant CA124766

SU‐GG‐J‐105: An Analysis of CyberKnife Prediction Accuracy

Purpose: To evaluate the prediction model used by the CyberKnife radiation therapy system in marginal terms. Method and Materials: The CyberKnife uses an online prediction model to improve radiation delivery. Modeler points indicate the tracked position of the tumor and Predictor points predict the position ≈115 ms in the future. The discrepancy between Predictor points and their corresponding Modeler points was investigated. For 100 data sets from 23 de‐identified lung patients, margins were determined in each anatomic direction about the Predictor points so as to minimally increase coverage to an arbitrary percentage of Modeler points. Each data set had about 30 minutes of motion data, of which about 10 minutes constituted treatment time; only these 10 minutes were used in the analysis. The frequencies of margin sizes were analyzed and truncated Gaussian normal functions were fit to each direction's distribution. The standard deviation of each Gaussian distribution was then used to describe the necessary margin expansions in each signed dimension in order to achieve the desired coverage. In this study 95% coverage was compared to 99% coverage. Results: Considering the magnitude of 2 from the mean of the Gaussian in each signed dimension, the margin expansions needed for 95% coverage were 1.2 mm in the lateral directions and 1.7 mm in the AP directions. For the SI directions the fit was poor; but empirically, the expansions were 3.5 mm. For 99% coverage, the AP margin was 3.6 mm and the lateral margin was 2.9 mm. The SI margins for 99% coverage were highly variable. Conclusion: The Predictor points follow the Modeler points closely. Margins were found in each clinical direction that would provide 95% coverage for 95% of the fractions reviewed in this study. Similar margins were found in two clinical directions for 99% coverage in 95% of fractions.

A Study of Structural Defects in 3C-SiC Hetero-Epitaxial Films

A wide characterization of crystalline defects involved in the 3C-SiC heteroepitaxy on Si is here presented. The aim of this work is to show how analysis techniques, such as transmission electron microscopy (TEM) and x-ray diffraction (XRD), can help the researcher in the study of structural defects. The work is focused on stacking faults and microtwins since both of them influence the atomic stacking along the {111} 3C-SiC planes. Their distinction can indeed be troublesome. It will be shown that TEM can be helpful, by choosing a determined zone axis of observation, for defect characterization and distinction. Moreover, the impact of microtwins on the crystal quality of 3C-SiC films is studied by performing XRD pole figures. By means of this technique and simulations, we found that the <111> direction of the SiC crystal is not aligned to the <110> Si direction, but it is shifted of 3.5° along the <002> Si direction, due to second-order twinnings in the 3C-SiC crystal.

InSb films grown on the V-grooved Si(001) substrate with InSb bilayer

V-shaped grooves were prepared by patterning of line and space (LS) using photolithography and Reactive Ion Etching (RIE), and anisotropic etching using hot KOH solution on the patterned 100 nm- SiO2/Si(001) substrate. The V-shaped grooves consist of two 〈111〉 planes. The width of grooves was varied from 1 to 10 μm while keeping intervals (line-shaped spaces) between the grooves. The InSb bi-layer was prepared onto the 〈111〉 surfaces and the heteroepitaxial growth of InSb films was performed using two-step growth procedure via the InSb bi-layer. Compared with the samples directly grown on the V-grooved Si(001) substrate, the samples grown via the InSb bi-layer showed higher crystal quality. The weak InSb(004) peak was observed in the 2θ/ω scan at χ=0o, indicating the 〈001〉 planes of the InSb films don’t face toward to the normal direction of Si(001) substrate. This means that the InSb films grown via InSb bi-layer rotate by 30∘ with respect to the 〈111〉 surfaces of the V-shaped grooves. The confirmation of the rotated InSb(111) peak was difficult, because the incident and diffracted X-ray were obstructed by the grooves. The full width at half maximum (FWHM) of the InSb(111) peak in the 2θ/ω scan at χ=54.7∘ became broader with decrease in the space width between the line-shaped 〈001〉 surfaces. This may be related to the areal ratio of the line-shaped 〈001〉 surfaces, on which the InSb bi-layer don’t form under the present growth condition.

Sliding characteristic and material compressibility of human lung: Parametric study and verification

To find and verify the optimum sliding characteristics and material compressibility that provide the minimum error in deformable image registration of the lungs. A deformable image registration study has been conducted on a total of 16 lung cancer patients. Patient specific three dimensional finite element models have been developed to model left and right lungs, chest (body), and tumor based on 4D CT images. Contact surfaces have been applied to lung-chest cavity interfaces. Experimental test data are used to model nonlinear material properties of lungs. A parametric study is carried out on seven patients, 20 conditions for each, to investigate the sliding behavior and the tissue compressibility of lungs. Three values of coefficient of friction of 0, 0.1, and 0.2 are investigated to model lubrication and sliding restriction on the lung-chest cavity interface. The effect of material compressibility of lungs is studied using Poisson's ratios of 0.35, 0.4, 0.45, and 0.499. The model accuracy is examined by calculating the difference between the image-based displacement of bronchial bifurcation points identified in the lung images and the calculated corresponding model-based displacement. Furthermore, additional bifurcation points around the tumor and its center of mass are used to examine the effect of the mentioned parameters on the tumor localization. The frictionless contact model with 0.4 Poisson's ratio provides the smallest residual errors of 1.1 +/- 0.9, 1.5 +/- 1.3, and 2.1 +/- 1.6 mm in the LR, AP, and SI directions, respectively. Similarly, this optimum model provides the most accurate location of the tumor with residual errors of 1.0 +/- 0.6, 0.9 +/- 0.7, and 1.4 +/- 1.0 mm in all three directions. The accuracy of this model is verified on an additional nine patients with average errors of 0.8 +/- 0.7, 1.3 +/- 1.1, and 1.7 +/- 1.6 mm in the LR, AP, and SI directions, respectively. The optimum biomechanical model with the smallest registration error is when frictionless contact model and 0.4 Poisson's ratio are applied. The overall accuracies of all bifurcation points in all 16 patients including tumor points are 1.0 +/- 0.7, 1.2 +/- 1.0, and 1.7 +/- 1.4 mm in the LR, AP, and SI directions, respectively.

Automatic marker detection and 3D position reconstruction using cine EPID images for SBRT verification

In previous studies, an electronic portal imaging device (EPID) in cine mode was used for validating respiratory gating and stereotactic body radiation therapy (SBRT) by tracking implanted fiducials. The manual marker tracking methods that were used were time and labor intensive, limiting the utility of the validation. The authors have developed an automatic algorithm to quickly and accurately extract the markers in EPID images and reconstruct their 3D positions. Studies have been performed with gold fiducials placed in solid water and dynamic thorax phantoms. In addition, the authors have examined the cases of five patients being treated under an SBRT protocol for hepatic metastases. For each case, a sequence of images was created by collecting the exit radiation using the EPID. The markers were detected and recognized using an image processing algorithm based on the Laplacian of Gaussian function. To reduce false marker detection, a marker registration technique was applied using image intensity as well as the geometric spatial transformations between the reference marker positions produced from the projection of 3D CT images and the estimated marker positions. An average marker position in 3D was reconstructed by backprojecting, towards the source, the position of each marker on the 2D image plane. From the static phantom study, spatial accuracies of <1 mm were achieved in both 2D and 3D marker locations. From the dynamic phantom study, using only the Laplacian of the Gaussian algorithm, the marker detection success rate was 88.8%. However, adding a marker registration technique which utilizes prior CT information, the detection success rate was increased to 100%. From the SBRT patient study, intrafractional tumor motion (3.1-11.3 mm) in the SI direction was measured using the 2D images. The interfractional patient setup errors (0.1-12.7 mm) in the SI, AP, and LR directions were obtained from the average marker locations reconstructed in 3D and compared to the reference planning CT image. The authors have developed an automatic algorithm to extract marker locations from MV images and have evaluated its performance. The measured intrafractional tumor motion and the interfractional daily patient setup error can be used for off-line retrospective verification of SBRT.

Prostate positioning errors associated with two automatic registration based image guidance strategies

Daily image guidance for helical tomotherapy prostate patients is based on the registration of pretreatment megavoltage CT (MVCT) images and the original planning CT. The goal of registration, whether manual or automatic, is the overlap of the prostate; otherwise prostate misplacement may compromise the efficacy of treatment or lead to increased toxicity. A previous study demonstrated that without the aid of implanted fiducials, manual registration results in inaccurate prostate positioning. The objective of this work is to quantify prostate misplacement that results from automatic bone matching (BM) and image matching (IM) registration algorithms. 204 MVCT images from eight high‐risk tomotherapy prostate patients were incorporated into this retrospective study. BM and IM registration algorithms – based on maximization of mutual information of bony anatomy only and the entire image, respectively – were used to independently register MVCT images to their respective planning images. A correlation coefficient based algorithm that uses known planning CT contour information was used for automatic prostate localization in each MVCT image. Daily prostate misplacement was determined by repositioning as calculated from the BM and the IM algorithms. Mean (±SD) and maximum 3D prostate positioning errors were 3.7±2.1mm and 11.8 mm for bone matching, and 4.6±2.3mm and 11.5 mm for image matching. In terms of translational directions, IM would lead to prostate positioning error ≥3mm in any of the LR, AP or SI directions in 62% of treatment fractions. The corresponding value for BM is 51%. The values for positioning errors ≥5mm were 29% and 17% for IM and BM, respectively. This data suggests automatic daily image guidance for tomotherapy prostate patients should be based on bone matching instead of image matching.PACS number: 87.19.xj, 87.57.nj

Characterization of the quality of ZnO thin films using reflective second harmonic generation

A polar mirror symmetrical contribution originated from the arrangement of grain boundaries existing in the ZnO film is detected by reflective second harmonic generation pattern. The ordering of ZnO grain boundary is dependent on the kinetic energy of deposited atoms and affects the quality of ZnO films. The net direction of the grain boundary in ZnO film trends toward the [1¯10] direction of Si(111) to reach the minimum grain energy for better quality ZnO film. The polar structure of the mirrorlike boundaries under the optically macroscopic viewpoint presents a correlation with film quality.

Azimuthal dependence and accurate determination of the half-angle in RBS channeling

Rutherford Backscattering Channeling (RBSC) is often used in the studies of single crystalline materials with regard to issues hinging upon the location of specific atomic species within the crystal lattice. Results are deduced from the characteristics of an angular scan curve representing the yield of close collision events from the constant scattering of the probing projectiles from the crystalline sample in a sequence of directions lying in a plane perpendicular to one particular crystal lattice direction. Such angular scans exhibit dips near major channels, and their angular widths are of concern. On a fundamental level, accurate determination of these angular widths in the case of axial channeling and their interpretation come up against uncertainties arising out of the absence of a preferred choice for the plane of scan. Here, we shall illustrate these uncertainties in a representative case, channeling of 2 MeV He + ions in the 〈1 0 0〉 direction of Si.

SU‐DD‐A3‐06: Fully Automated Internal Marker Tracking Algorithm for Cine EPID Images

Introduction: An electronic portal imaging device (EPID) in cine mode can be used for validating respiratory gating and stereotactic body radiation therapy (SBRT) by tracking implanted fiducials. Manual tracking methods are time and labor intensive, limiting the utility of the validation. We have developed a fully automated algorithm to quickly and accurately extract the markers in EPID images and reconstruct their 3D positions. Materials and Methods: The markers were detected and recognized using an image processing algorithm based on the Laplacian of Gaussian (LoG) filter. To reduce false marker detection, a marker registration technique was applied using image intensity as well as the geometric spatial transformations between the reference marker positions produced from the projection of 3D CT images and the estimated marker positions. An average marker position in 3D was reconstructed by backprojecting, towards the source, the position of each marker on the 2D image. Results: From phantom studies, spatial accuracies of &lt;1 mm were achieved in both 2D and 3D marker locations. Using only the LoG algorithm, the marker detection success rate was 88.8%. However, adding the registration technique which utilizes prior CT information, the success rate was increased to 100%. In addition, we have examined the cases of 5 patients being treated under an SBRT protocol for hepatic metastases. The intrafractional tumor motion (3.1–11.3 mm) in the SI direction was measured using the 2D images. The interfractional patient setup errors (0.1–12.7 mm) in the SI, AP, and LR directions were obtained from the marker locations reconstructed in 3D and compared to the reference planning CT image. Conclusions: The measured intrafractional tumor motion and the interfractional daily patient setup error can be used for off‐line retrospective verification of SBRT.This work was partially supported by a grant from Varian Medical Systems, Inc.

TH-D-210A-08: On the Importance of Correcting Anatomical Deformations in Prostate Cancer Patients

Purpose: To assess the effects of target, rectum and bladder anatomical deformations on targeting accuracy of prostate and postprostatectomy cancer patients undergoing IG‐IMRT. Methods: Localization with online kV‐CBCT was performed. The target and OARs positional/volumetric changes were evaluated and couch shifts were applied. For patients with large target/OARs volumetric changes compared to planning CT, arising from medications, diet, and/or ongoing RT, repeated localization CB scans were performed following an interventional procedure, shifts were then evaluated, and the IMRTtreatment was subsequently delivered. The interventional procedure involves the insertion of a rectal catheter or rectal deflation, and/or bladder filling. A total of 160 pre‐/post‐intervention shifts from 14 patients in the lateral/LR, vertical/AP, and longitudinal/SI directions were compared. The percentage of shifts larger than 5 mm in all directions was also compared. CTV‐to‐PTV expansion margins were estimated based on the pre‐ and post‐intervention localization data. Results: Systematic/S and random/s shifts from pre‐versus post‐intervention data (in mm) were: LR, 0.2±2.8 vs. 0.4±2.9; AP, −0.7±5.3 vs. −1.1±3.6; SI, 0.6±3.7 vs. −0.5±2.5. The mean 3D shift distance was 6.4±3.1 vs. 4.8±2.4 with a p‐value < 0.05. The percentage of preintervention shifts larger than 5 mm were 7.5%, 31.3%, and 16.3% in the LR, AP, and SI directions, respectively, compared to 8.8%, 15.0%, and 6.3% for post‐intervention. Large anatomical variations were observed for rectum and/or bladder, suggesting that localization without intervention may not be sufficient to ensure accurate targeting and sparing of rectum/bladder. Conclusion: Localization data from pre‐ and post‐intervention procedures show that for treatments that do not include intervention to correct for rectum/bladder anatomical variations, the CTV‐to‐PTV margin required is larger by 5 mm, and more rectum/bladder volumes are potentially at risk of radiation‐induced acute or late toxicity..

SU‐FF‐J‐66: Deriving Correlation of External Surface and Internal Tumor Motion Using 4D‐CT Images

Purpose: To quantify the correlation between patient surface motion and internal tumor motion. Method and Materials: Thoracic 4D‐CT scans are used for the study. The CT image at the end of inhale is set as the target image. On each CT slice of the target image, 201 points are evenly placed on the skin contour (excluding the patient's back). The other nine CT images, corresponding to nine different respiratory phases, are set as moving images. An optical flow deformable image registration algorithm is used to map the moving images to the target image. Then the surface points in the target image are transferred to the moving images using the deformation vector field. The motion trajectory of each surface point is quantified for the ten respiratory phases, and the correlation between each surface point motion and internal tumor movement in the superior‐inferior direction is computed. Results: Color maps of the surface point motion amplitude and correlation with internal tumor motion have been derived for each patient. Color maps highlight the high‐correlation and high‐amplitude motion regions on the surface. It has been observed that 88% of body surface has a correlation of 0.9 or higher with internal tumor motion in the SI direction but only 17% of surface points have motion amplitude bigger than 2mm. Conclusion: Color maps of the surface point motion amplitude and correlation with internal tumor motion have been derived for each patient from a 4D‐CT scan. These color maps can be used to identify patient specific surface regions of interest to be used as surrogates for external gating.

SU-FF-J-79: Markerless Lung Tumor Tracking in Rotational Radiotherapy

Purpose: To develop a method to track lungtumors in rotational cone beam projections during rotational radiotherapy and cone beam CT scanning.Method and Materials: A multiple template based tracking algorithm was developed and used to track tumors in rotational cone beam projections. Templates were generated by creating DRRs of ten phases of 4DCT. These templates were generated for a sequential set of angles matching the projections of a CBCT scan. The position of the tumor in each template was derived from contours drawn on 4DCT. Shifting of the templates was used to allow for a greater tumor motion ranges. The mutual information between projections and templates was computed and used as one parameter in a probability function used to track tumor position. Tumor distance traveled and phase change between successive projections were also incorporated into the probability function. This output was compared to physician specified tumor locations on each cone beam projection. In addition to a patient study, the method was tested on a respiratory motion phantom programmed to exhibit sinusoidal motion in the SI direction. Results: In the phantom study, the SI motion of the tumor was tracked with a mean absolute error (MAE) ranging from 1.2mm to 1.6mm and a 95th percentile absolute error (P95) ranging from 3.2mm to 3.7mm. For the patient study, the SI motion was tracked with MAE ranging from 1.7mm to 1.9mm and P95 ranging from 3.4mm to 3.9mm. Conclusion: The algorithm has demonstrated the feasibility of tracking tumors in rotational x‐ray images. Further development is needed in order to achieve accuracy similar to that of fixed gantry fluoroscopic tracking.

SU‐FF‐I‐144: Compare with Phase Correlated Image Registration to 4D CT for Liver Stereotactic Radiosurgery with Image Guidance Cone Beam CT

Purpose: Stereotactic radiosurgery (SRS) is characterized by high doses delivered to small targets in a few fractions. Image guidance systems, cone beam computed tomography (CBCT), that yield volume imaging and sufficient soft‐tissue contrast have permitted daily imaging of bony anatomy and target permitting online correction of tumor position prior to treatment delivery, but CBCT is not perfectly support in phase correlated image guidance for targeting. The main objective was to overcome breathing induced uncertainties in the process of image guided RT using phase correlated image registration both the planning CT scan and CBCT for verification imaging scan. Methods and materials: For 20 patients with liver SRS, respiratory correlated 4D CT scan and CBCT for image guidance was acquired. Internal target volume (ITV) is determined utilizing 50% phase end exhalation reconstructed images by 4DCT, In order to check the accuracy of maximum intensity projection for various target motion, especially for target moving irregularly with three implanted gold seed. The data analyzed were respiratory‐induced peak‐to‐trough distance, 3D motion from LR, AP and SI directions and motion nonlinearity and hysteresis. Motion hysteresis occurs when three seed follows different paths between inhale and exhale phases during a respiratory cycle. Results: The overall mean respiratory‐induced peak‐to‐trough distance is 0.48 cm, with individual treatment fraction means ranging from 0.02 to 1.44 cm. The overall mean respiratory period is 3.8 s, with individual treatment fraction means ranging from 2.2 to 6.4 s. Conclusions: The motion nonlinearity and hysteresis are important characteristics of respiratory tumor motion. Seppenwoolde et al (2002) calculated the hysteresis as a phase difference between the fitted parameterized curves of the average breathing cycles of two directions. From 3D tumor trajectories, they showed that the hysteresis ranged from 0.1 to 0.6 cm for 20 tumors.

SU-FF-J-81: A Feasibility Study for Real-Time Tomosynthesis-Guided Rapid Arc Therapy

Purpose: To investigate the feasibility of real‐time mis‐alignment correction in Rapid Arc treatment and design a corresponding tomosynthesis acquisition protocol. Method and Materials: A CTimage set of an anthropomorphic pelvic phantom was used in the study. Simulated projection images were produced to resemble a simultaneous kV fluoro in Rapid Arc treatment. A modified Feldkamp algorithm was used to reconstruct the tomosynthesisimages. Various combinations of imagingreconstruction parameters including scan angle, angular interval, and slice thickness (mm) were tested: 1) 60°, 6°, 2.4; 2) 60°, 6°, 0.8; 3) 60°, 3°, 2.4; and 4) 30°, 3°, 2.4. A predefined 5 mm displacement in all three orthogonal directions modeled patient motion during treatment. After each successive tomosynthesis acquisition, registrations were performed between current reconstructions and reference images. The phantom position was corrected accordingly by shifting the treatment couch. Residual errors and their root mean square (RMS) values were recorded for evaluation. Results: The residual errors (L‐R, A‐P and S‐I directions in mm) for the 4 schemes after the first tomosynthesis acquisition were (1.2, 2.5, −0.2), (−1.2, 1.1, 0.0), (1.1, 1.9, 0.0) and (−1.2, 3.1, 0.0), and the corresponding RMSs were 1.6, 0.9, 1.3 and 1.9 respectively. The RMSs after a full arc delivery were 0.7, 0.5, 0.5 and 0.7. All schemes tested accurately corrected displacement in the SI direction after first acquisition. Scheme 2 performed better than scheme 1 at the expense of more computation time. By doubling projection numbers in scheme 1, scheme 3 improved correction ability in the L‐R direction. With a smaller 30° scan angle, scheme 4 was acceptable and will be improved after several acquisitions. Conclusions:Tomosynthesis scans can be used for real‐time mis‐alignment correction in Arc therapy after 30° gantry rotation.

SU-FF-J-80: Evaluation of Rotational and Translational Setup Variations for Brain Tumor Patients with Mask for Immobilization

Purpose: Rotational setup errors for braintumor patients undergoing external beam radiotherapy with a thermoplastic mask system were not well investigated and often ignored during the planning and treatment process. In this study, both translational and rotational setup variations are evaluated for braintumor patients undergoing external beam radiotherapy, using daily helical tomotherapy megavoltage computed tomography (MVCT) scans. Method and Materials: 15 primary braintumor patients who finished fractionated helical tomotherapy treatment were retrospectively selected. The number of fractions ranged from 29 to 34. Prior to each treatment fraction, an MVCT scan was performed over the brain volume. The MVCT images were registered with the planning CTimages based on bony structure using rigid translations in three dimensions and rotations in pitch, roll, and yaw. Results: The standard deviations (SD) for translations ranged 0.78 to 2.32 mm, 1.11 to 2.65 mm, and 0.52 to 6.65 mm in the RL, SI, and AP directions, respectively. The SD's ranged 0.28° to 0.83°, 0.35° to 1.04°, and 0.41° to 2.41° in pitch, roll, and yaw, respectively. 20.2%, 20.9%, and 38.9% of all the daily setups had more than 1° setup errors in pitch, roll, and yaw, respectively; and 58.4% had more than 1° errors in at least one rotational directions. 2.2%, 3.1%, and 20.9% had more than 2° setup errors in pitch, roll, and yaw, respectively, and 24.3% had more than 2° setup errors in at least one rotational directions. Residual translational setup errors in the absence of rotational setup corrections were also evaluated. Conclusion: Rotational setup variations can be significant for braintumor patients immobilized with masks, and setup corrections without rotations could lead to significant overall setup errors. Margins for treatment planning should take rotational setup variations into account. Daily image‐guided setup can be used to position the target volume.

TU-D-BRC-02: Determination of Skin Mark Based Patient Setup Errors Using OBI and CBCT for Head and Neck Patients and Investigation of the Dosimetric Impact of the Errors On IMRT Treatment

Purpose: To determine the skin-mark-based patient setup error using On Board Imaging (OBI) and cone-beam CT (CBCT) for head and neck patients, and to determine the CTV-PTV margin necessary to compensate for this setup error for IMRT treatments. Method and Materials: A total of 50 patients were analyzed for this study. All patients were first set up by aligning skin marks with lasers. Before the delivery of first treatment, a CBCT image was acquired to determine the final treatment position. OBI images were acquired for the subsequent 4 fractions. This weekly image acquisition protocol was followed for all patients until treatments were completed. Shift data were measured for a total of 1064 OBI fractions and 313 CBCT fractions. These data were compared statistically. Correlations between the CBCT and OBI data and among the OBI data were analyzed. The top 10 patients showing the largest setup error were selected to investigate the dosimetric impact of setup error using a 5 mm CTV-PTV margin. Results: The mean ± sd of the shifts measured by OBI in the SI, lateral and AP direction were 0.6±2.9, −1.6±3.5 and −0.7±2.9 mm and those measured by CBCT were 0.1±2.9, −1.9±3.3 and 0.3±3.3 mm respectively. The correlation indices between the CBCT and OBI were 0.43, 0.59, and 0.38, and those among the OBI were 0.46, 0.34, and 0.53 along SI, lateral and AP directions respectively. For 9 of the 10 patients investigated, at least 99.5% of CTV volume received more than 95% of the prescription dose after accounting for the setup errors. Conclusion: There is no significant difference between the patient shifts measured by OBI and CBCT. No correlation was found between the shifts for different fractions. A 5 mm CTV-PTV margin was found to be adequate to account for the setup errors measured in this study.

SU-FF-J-100: Margins for Hypofractionated Lung SBRT Using CyberKnife Synchrony

Purpose: Real time tumor tracking can be performed using the Synchrony respiratory tracking (CyberKnife, Accuray). This study evaluates the range of target correction margins required for treatment of patients with lungtumors using this system. Method and Materials: In this study, twenty‐eight lung patients treated using Synchrony were selected. The moving average of the correction margin (CM), i.e., the difference between the predicted tumor positions versus on‐line x‐ray measured tumor positions, was extracted for each fraction as well as the entire course of the treatment for each patient. The population‐based CM was then derived using the modified van Herk margin formula taking into account the hypofractionated regimen. This modified margin recipe also incorporated the situations of small number of patients, N<30. Results: For tumors of different locations, CMs for upper‐lobe tumor margins (90% confidence level) were 0.6mm (AP), 0.7mm (RL), and 0.6mm (SI); CMs for lower‐lobe tumors were 2.7 mm (AP), 2.6mm (RL), and 1.3mm (SI); CMs for hilar tumors were 1.4mm (AP), 1.3mm (RL), and 1.3mm (SI); CMs for mediastinum tumors were 1.7mm (AP), 1.2mm (RL), and 0.8mm (SI), respectively. Statistical significance on these value differences was found between the upper and the lower lobe group using student t‐test (p=<0.01). The uncorrected tumor margins were 4.3mm (AP), 4.2mm (RL) and 8.7mm (SI) for upper‐lobe tumors; 9.3mm (AP), 8.7mm (RL) and 9.8mm (SI) for lower‐lobe tumors; 8.0mm (AP), 7.4mm (RL) and 14.1mm (SI) for hilar tumors; 8.8mm (AP), 7.4mm (RL) and 7.6mm (SI) for mediastinum tumors, respectively. Conclusions: Based on the tumor location and the range of correction margins for real time respiratory tracking using Synchrony, the addition of a target margin of 1 to 3 mm in the AP, RL and SI directions for lungtumors would be adequate throughout the course of the treatment.