Anthropomorphic Pelvis Phantom Research Articles

TH‐C‐BRB‐11: Range‐Guided Proton Prostate Treatment by Anterior Fields

Purpose: Only bi‐lateral fields are used for proton prostate treatment in current practice, often giving less favorable dose conformity and organ sparing than IMRT. Prostate treatment by anterior fields can utilize the sharp distal fall‐off of the Bragg peaks to spare the rectum, but this will require an accurate pre‐treatment “range check” for every fraction. We have developed and validated a range‐guided proton therapy (RGPT) technique for such treatments. Methods: Our technique utilizes the fact that in treatment by passively scattered beams, the time‐resolved dose rate signal at a point in patients encodes the corresponding water‐equivalent pathlenght (WEPL). A characteristic relationship between the r.m.s widths of signals and the corresponding WEPLs is calibrated and validated in a water tank. By measuring the signals at critical points with in‐vivo dosimeters, the WEPLs to these points can be obtained. An anthropomorphic pelvic phantom with a rectal cavity and water‐filled bladder was used in the study. Anterior treatment fields were planned using the CMS/XiO planning system. A detector array (6×3 cm) consisting of 16 diodes was placed directly below the anterior rectal wall supported by an endorectal water balloon. A range‐ check beam of 18 cm was first used, with the treatment hardware (aperture and compensator) in place, to verify the WEPLs to the detectors and hence the beam range for treatment. Finally the treatment beam with the corrected range was used to deliver the prescribed dose. Results: The differences in WEPLs between the measured and those calculated by the planning system were <2 mm at points behind the prostate target volume. The dose used for the range verification was <1 cGy. Potential improvements in data acquisition and analysis could achieve millimeter accuracy. Conclusions: Our study indicates that the RGPT technique is simple, effective and reliable for prostate treatment by anterior fields.

SU-E-J-20: Dose Calculation for Pelvic Megavoltage Cone Beam CT Image with a Quadratic-Fit Based Correction

Purpose: The accurate dose recalculation requires that the original pelvic megavoltage cone beam CT (MVCBCT) images be corrected for cupping and truncation artifacts caused by beam scattering and field‐of‐view limitation. Our group previously developed a pelvic phantom‐based correction algorithm. The purpose of this study is to introduce a simple yet accurate pelvic MVCBCT image correction strategy based only the planning kVCT and treatment‐day MVCBCT images to allow accurate dose calculation. Methods: The quadratic‐fit (QF) based correction algorithm uses the planning kVCT as a reference, and individually corrects the MVCBCT images slice by slice. A realistic pelvic‐size water phantom, an anthropomorphic pelvic phantom, and a clinical pelvic patient were used to validate the method. Furthermore, a prostate patient with large anatomy changes in planning CT and a treatment‐day MVCBCT was selected for evaluating the clinical applicability of the method. The dose differences with and without QF correction were compared. Results: The QF correction reduced the cupping artifact significantly. The maximum mean CT number deviations in the soft tissue part of the anthropomorphic phantom between the conventional CT and MVCBCT without and with QF correction were 15% and 2%, respectively. After applying the QF and data missing correction, 96.5% of the voxels showed a dose agreement better than 3% between the planning CT and MVCBCT. In a clinical patient without significant anatomy changes, the dose difference in the targets and organ at risks between the planning CT and a treatment‐day MVCBCT was within 1.1%. Conclusions: The QF based method is feasible for correcting the cupping artifact in the pelvic MVCBCT. And, based on the corrected MVCBCT images, the monitoring of treatment‐day dose distributions is possible. Research supported in part by Siemens

SU-E-J-22: Measurement-Based Cross-Scatter Correction in Dual Detector Cone-Beam CT

Purpose: A dual detector cone-beam CT (CBCT) system could potentially allow for dual energy CBCT and dual-view DTS. However, image quality in this system is severely degraded by the presence of scatter between the two imaging chains, i.e. cross-scatter. The aim of this work is to develop a measurement-based method for correcting cross-scatter without increasing scan-time or exposure and without adding additional hardware. Methods: The dual detector CBCT imaging system has two tube/detector pairs mounted orthogonally; each 40×30 cm detector has an anti-scatter grid. The cross-scatter distribution was measured at a certain angular intervals by firing a single x-ray tube and reading out both detectors. Cross-scatter at intermediate angles was estimated by cubic spline interpolation. The cross-scatter estimates were subtracted from the projections prior to reconstruction. The angular interval between cross-scatter measurements was optimized for an anthropomorphic pelvic phantom. Accuracy of scatter interpolation was evaluated by comparing to directly measured cross-scatter. Effectiveness of scatter correction was evaluated by measures of contrast and contrast-to-noise ratio (CNR) in reconstructions of an image quality phantom from projection data acquired with and without cross-scatter. Results: For the pelvic phantom and an angular interval of 11 degrees, interpolated cross-scatter distributions were within 2.5% of measured cross-scatter distributions. This error remained constant as the angular interval decreased below 11 degrees and rose sharply to about 90% as the angular interval increased to 34 degrees. The contrast was 58.0%, 70.8% and 70.8%, in the uncorrected, corrected, and cross-scatter free reconstructions and similarly the CNR was 23.6, 22.8 and 24.9. Conclusions: This measurement-based method effectively corrects for cross-scatter without any additional hardware or imaging dose. This work is partially supported by a research grant from Varian Medical Systems.

Combining scatter reduction and correction to improve image quality in cone‐beam computed tomography (CBCT)

The authors propose a combined scatter reduction and correction method to improve image quality in cone-beam computed tomography (CBCT). Although using a beam-block approach similar to previous techniques to measure the scatter, this method differs in that the authors utilize partially blocked projection data obtained during scatter measurement for CBCT image reconstruction. This study aims to evaluate the feasibility of the proposed approach. A 1D grid, composed of lead septa, was placed between the radiation source and the imaging object for scatter measurement. Image data were collected from the grid interspace regions while the scatter distribution was measured in the blocked regions under the grid. Scatter correction was performed by subtracting the measured scatter from the imaging data. Image information in the penumbral regions of the grid was derived. Three imaging modes were developed to reconstruct full CBCT images from partial projection data. The single-rotation half-fan mode uses interpolation to fill the missing data. The dual-rotation half-fan mode uses two rotations, with the grid offset by half a grid cycle, to acquire two complementary sets of projections, which are then merged to form complete projections for reconstruction. The single-rotation full-fan mode was designed for imaging a small object or a region of interest. Full-fan projection images were acquired over a 360 degrees scan angle with the grid shifting a distance during the scan. An enlarged Catphan phantom was used to evaluate potential improvement in image quality with the proposed technique. An anthropomorphic pelvis phantom was used to validate the feasibility of reconstructing a complete set of CBCT images from the partially blocked projections using three imaging modes. Rigid-body image registration was performed between the CBCT images from the single-rotation half-fan mode and the simulation CT and the results were compared to that for the CBCT images from dual-rotation mode and conventional CBCT images. The proposed technique reduced the streak artifact index from 58% to 1% in comparison with the conventional CBCT. It also improved CT number linearity from 0.880 to 0.998 and the contrast-to-noise ratio (CNR) from 4.29 to 6.42. Complete sets of CBCT images with overall improved image quality were achieved for all three image modes. The longitudinal resolution was slightly compromised for the single-rotation half-fan mode. High resolution was retained for the dual-rotation half-fan and single-rotation full-fan modes in the longitudinal direction. The registration error for the CBCT images from the single-rotation half-fan mode was 0.8 +/- 0.3 mm in the longitudinal direction and negligible in the other directions. The proposed method provides combined scatter correction and direct scatter reduction. Scatter correction may eliminate scatter artifacts, while direct scatter reduction may improve the CNR to compensate the CNR degradation due to scatter correction. Complete sets of CBCT images are reconstructed in all three imaging modes. The single-rotation mode can be used for rigid-body patient alignment despite degradation in longitudinal resolution. The dual-rotation mode may be used to improve CBCT image quality for soft tissue delineation in adaptive radiation therapy.

Image-Guided Localization Accuracy of Stereoscopic Planar and Volumetric Imaging Methods for Stereotactic Radiation Surgery and Stereotactic Body Radiation Therapy: A Phantom Study

To evaluate the positioning accuracies of two image-guided localization systems, ExacTrac and On-Board Imager (OBI), in a stereotactic treatment unit. An anthropomorphic pelvis phantom with eight internal metal markers (BBs) was used. The center of one BB was set as plan isocenter. The phantom was set up on a treatment table with various initial setup errors. Then, the errors were corrected using each of the investigated systems. The residual errors were measured with respect to the radiation isocenter using orthogonal portal images with field size 3 × 3 cm(2). The angular localization discrepancies of the two systems and the correction accuracy of the robotic couch were also studied. A pair of pre- and post-cone beam computed tomography (CBCT) images was acquired for each angular correction. Then, the correction errors were estimated by using the internal BBs through fiducial marker-based registrations. The isocenter localization errors (μ ±σ) in the left/right, posterior/anterior, and superior/inferior directions were, respectively, -0.2 ± 0.2 mm, -0.8 ± 0.2 mm, and -0.8 ± 0.4 mm for ExacTrac, and 0.5 ± 0.7 mm, 0.6 ± 0.5 mm, and 0.0 ± 0.5 mm for OBI CBCT. The registration angular discrepancy was 0.1 ± 0.2° between the two systems, and the maximum angle correction error of the robotic couch was 0.2° about all axes. Both the ExacTrac and the OBI CBCT systems showed approximately 1 mm isocenter localization accuracies. The angular discrepancy of two systems was minimal, and the robotic couch angle correction was accurate. These positioning uncertainties should be taken as a lower bound because the results were based on a rigid dosimetry phantom.

Poster — Thur Eve — 47: Automatic Comparison of Portal Images for the Detection of Radiotherapy Treatment Delivery Errors

The goal of this project is to develop a real‐time automatic comparison tool to detect portal dose image errors and has two logical parts:1. A mechanism to detect the presence of a significant discrepancy.2. A set of components to identify the probable cause(s) of the discrepancy including geometrical positioning errors as well as machine specific delivery errors.Initially, we have only considered in‐plane rotations and translations. A total of ∼700 portal images of an anthropomorphic pelvic phantom were obtained using a Varian aS1000 EPID. The images contained combinations of three types of geometrical patient set‐up errors. The angle of rotation (about the axis perpendicular to the EPID) was varied from [−5, +5] degrees and the translational distance was varied from [−10, +10] mm. The majority of the images were acquired from the AP direction however ∼130 images were of a lateral view. The system was found to be sensitive to in‐plane rotations down to ∼ 2°, out of plane rotations greater than ∼4° and displacements 3mm. For the lateral views, the calculated rotation was correct to within 1/2° and translations to within 5mm 53% of the time. Considering only translations, the accuracy increases to ∼82%. The results are much better for the AP views. The in‐plane rotation was within ±1/2deg; for all cases and the displacement error was greater than 1.5 mm in only 1 case. The system was able to analyze an image pair in ∼10s, however, no effort was put toward optimization of the system.

TU‐A‐204B‐02: On the Potential of CBCT for Range Verification in Proton Therapy

Purpose: We have investigated the potential use of cone beam CT (CBCT) for beam range verifications in proton therapy treatment, in addition to its primary role in geometric targeting. Specifically, we studied the intrinsic imaging variability of a CBCT and its effect on the water equivalent path length (WEPL) calculations, in the context of daily beam range verification/correction required for a recently proposed method of treating prostate using anterior fields. The current approach uses only lateral fields due to the lack of precise range control in patient. Materials and Methods: An anthropomorphic pelvic phantom was scanned using CBCT, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then with a random shift of one centimeter in lateral directions. The Xio treatment planning system was used to perform the analysis. The average Hounsfield unit (HU) numbers for the water column in the rectal balloon was used to perform a linear calibration of the stopping power ratio, independently for each scan, as supported by the planning system. A number of WEPL values vertically from the anterior skin surface to the anterior surface of the water balloon were calculated on slices covering the region of the prostate, in relevance to a prostate treatment using an anterior field. Results: The HU number in the water column varied significantly even within the same CBCT. The average value also varied from day to day for up to 20 units. However, when these average values are used to calibrate the stopping power ratio, the variations in WEPL values along the anterior beam path are mostly within 2 mm. Conclusions: In‐room CBCT can be used in proton therapy to make online verification of protons range in patients with 2mm accuracy.

TH‐C‐BRB‐01: Characterization of Optically‐Stimulated Luminescent Detectors (OSLDs) in Photon & Proton Beams

Purpose: To investigate characteristics of optically‐stimulated luminescent detectors (OSLDs) to protons thus allowing comparison to thermoluminescent detectors and implementation into a quality assurance program. Methods & Materials: The OSLDs used were aluminum oxide (Al2O3) nanodots (Landauer Inc. Glenwood IL) measuring 10×10×2 mm3. A 20(L)×20(W)×0.5(H) cm3 volume of solid water was fabricated with pockets to allow OSLDs and TLDs to be irradiated simultaneously and perpendicular to the beam. Irradiations were performed at a 5 cm depth for photons and in the center of a 10 cm SOBP for a 200 MeV proton beam. Additionally the Radiological Physics Center's (RPC) anthropomorphic pelvic phantom was used to test the angular dependence of OSLDs for photons and protons. A cylindrical insert in the phantom allows the dosimeters to be rotated to any angle with a fixed gantry angle. OSLDs were irradiated at 8 angles between 0 and 360 degrees. OSLDs were read out with a Landauer Microstar reader. Results: Dose response measurements indicate for photons that at angles where the dosimeter is near parallel with the radiation beam response is slightly reduced. Results in protons do not seem to show significant angular dependence. OSLDs and TLDs are comparable within 3% for both photons and protons during perpendicular measurements. Conclusion: OSLDs and TLDs are comparable dosimeters in 6 MV photon and 200 MeV protons beams in a perpendicular irradiation. Preliminary results show that OSLDs might have some angular dependence to photons but not protons. With angular dependence characteristics defined OSLDs can be implemented into multiple‐field treatment plans in photons and protons and used in the RPC's quality assurance program.

TU‐C‐204B‐01: Reduce Patient Dose and Improve Image Quality in CBCT for Image Guided Radiotherapy

Introduction: We have demonstrated that cone beam CT (CBCT) image quality was substantially improved in terms of eliminating scatter related artifacts and enhancing contrast‐to‐noise ratio, using a 1D grid placed between the x‐ray source and the imaging object. The grid serves to physically reduce scatter while also enabling measurements of scatter simultaneously for post‐scan scatter correction. A full set of CBCT images can be generated from the partially‐blocked imaging data using either one‐ or dual‐rotation modes. Using one‐rotation has particularly the advantage of reducing imaging dose by a factor of two in comparison to conventional CBCT. The tradeoff is that longitudinal resolution is degraded. This study evaluates the feasibility of using CBCT images from the single‐rotation mode for image‐guided radiotherapy (IGRT). Methods and material: A lead grid of approximately 1‐mm septa width and interspace was placed at a source‐to‐grid distance of approximately 30‐cm in a Varian Trilogy CBCT system for the single‐rotation mode. An anthropomorphic pelvis phantom was used for the test. Various steps in imaging processing were performed for each projection, including measuring the scatter distribution, performing scatter correction, deriving imaging information in the grid penumbral regions by dividing the projection with a blank scan, and interpolating the remaining blocked information. Rigid body image fusion was performed for the reconstructed CBCT images with the simulation CT, and compared with CBCT images from the two‐rotation and conventional CBCT. Results: The single‐rotation CBCT images showed substantially reduced scatter‐related artifacts. Despite the degradation in longitudinal resolution, the visibility of anatomic structures (an important requirement of IGRT) was not significantly affected. The auto‐registration error was 0.8±0.3 mm in longitudinal direction, and was negligible in AP and lateral directions and three rotational directions. Conclusion: CBCT images from the single‐rotation mode can achieve reasonable longitudinal resolution for anatomic structure visualization and accurate auto‐registration in IGRT.

SU‐GG‐J‐30: A Novel Scatter Reduction and Correction Method to Improve Cone‐Beam CT (CBCT) Image Quality

Purpose: To develop a novel scatter reduction and correction method to improve the CBCT image quality for IGRT applications. Method and Materials: A one‐dimensional grid composed of lead with equal interspacing was positioned between the x‐ray source and the imaging object during CBCT acquisition. The grid provides direct scatter reduction by blocking half of the beams. Partial image data were obtained from the grid inter‐space (unblocked) region while scatter was measured from the blocked region beneath the grid in the projection images. Scatter was corrected by subtracting the measured scatter from the original projection image. Information in the penumbra region of each grid septa was derived. Three modes were developed to reconstruct full CBCT images from partially blocked projections. In single rotation axial mode, interpolation was used to fill in the missing data in the half‐fan projections. In dual rotation mode, two‐rotation half‐fan scans were acquired with the grid offset by half a grid cycle, and complimentary projections were merged to form complete projections for reconstruction. In single rotation helical mode, one‐rotation full‐fan scan was acquired with the grid offset by half a grid cycle at the scan center. Both CatPhan and anthropomorphic pelvis phantoms were used to evaluate the method. Results: In the CatPhan study, CNR was improved using our approach from 4.3 to 6.4 in comparison to conventional CBCT, and CT‐number linearity correlation coefficient increased from 0.88 to 0.998. Scatter induced streak artifacts were significantly reduced. In the pelvis phantom study, complete sets of CBCT images were reconstructed and scatter related artifacts were significantly reduced in all three modes. Conclusion: Our method enhances CNR and provides much more accurate measurement of scatter with reduced or equivalent dose to conventional CBCT. Scatter‐related artifacts are substantially reduced, and the CBCT images generated are of appropriate quality for adaptive radiation therapy.

SU‐HH‐BRB‐11: Range Adaptive Proton Therapy for Prostate Cancer

Purpose: The rapid distal falloff of a proton beam allows for sparing of normal tissues distal to target organs. However proton beams that aim directly towards critical structures are avoided due to concerns of range uncertainties, such as CT number conversion and anatomy variations. We propose to eliminate range uncertainty and enable prostate treatment with a single anterior beam by detecting the proton's range at the prostate‐rectal interface and adaptively adjusting the range in vivo and in real‐time. Materials and Methods: A prototype device, consisting of an endorectal liquid scintillation detector and dual‐inverted lucite wedges for range compensation, was designed to test the feasibility and accuracy of the technique. Liquid scintillation filled volume was fitted with optical fiber and placed inside the rectum of an anthropomorphic pelvic phantom. Photodiode‐generated current signal was generated as a function of proton beam depth, and the spatial resolution of this technique was calculated by relating the variance in detecting proton spills to the penetration depth. The relative water‐equivalent thickness of the wedges was measured in a water phantom and prospectively tested to determine the accuracy of range corrections. Treatment simulation studies were performed to test the potential dosimetric benefit in sparing the rectum.Results: The spatial resolution of the detector in phantom measurement was 0.5 mm. The precision of the range correction was 0.04 mm. The residual margin to ensure CTV coverage was 1.1 mm. The composite distal margin for 95% treatment confidence was 2.4 mm. Planning studies based on a previously estimated 2mm margin on 27 patients showed a rectal sparing up to 51% at 70Gy and 57% at 40Gy relative to IMRT and bilateral proton treatment. Conclusion: We demonstrated the feasibility of our design. Use of this technique allows for proton treatment using a single anterior beam, significantly reducing the rectal dose.

SU-GG-T-476: Effect of Film Orientation on Proton Beam Dosimetry

Purpose: To evaluate the dosimetric effect of film irradiations in a pelvis phantom treated with protons.Method and Materials: A CT simulation of an anthropomorphic pelvis phantom was performed and the Hounsfield Units were converted to the measured relative stopping power for each material. A protontreatment plan with lateral fields was devised. TLD and Gafchromic© EBT film were inserted in the phantom with the film orientation at 0° and 90° relative to the beam axes and the plan was delivered. The process was repeated twice more. The phantom was re‐loaded with the film rotated 10° from the beam axes. The treatment plan was delivered three more times. Dose calculations from the planning system were compared to the measured dose from the TLD and film. Profiles along the film planes were compared to calculations from the planning system. An acceptable displacement was defined as < 3 mm. Results: The film aligned along the axes of the beams showed a displacement between the measured and calculated dose profiles of −7 mm on the left side and 5 mm on the right side. After rotating the film 10°, the new displacements were −4 mm on the left side and 3 mm on the right side. Conclusion: Rotating the film from the axis of the beam reduced the displacements between the measured and calculated dose profiles in the right‐left direction by as much as 3 mm. The difference in density between the film and phantom material is believed responsible for the differences in the dose profiles. The planning system did not account for the density of film and underestimated the range of the protons. This work was supported by PHS CA010953 and CA081647, awarded by NCI, DHHS, and funds from the RTOG.

SU‐GG‐J‐71: Cone Beam CT Hounsfield Unit to Electron Density Calibration and Its Impact on Dose Calculation Accuracy

Purpose: Accuracy of direct dose calculation on CBCT datasets is compromised by inaccuracy of reconstructed Hounsfield unit (HU) numbers in CBCT. Establishing HU to electron density (ED) calibration curves for CBCT can improve dose calculation accuracy on CBCT images. The goal of this study is to investigate effects of phantom volume and radial positions of inserting materials on HU‐to‐ED calibration for CBCT and evaluate impact of the calibration on dose calculation accuracy on CBCT datasets. Method and Materials: Catphan 600 and Gammex RMI 467 phantoms were used to characterize HU‐to‐ED calibration curves for both FBCT and CBCT. The effect of scatter volume on the HU‐to‐ED calibration was evaluated using the Catphan phantom fitted with annuli and the Gammex phantom in water. The CBCT HU‐to‐ED calibration was conducted by varying the radial positions of inserts in the Gammex phantom. The determined calibration curves were input into the Pinnacle TPS to calculate dose for an anthropomorphic pelvis phantom. Results: The scatter volume has negligible effects on the HU‐to‐ED calibration for FBCT. The CBCT HU‐to‐ED calibration curves vary significantly with the scatter volume and differ largely from that of FBCT. Large deviations of 330 HU and 700 HU in high density materials are observed for CBCT with the use of the modified Catphan and Gammex phantoms. In comparison to FBCT, the HU numbers on the CBCT HU‐to‐ED calibration curves are larger and smaller for materials less dense and denser than water, respectively. Radial position variations of inserts can cause HU number to change by up to 200 HU. The doses computed based on FBCT and CBCT of the pelvis phantom agree to within 3%. Conclusion: The CBCT and FBCT HU‐to‐ED calibration curves differ significantly. A fairly good dose calculation accuracy can be achieved using a single CBCT HU‐to‐ED calibration curve.

Evaluation of an a-Si EPID in direct detection configuration as a water-equivalent dosimeter for transit dosimetry

A major problem associated with amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) for transit dosimetry is the presence of a phosphor layer, which can introduce large deviations from water-equivalent behavior due to energy-dependent response and visible light scattering. In this study, an amorphous silicon EPID was modified to a direct detection configuration by removing the phosphor layer, and the accuracy of using it for transit dosimetry measurements was investigated for 6 and 18 MV treatment beams by comparison to ion-chamber in water measurements. Solid water and copper were both evaluated as buildup materials. Using the optimum buildup thickness in each case, effects of changes in radiation field size, source to detector distance, and patient/phantom thickness were investigated by comparison to reference measurements made by an ionization chamber on the central axis. The off-axis response of the imager was also investigated by comparison of EPID image profiles to dose profiles obtained by a scanning ionization chamber in a water tank with various thicknesses of slab phantoms, and an anthropomorphic phantom in the beam using Gamma evaluation (3%, 3 mm criteria). The imaging characteristics of the direct EPID were investigated by comparison to a commercial EPID using QC3V phantom, and by taking images of an anthropomorphic pelvic phantom containing fiducial gold markers. Either 30 mm of solid water or 3.3 mm of copper were found to be the most suitable buildup thicknesses with solid water providing more accurate results. Using solid water buildup, the EPID response compared to the reference dosimeter within 2% for all conditions except phantom thicknesses larger than 25 cm in 6 MV beams, which was up to 6.5%. Gamma evaluation results comparing EPID profiles and reference ionization chamber profiles showed that for 6 and 18 MV beams, at least 91.8% and 90.9% of points had a Gamma <1 for all phantoms, respectively. But using copper buildup, the EPID response had more discrepancies from the ionization chamber reference measurements, including: More than 2% difference for small air gaps using 6 MV beams, up to 8% difference for phantom thicknesses larger than 25 cm in 6 MV beams, and large differences (up to 9.3%) for increasing phantom thicknesses in 18 MV beams. The percentage of points with Gamma <1 with copper buildup were at least 96.6% and 99.8% in 6 and 18 MV beams, respectively. The direct EPID performs as an ion-chamber detector for transit dosimetry applications in all geometries studied except for small discrepancies at 6 MV for thick phantoms. This can be ameliorated by the calibration of the EPID to dose at an intermediate phantom thickness. The major current limitation of the direct EPID is poor quality of images compared with the clinical configuration, which could be overcome by a method to interchange between imaging and dosimetry setups.

A Dosimetric Evaluation of Organs at Risk in Prostate Radiation Therapy using a MAGIC Gel Dosimeter

Introduction: Multiple fields and presence of heterogeneities create complex dose distributions that need three dimensional dosimetry. In this work, we investigated MR-based MAGIC gel dosimetry as a three-dimensional dosimetry technique to measure the delivered dose to bladder and rectum in prostate radiation therapy. Materials and Methods: A heterogeneous slab phantom including bones was made. Paired cubes in the phantom representing bladder and prostate and a cylindrical container representing rectum were filled with MAGIC gel and placed in the anthropomorphic pelvic phantom. The phantom was irradiated with four beams as planned using a treatment planning system (TPS). Magnetic resonance transverse relaxation rate images were acquired and turned into dose distribution maps using a calibration curve. This calibration curve was obtained by linear fitting to R2 values of 4 test tubes against their given known doses. Image processing and data analysis were performed in MATLAB7 software. The gel dosimeter was validated using an ionization chamber. Dose maps and dose volume histograms (DVHs) were compared with dose distributions and DVHs of the TPS. Results: Mean “distance-to-agreement” and mean “dose difference” were 2.98 mm and 6.2%, respectively, in the comparison of profiles obtained from ionization chamber and gel dosimetry. Mean relative difference of DVHs between gel dosimetry and TPS data were 3.04%, 10.4% and 11.7%, for prostate, bladder and rectum, respectively. Discussion and Conclusions: Gel dosimetry is a good method for three dimensional dosimetry although it has a low precision in high dose gradient regions. This method can be used for evaluation of complicated dose distribution accuracy in 3D conformal radiotherapy, especially in presence of heterogeneities.

Evaluation of an anthropomorphic male pelvic phantom for image-guided radiotherapy

Evaluation of an anthropomorphic male pelvic phantom for image-guided radiotherapy B Schaly1, V Varchena2, P Au3,&nbsp;G Pang3,41Grand River Regional Cancer Centre, Kitchener, ON, Canada; 2CIRS Inc., Norfolk, VA, USA; 3Odette Cancer Centre, Toronto, ON, Canada; 4Departments of Radiation Oncology and Medical Biophysics, University of Toronto, Toronto, ON, Canada Abstract: Soft-tissue imaging in the treatment room is one of the main challenges faced today in high precision radiotherapy. The objective of this work is to evaluate a new anthropomorphic male pelvic phantom (CIRS Inc., Norfolk, VA, USA) that can be used in a radiotherapy department to assess the ability of an X-ray imaging system for imaging soft-tissue targets in the treatment room. To this end, we evaluated the tissue-equivalency of the phantom materials in terms of the linear attenuation and energy absorption coefficients. X-ray computed tomography (CT) images of the phantom were also obtained and compared with that of patients. Our results demonstrated that the male pelvic phantom is a good representation of actual prostate cancer patients and can be a valuable tool for image-guided radiotherapy. Keywords: image-guided radiotherapy, X-ray imaging, anthropomorphic phantomPACS numbers: 87.56.Fc, 87.59.bd, 87.85.Lf

MO‐FF‐A1‐04: Toward a True Real‐Time in Vivo Dosimetry System Using Plastic Scintillators

Purpose: Plastic scintillation detectors (PSDs) have shown great promise for offline applications such as IMRT QA. Because of their fast response and high sensitivity they could also be used as in vivo detectors. In this work we present and validate a PSD system designed for multi‐probe in vivo measurements with an electron‐multiplying CCD for real‐time photodetection. Method and Materials: The detectors were built with a dose sensitive volume of 0.4 cubic millimeters. Individual PSDs were assembled in modular detector patches each containing 5 closely packed detector elements. Continuous dose readings were performed every 150 ms (1.5 cGy) with a dead time of less than 0.3 ms between consecutive readings. We first studied the signal‐to‐noise ratio for different electron multiplication gain factors. We then analyzed the precision and accuracy of the detectors in acrylic and anthropomorphic pelvic phantoms. Results: The PSDs were found to be compatible with two clinical models of rectal balloons and could easily be inserted into the anthropomorphic phantom. A twofold increase in signal‐to‐noise ratio was seen by setting the electron‐multiplication gain factor to 40 therefore making near real‐time dosimetry feasible. Under calibration conditions, the PSDs agreed with ion chamber measurements to 0.08%. Precision was a function of the total dose delivered, ranging from 2% at 2 cGy to 0.4% at 200 cGy. Conclusion: We have shown PSD read in 150 ms with high accuracy and precision. We have also shown that these detectors can be mounted on different types of rectal balloons therefore transforming these clinical devices in dose detectors without modifying current clinical standard of practice and care. Real‐time monitoring of the dose delivered close to the rectum during prostate treatments will be invaluable to protect this sensitive normal structure while keeping small margins around the prostate.Supported by the NCI (1R01CA120198‐01A2)

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‐T‐370: Evaluation of An Anthropomorphic Pelvis Phantom for Proton Therapy

Purpose: To design and implement an anthropomorphic pelvis phantom to audit proton therapy treatment procedures. Method and Materials: A pelvis phantom already in use for independent audits of photon IMRT treatments was retrofitted for use with protons. The relative stopping power of each material used to construct the phantom was measured. Hounsfield Units were determined for each material with a clinical CT scanner. The tissue equivalence of the materials was determined by comparing with the CT calibration curve used clinically for human tissues. A CT simulation of the phantom was then performed and a proton treatment plan was devised. TLD and radiochromic film were inserted in the phantom and the treatment plan was delivered. The measurements from the TLD and film were compared to calculations made with the treatment planning system. Profile plots through the coronal and sagittal planes were compared to confirm agreement between the treatment plan and delivery. Results: Measured relative stopping powers differed by as much as 10% from values used by the planning system. The comparison between the plan and the TLD showed a difference in dose of less than 3%. The film showed a 2 mm shift in the anterior‐posterior profile and a 6 mm shift in the superior‐inferior profile. The width of the delivered high‐dose region shown by the right‐left film profile differed by as much as 8 mm compared to the plan profile. Conclusion: Preliminary results show the phantom will be able to confirm agreement between measured and calculated dose within 5%/3mm. Stopping power differences between tissue and phantom materials might account for discrepancies between the treatment plan and delivery in the left‐right direction, as lateral fields were used. Conflict of Interest: Work supported by PHS CA010953 and CA081647, awarded by NCI, DHHS, and funds from the RTOG.

An anthropomorphic pelvis phantom for optimization of the diagnosis of lymph node metastases in the pelvis

An anthropomorphic pelvis phantom was made by the modification of a National Electrical Manufacturers Association (NEMA) phantom, currently the most popular one, and its clinical usefulness was evaluated. The NEMA plus bone phantom was made by placing the pelvic bone model in the NEMA phantom. The NEMA plus bladder phantom was made by placing an imitation of the urinary bladder in the NEMA phantom. The pelvis phantom was also made by placing both the pelvic bone model and the bladder in the NEMA phantom. Four kinds of phantoms were imaged by both 2D and 3D dynamic modes, and for each phantom, prompt coincidence count rates, random ones, true plus scatter ones, and single photon rates were measured and these coincidence count rates were compared with those from the actual clinical data. After image reconstruction, the contrast ratio and image noise were also investigated. For the random coincidence count rate, the data obtained from each phantom showed good correspondence to the clinical data. The prompt coincidence count rates and true plus scatter ones of the clinical data were different from those obtained from NEMA phantom, NEMA plus bone one and NEMA plus bladder one, whereas there was a good correspondence between the data of the pelvis phantom and the clinical data. For the contrast ratio and image noise, there were discrepancies between the data of NEMA phantom and pelvis phantoms. We made an anthropomorphic pelvis phantom by the simple modification of a NEMA phantom. This phantom showed performance similar to that of the actual human pelvis, suggesting clinical usefulness in the evaluation of new acquisition protocols and reconstruction algorithms.