Anthropomorphic Lung Phantom Research Articles

A High-Resolution Imaging Technique using a Whole-body, Research Photon Counting Detector CT System.

A high-resolution (HR) data collection mode has been introduced to the whole-body, research photon-counting-detector CT system installed in our laboratory. In this mode, 64 rows of 0.45 mm × 0.45 mm detectors pixels were used, which corresponded to a pixel size of 0.225 mm × 0.225 mm at the iso-center. Spatial resolution of this HR mode was quantified by measuring the MTF from a scan of a 50 micron wire phantom. An anthropomorphic lung phantom, cadaveric swine lung, temporal bone and heart specimens were scanned using the HR mode, and image quality was subjectively assessed by two experienced radiologists. Comparison of the HR mode images against their energy integrating system (EID) equivalents using comb filters was also performed. High spatial resolution of the HR mode was evidenced by the MTF measurement, with 15 lp/cm and 20 lp/cm at 10% and 2% MTF. Images from anthropomorphic phantom and cadaveric specimens showed clear delineation of small structures, such as lung vessels, lung nodules, temporal bone structures, and coronary arteries. Temporal bone images showed critical anatomy (i.e. stapes superstructure) that was clearly visible in the PCD system but hardly visible with the EID system. These results demonstrated the potential application of this imaging mode in lung, temporal bone, and vascular imaging. Other clinical applications that require high spatial resolution, such as musculoskeletal imaging, may also benefit from this high resolution mode.

Low-dose lung CT processing using weighted intensity averaging over large-scale neighborhoods.

The aim of the proposed work is to improve low-dose lung CT (LDCT) screening using the processing of weighted intensity averaging over large-scale neighborhoods (WIA-LN). Both current and voltage reductions were considered for LDCT imaging. In the WIA-LN method, the processed pixel intensities are calculated by weighted averaging intensities among a large neighboring region. The weights are determined by the inter-similarity of the surrounding textures. A compute unified device architecture based parallelization was applied to accelerate the implementation. To evaluate the effectiveness of the proposed processing, low-dose lung CT images were obtained under both 75% reduced tube current and 33.3% reduced tube voltage condition respectively from a 16 detector rows Siemens CT. The standard routine standard-dose CT images were also collected as the reference images. In addition to clinical data from patients, an anthropomorphic lung phantom was also used in the study. Visual comparison and statistical qualitative analysis of image quality scores on the datasets are made in validation. Compared to the original LDCT images, improved visual and qualitative performance can be observed for the processed images. Statistically significant improvement of noise/artifacts suppression and nodule structure enhancement are achieved by using the proposed method (P<0.05). The proposed method is capable of providing LDCT images under significantly reduced tube current and voltage settings in low-dose condition. Quality of the processed images was assessed by radiology specialists. Parallelization based algorithm optimization was also performed to increase the clinical applicability of the proposed processing.

Stationary chest tomosynthesis using a carbon nanotube x-ray source array: a feasibility study

Chest tomosynthesis is a low-dose, quasi-3D imaging modality that has been demonstrated to improve the detection sensitivity for small lung nodules, compared to 2D chest radiography. The purpose of this study is to investigate the feasibility and system requirements of stationary chest tomosynthesis (s-DCT) using a spatially distributed carbon nanotube (CNT) x-ray source array, where the projection images are collected by electronically activating individual x-ray focal spots in the source array without mechanical motion of the x-ray source, detector, or the patient. A bench-top system was constructed using an existing CNT field emission source array and a flat panel detector. The tube output, beam quality, focal spot size, system in-plane and in-depth resolution were characterized. Tomosynthesis slices of an anthropomorphic chest phantom were reconstructed for image quality assessment. All 75 CNT sources in the source array were shown to operate reliably at 80 kVp and 5 mA tube current. Source-to-source consistency in the tube current and focal spot size was observed. The incident air kerma reading per mAs was measured as 74.47 uGy mAs−1 at 100 cm. The first half value layer of the beam was 3 mm aluminum. An average focal spot size of 2.5 × 0.5 mm was measured. The system MTF was measured to be 1.7 cycles mm−1 along the scanning direction, and 3.4 cycles mm−1 perpendicular to the scanning direction. As the angular coverage of 11.6°–34°, the full width at half maximum of the artifact spread function improved greatly from 9.5 to 5.2 mm. The reconstructed tomosynthesis slices clearly show airways and pulmonary vascular structures in the anthropomorphic lung phantom. The results show the CNT source array is capable of generating sufficient dose for chest tomosynthesis imaging. The results obtained so far suggest an s-DCT using a distributed CNT x-ray source array is feasible.

Automated marker tracking using noisy X-ray images degraded by the treatment beam

This study demonstrates the feasibility of automated marker tracking for the real-time detection of intrafractional target motion using noisy kilovoltage (kV) X-ray images degraded by the megavoltage (MV) treatment beam. The authors previously introduced the in-line imaging geometry, in which the flat-panel detector (FPD) is mounted directly underneath the treatment head of the linear accelerator. They found that the 121 kVp image quality was severely compromised by the 6 MV beam passing through the FPD at the same time. Specific MV-induced artefacts present a considerable challenge for automated marker detection algorithms. For this study, the authors developed a new imaging geometry by re-positioning the FPD and the X-ray tube. This improved the contrast-to-noise-ratio between 40% and 72% at the 1.2 mAs/image exposure setting. The increase in image quality clearly facilitates the quick and stable detection of motion with the aid of a template matching algorithm. The setup was tested with an anthropomorphic lung phantom (including an artificial lung tumour). In the tumour one or three Calypso® beacons were embedded to achieve better contrast during MV radiation. For a single beacon, image acquisition and automated marker detection typically took around 76±6 ms. The success rate was found to be highly dependent on imaging dose and gantry angle. To eliminate possible false detections, the authors implemented a training phase prior to treatment beam irradiation and also introduced speed limits for motion between subsequent images.

Poster - Thur Eve - 76: Dosimetric Comparison of Pinnacle and iPlan Algorithms with an Anthropomorphic Lung Phantom

Our goal is to compare the dosimetric accuracy of the Pinnacle-3 9.2 Collapsed Cone Convolution Superposition (CCCS) and the iPlan 4.1 Monte Carlo (MC) and Pencil Beam (PB) algorithms in an anthropomorphic lung phantom using measurement as the gold standard. Ion chamber measurements were taken for 6, 10, and 18 MV beams in a CIRS E2E SBRT Anthropomorphic Lung Phantom, which mimics lung, spine, ribs, and tissue. The plan implemented six beams with a 5×5 cm2 field size, delivering a total dose of 48 Gy. Data from the planning systems were computed at the treatment isocenter in the left lung, and two off-axis points, the spinal cord and the right lung. The measurements were taken using a pinpoint chamber. The best results between data from the algorithms and our measurements occur at the treatment isocenter. For the 6, 10, and 18 MV beams, iPlan 4.1 MC software performs the best with 0.3%, 0.2%, and 4.2% absolute percent difference from measurement, respectively. Differences between our measurements and algorithm data are much greater for the off-axis points. The best agreement seen for the right lung and spinal cord is 11.4% absolute percent difference with 6 MV iPlan 4.1 PB and 18 MV iPlan 4.1 MC, respectively. As energy increases absolute percent difference from measured data increases up to 54.8% for the 18 MV CCCS algorithm. This study suggests that iPlan 4.1 MC computes peripheral dose and target dose in the lung more accurately than the iPlan 4.1 PB and Pinnicale CCCS algorithms.

SU‐E‐CAMPUS‐J‐01: TG142 Complied Comprehensive Commissioning and Quality Assurance Procedure for Respiratory Gating

Purpose:To develop and establish a comprehensive gating commissioning and quality assurance procedure in compliance with TG142.Methods:Quality assurance tests on three Varian LINACs included beam output and energy constancy, calibration of surrogate, as well as phase and amplitude gating temporal accuracy. A diode array (MapCHECK 2) and film (Gafchromic EBT2) were used to measure the temporal accuracy of phase and amplitude gating windows. A motion simulation device (MotionSim) was used to simulate respiratory motion for both detectors. An end‐to‐end test was also performed on all three machines. The overall accuracy and uncertainty was analyzed and compared.Results:The end‐to‐end test using an anthropomorphic lung phantom (CIRS) results in an OSL dose difference reading within 5% (within measurement uncertainty) for both phase and amplitude gated treatment. Film results showed &lt; 1% agreement between profiles for gated delivery and predicted dose. The diode array demonstrated an 80% passing rate for gamma criteria of 2%/0.2 mm, which results in a 111 msec temporal accuracy. However, the diode array is limited by its spatial resolution of measurements, due to its 7.07 mm diode spacing. Film provided higher measuring resolution, thus demonstrated a temporal accuracy of &lt;100 msec.Conclusion:Results showed consistent respiratory gating stability and accuracy. MapCHECK 2 may not be sufficient for the temporal accuracy test in the respiratory gating treatment in order to meet the corresponding tolerance in TG142. One would need to decrease respiratory motion speed from the surrogate or tighten the gating window in order to be within tolerance of 100 msec temporal accuracy per TG‐142. The end‐to‐end test offers insight to the overall accuracy and uncertainties with a gated protocol. Compared to static delivery, respiratory motion increases the overall uncertainty of treatment delivery from 3% to 5% dose difference.

SU‐E‐J‐81: Interplay Effect in Non‐Gated Dynamic Treatment Delivery of a Lung Phantom with Simulated Respiratory Motion

Purpose:To quantify the interplay effect in non‐gated VMAT external beam delivery using realistic, clinically relevant 3D motion in an anthropomorphic lung phantom, and to determine if adding margins is sufficient to account for motion or if gating is required in all cases.Methods:A 4D motion stage was used to move a Virtual Water (VW) lung target containing a piece of radiochromic EBT3 film in an anthropomorphic chest phantom. A five‐arc stereotactic body radiation therapy (SBRT) treatment was planned using a CT scan of the phantom in its stationary position, using planning parameters chosen to push the optimizer to achieve a highly‐modulated plan. Two scenarios were delivered using a Varian TrueBeam: the first was delivered with the phantom and target both stationary and the second was delivered with the phantom stationary but the target moving in a realistic, irregular 3D elliptical pattern. A single piece of 4×4 cm2 film was used per fraction, located in the central coronal plane of the target. Film was calibrated on a 6 MV beam with dose values from 0.20 to 20 Gy.Results:Preliminary test films were analyzed in ImageJ and MatLab software. Dose maps were calculated on a central region of interest (ROI) delineated on both the motion‐induced and stationary films. Both static and dynamic film dose maps agreed with planning values within acceptable uncertainty.Conclusion:Including a large number of arcs in a clinically realistic SBRT treatment could reduce the effect of motion interplay due to averaging. Because all clinics do not employ multiple arcs for SBRT lung treatments, it is still important to consider the effects of motion on treatment delivery. Further analysis on the treatment films, as well as a broader investigation other planning parameters, will be conducted.

Improved visualization of perfusion defects by respiratory-gated SPECT

Single-photon emission computed tomography ventilation/perfusion (SPECT V/Q) imaging is recommended both by the Society of Nuclear Medicine and by the European Association of Nuclear Medicine for the diagnosis of pulmonary embolism. However, respiratory motion produces image blurring and degradation of detail in the lungs. We have investigated respiratory gating of SPECT images, correcting for motion to reduce blur and improve image definition. Wedge-shaped defects of different sizes ranging from 15 to 4 mm were fixed in the lung cavities of an anthropomorphic lung phantom to simulate perfusion defects. Gated and nongated SPECT images were obtained using a double-headed SPECT system. Three-dimensional movement was introduced using a purpose-built moving platform with two motion frequencies of 10 and 20 cycles/min. Motion was tracked with a respiratory-gating system. Gated SPECT data were acquired in 16 discrete data bins in synchronization with the breathing cycle. The images were reconstructed using ordered-subset expectation maximization algorithms and corrected for rigid motion. Contrast and contrast-to-noise ratios (CNRs) were measured to quantify any improvement in the gated motion-corrected images. Visualization of defects in the reconstructed images was performed by seven observers and analyzed using alternative free-response receiver operating characteristic analysis. Assessment of gated and nongated SPECT phantom images demonstrated that motion adversely affected the detectability of defects. Quantification of data demonstrated that, in the controlled simulation, image quality, defect definition, observer confidence, contrast, and CNR were increased after applying motion correction. Improvement in CNRs was found to be significant using alternative free-response receiver operating characteristic analysis (P=0.0002). Respiratory-gated motion-corrected SPECT images enhanced the visualization of defects compared with matched moving/nongated images in a realistic moving phantom. This approach may be particularly valuable for SPECT V/Q imaging and may improve the diagnosis of pulmonary embolism.

Lung cancer screening with CT: Evaluation of radiologists and different computer assisted detection software (CAD) as first and second readers for lung nodule detection at different dose levels

ObjectivesTo find the best pairing of first and second reader at highest sensitivity for detecting lung nodules with CT at various dose levels. Materials and methodsAn anthropomorphic lung phantom and artificial lung nodules were used to simulate screening CT-examination at standard dose (100mAs, 120kVp) and 8 different low dose levels, using 120, 100 and 80kVp combined with 100, 50 and 25mAs. At each dose level 40 phantoms were randomly filled with 75 solid and 25 ground glass nodules (5–12mm). Two radiologists and 3 different computer aided detection softwares (CAD) were paired to find the highest sensitivity. ResultsSensitivities at standard dose were 92%, 90%, 84%, 79% and 73% for reader 1, 2, CAD1, CAD2, CAD3, respectively. Combined sensitivity for human readers 1 and 2 improved to 97%, (p1=0.063, p2=0.016). Highest sensitivities – between 97% and 99.0% – were achieved by combining any radiologist with any CAD at any dose level. Combining any two CADs, sensitivities between 85% and 88% were significantly lower than for radiologists combined with CAD (p<0.03). ConclusionsCombination of a human observer with any of the tested CAD systems provide optimal sensitivity for lung nodule detection even at reduced dose at 25mAs/80kVp.

Actively triggered 4d cone‐beam CT acquisition

4d cone-beam computed tomography (CBCT) scans are usually reconstructed by extracting the motion information from the 2d projections or an external surrogate signal, and binning the individual projections into multiple respiratory phases. In this "after-the-fact" binning approach, however, projections are unevenly distributed over respiratory phases resulting in inefficient utilization of imaging dose. To avoid excess dose in certain respiratory phases, and poor image quality due to a lack of projections in others, the authors have developed a novel 4d CBCT acquisition framework which actively triggers 2d projections based on the forward-predicted position of the tumor. The forward-prediction of the tumor position was independently established using either (i) an electromagnetic (EM) tracking system based on implanted EM-transponders which act as a surrogate for the tumor position, or (ii) an external motion sensor measuring the chest-wall displacement and correlating this external motion to the phase-shifted diaphragm motion derived from the acquired images. In order to avoid EM-induced artifacts in the imaging detector, the authors devised a simple but effective "Faraday" shielding cage. The authors demonstrated the feasibility of their acquisition strategy by scanning an anthropomorphic lung phantom moving on 1d or 2d sinusoidal trajectories. With both tumor position devices, the authors were able to acquire 4d CBCTs free of motion blurring. For scans based on the EM tracking system, reconstruction artifacts stemming from the presence of the EM-array and the EM-transponders were greatly reduced using newly developed correction algorithms. By tuning the imaging frequency independently for each respiratory phase prior to acquisition, it was possible to harmonize the number of projections over respiratory phases. Depending on the breathing period (3.5 or 5 s) and the gantry rotation time (4 or 5 min), between ∼90 and 145 projections were acquired per respiratory phase resulting in a dose of ∼1.7-2.6 mGy per respiratory phase. Further dose savings and decreases in the scanning time are possible by acquiring only a subset of all respiratory phases, for example, peak-exhale and peak-inhale only scans. This study is the first experimental demonstration of a new 4d CBCT acquisition paradigm in which imaging dose is efficiently utilized by actively triggering only those projections that are desired for the reconstruction process.

SU-E-P-21: To Investigate CT and KV CBCT Image Deformable Contour Registration Method Using Lung Phantom

Purpose: To analyze CT and KV CBCT image deformable contour registration method and explore calibration CBCT image quality method to achive the goal of Adaptive Radiotherapy(ART) using lung phantom Methods: Acquiring CT, CBCT and modified CBCT (mCBCT) images(calibtated by electron density and image quality) of the anthropomorphic lung phantom. The body contour, simulated targets volume (a sphere with diameter 3.0 centimeters located on left lung, mediastinum and right lung) and simulated organ at risks(OARs) on CT image were deformed to CBCT and mCBCT images with Mimivista software. We achieved the auto deformable conntours and compared with manual contours. We anlyzed the DSC(dice similarity coefficient), PPV(positive predictive value) and SENS(sensitivity) of the body, targets and OARs for three images. Results: Compared to auto contours on CBCT, auto contours on mCBCT had a higher value of 0.7% Dice index and a higher value of 2.51% PPV for body; a higher DSC of 32.50%,a higher PPV of 21.30% and a higher of SENS 30.39% for simulation PTV;a higher DSC of 17.35% and a higher SENS of 32.82% for heart. There were no difference for other parameters of simulation organs. Conclusion: KV CBCT images callibtated by electron density and image quality with the anthropomorphic lung phantom have a good deformable registration contours compared to CBCT auto contours without calibration.

SU‐E‐CAMPUS‐T‐01: Results of Irradiations Performed On Radiological Physics Center's Anthropomorphic Lung Phantom and Respiratory Simulating Motion Table

Purpose: To present the results of the irradiations performed on Radiological Physics Center's anthropomorphic lung phantom using a respiratory simulating motion table. Methods: The RPC's lung phantom is used to credential institutions to participate in NCI sponsored lung clinical trials. When institutions use breath hold, gating, ITV or tracking techniques to account for patient motion, a motion table was included with the phantom. The institutions designed and delivered 3D or IMRT plans that covered an ovoid shaped target located in the left lung. All irradiations in this analysis included heterogeneity dose calculation algorithms approved by the RTOG. The TLD criteria were ±5% about RPC/Inst=0.97 (about 1.00 for Monte Carlo algorithms). Before March 2012 the film criteria required a DTA &lt;=5 mm in the fall off regions of orthogonal profiles taken through the center of the target. After March 2012 the film criteria changed to require &gt;=80% of the pixels in the ROI in each plane to pass a 5%/5mm gamma calculation. The average of the 3 planes must be &gt;=85%. Results: The lung phantom was irradiated 427 times by 304 institutions. 124 institutions irradiated it 157 times using the motion platform. 79% of the irradiations utilizing the motion platform and 87% of the static irradiations passed the criteria. The overall pass rate for the phantom is 84%. Thirty percent of the moving phantom failures and 67% of the static phantom failures were due to failing solely the film criteria. 4 of 4 (100%) breath hold irradiations, 54 of 68 (79%) gating irradiations, 43 of 61 (70%) ITV irradiations and 23 of the 24 (96%) tracking irradiations passed the criteria. Conclusion: The moving phantom has a pass rate that is statistically significantly different than that for a static phantom. Supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

Influence of smoothing algorithms in Monte Carlo dose calculations of cyberknife treatment plans: A lung phantom study

The Monte Carlo dose calculation algorithm yields accurate dose distributions in heterogeneous media and interfaces. The Monte Carlo calculation algorithm provided in the Multiplan Cyberknife treatment planning system (Accuray, Sunnyvale, CA, USA) has five different dose-smoothing algorithms in it. As the principle of smoothing of these algorithms is different, they can produce a disparity in the final dose distribution. The aim of the present study is to analyze the influence of these Monte Carlo smoothing algorithms in the final dose distribution of cyberknife treatment plans. An anthropomorphic lung phantom with a tumor mimicking ball target was taken for this study. The basic optimization was performed with the Ray tracing algorithm. The Monte Carlo calculations were introduced with each smoothing algorithm on the basic plan and the plans were compared. The Monte Carlo doses were found to be lesser than the Ray tracing doses. The dose conformity index was above 4 for all the smoothing algorithms, while it was only 1.19 for Ray tracing. The least coverage of 6.34 was obtained for a weighted average algorithm. The deviation between the V100% values of different smoothing algorithms was higher than the deviation in V80%. The deviations between the smoothing algorithms are higher in the high-dose regions, including the prescribing isodose, than the low-dose regions of the target, as well as in the organs at risk (OAR).

TU‐G‐BRB‐08: 3D Proton Dose Distributions in an Anthropomorphic Lung Phantom, Comparing Monte Carlo, Film, and TLD Measurements

Purpose: Evaluate and determine the feasibility of using thoracic phantoms in the credentialing of proton therapy facilities, by using irradiations, treatment planning calculations and Monte Carlo simulations.Methods: The Radiological Physics Center (RPC, Houston, TX) has developed an anthropomorphic lung phantom to be used for credentialing of lung trials in proton therapy. The phantom has structures like heart, spinal cord, right and left lungs and a cylindrical target located in the middle of the left lung. EBT2 films were placed cutting the target on the axial, coronal and sagittal planes for relative dosimetry. The absolute dose was measured with LiF TLDs located in the center of the target. The proton stopping power ratio of each material used in the phantom as well the respective CT number was also evaluated for this work. Based on a CT image of the phantom we created a plan in ECLIPSE with two fields delivering 6 Gy RBE to the target. A customized table was used to translate the phantom CT numbers into stopping power ratio for the ECLIPSE plan. The treatment plan was also simulated in Monte Carlo using a customized version of MCNPX. The phantom was irradiated at University of Texas MD Anderson Proton Therapy Center, Houston, Texas. Profiles and the Gamma tool were used to compare the film measurements to ECLIPSE and MCNPX calculations. Results: The gamma agreement of the Monte Carlo and ECLIPSE calculations was above 85% in the GTV region for all planes. The profiles of the Monte Carlo showed a better agreement with film measurements than ECLIPSE calculations. We also identified LET dependence on film measurements. Conclusion: We concluded that this method can give accurate results for passive scattering proton beams for the GTV areas. We are conducting further studies to correct for the film LET response.This project is supported in part by P01CA021239 from the National Cancer Institute.

SU-GG-T-442: Dose Verification of SRS Monte Carlo Plan with a Moving Anthropomorphic Phantom

Purpose: Dose verification of Stereotactic Radiosurgery plan using Monte Carlo for tissue heterogeneity correction with a moving anthropomorphic phantom Method and Materials: An anthropomorphic lung phantom has gold fiducials, TLD capsules and GAFChromic films imbedded in a target in the left lung, TLD capsules are also inserted in heart and cord structures. The phantom was scanned, CT images were sent to Cyberknife Multiplan for planning. Target and critical structures were contoured. A SRS treatment plan with no tissue heterogeneity correction was generated to give 6Gy to 95% of the target, 5.4Gy to 99% of the target, no point > 2cm from target > 3.5Gy, conformai index < 1.2, cord < 1.8Gy, heart < 3Gy, 10% of whole lung < 2Gy. The plan was recalculated with Monte Carlo with 2% uncertainty for tissue heterogeneity correction for dose delivery. During delivery the phantom moved in superior/inferior and anterior/posterior direction, Cyberknife Synchrony tracking system took orthogonal x-ray images of the phantom while infra-red camera tracked LED diodes placed on the phantom to track phantom motion in real time. After the fiducials were identified by the tracking program and correlated with the motion of LED diodes, a motion model of the target was built; radiation started with the Cyberknife robot following the predicted target motion and adjusted its position during irradiation. Throughout delivery, the motion model was updated and adjusted with new x-ray images of the fiducials. Results: Target TLD was 98.3% of Monte Carlo dose; heart and cord TLD were within 7% acceptability. Profiles from 3 orthogonal films were displaced L/R 0mm/3mm, P/A 1mm/5mm and I/S 1mm/2mm from treatment plan profiles, within 5mm acceptability. Conclusion: Cyberknife Multiplan with Monte Carlo accurately predicts dose in heterogeneous tissue; Cyberknife Synchrony tracking system delivers dose accurately to a target in a moving phantom.

SU‐GG‐T‐552: Commissioning an Anthropomorphic Spine and Lung Phantom for Remote Quality Assurance of Spinal Radiosurgery

Purpose: The RPC has developed a new phantom to ensure comparable and consistent radiation administration in spinal radiosurgery clinical trials. This study assesses the phantom's dosimetric utility. Methods: The ‘spine phantom’ is a water filled thorax with pertinent anatomy encountered in spinal radiosurgery: tumor, vertebral column, spinal canal, esophagus, heart, and lungs. The dose to the tumor volume is measured with axial and sagittal planes of radiochromic film and thermoluminescent dosimeters (TLD). The relative dose distribution is measured with the radiochromic film, and is calibrated to the absolute dose measured by the TLD. The dosimetric utility was assessed by administering a series of irradiations to the phantom. Three irradiations were administered: a four angle box plan, a seven angle conformal plan, and a nine angle IMRS plan. In each plan, at least 95% of the defined tumor volume received 8 Gy. The first two unmodulated plans were not clinically applicable; rather, they were used for dosimetric verification without intensity modulation. The third plan was tuned to the clinical specifications for organ tolerances delineated by the RTOG protocol 0631. The planned and administered doses were then registered via pinpricks, and compared using point dose measurements, dose profiles, isodose distributions, and gamma analyses. Results: The minimum number of pixels passing the gamma analyses was set at 95% at 5% dose difference and 3 mm distance to agreement, based on previous experience at the RPC. Each irradiation showed acceptable conformity in the qualitative assessments and exceeded the 95% passing rate at the 5% / 3mm criteria. Conclusions: Treatment planning and measurement in the new spinal phantom demonstrated acceptable conformality at the 5% / 3mm criteria, and the spine phantom should be a useful tool for the remote assessment of other institution's treatment planning and dose delivery regimen.

Dosimetric validation of a commercial Monte Carlo based IMRT planning system

Recently a commercial Monte Carlo based IMRT planning system (Monaco version 1.0.0) was released. In this study the dosimetric accuracy of this new planning system was validated. Absolute dose profiles, depth dose curves, and output factors calculated by Monaco were compared with measurements in a water phantom. Different static on-axis and off-axis fields were tested at various source-skin distances for 6, 10, and 18 MV photon beams. Four clinical IMRT plans were evaluated in a water phantom using a linear diode detector array and another six IMRT plans for different tumor sites in solid water using a 2D detector array. In order to evaluate the accuracy of the dose engine near tissue inhomogeneities absolute dose distributions were measured with Gafchromic EBT film in an inhomogeneous slab phantom. For an end-to-end test a four-field IMRT plan was applied to an anthropomorphic lung phantom with a simulated tumor peripherally located in the right lung. Gafchromic EBT film, placed in and around the tumor area, was used to evaluate the dose distribution. Generally, the measured and the calculated dose distributions agreed within 2% dose difference or 2 mm distance-to-agreement. But mainly at interfaces with bone, some larger dose differences could be observed. Based on the results of this study, the authors concluded that the dosimetric accuracy of Monaco is adequate for clinical introduction.

SU-FF-T-568: Clinical Implementation and Evaluation of Monte Carlo Treatment Planning for CyberKnife

Purpose: To evaluate the clinical implication for dose prescriptions, of adopting Monte Carlo treatment planning for the CyberKnife Radiosurgery System, and to directly compare Monte Carlo dose calculations to those made using the Ray-Tracing algorithm. Method and Materials: The anthropomorphic lung phantom from the Radiological Physics Center (RPC) was used to validate the accuracy of the CyberKnife Monte Carlo dose calculation algorithm (MultiPlan 2.1.0). A retrospective comparison of dose distributions calculated by the Ray-Tracing and Monte Carlo algorithms was made for a selection of CyberKnife patient treatment plans. In each case, a clinically acceptable treatment plan was developed using the Ray-Tracing algorithm. The beam sets for each Ray-Tracing plan were then recalculated using the Monte Carlo algorithm (Gaussian smoothing, 1%); beam orientations and monitor units for each respective pair of calculations were identical, only the resulting dose distributions changed. Results: For highly heterogeneous cases, such as those involving the lungs, the Ray-Tracing algorithm consistently overestimated the target dose. For each lung plan comparison, the isodose prescription line needed to be lowered for the Monte Carlo-calculated plans to achieve similar target coverage. The average isodose prescription line for the Ray-Tracing Plans was 82.5% compared to 74.9% for the Monte Carlo plans. In certain cases, this isodose reduction may result in unacceptably high maximum target doses and sub par conformality. Conclusion: There can be significant differences between dose distributions calculated by Monte Carlo and Ray-Tracing algorithms, particularly for treatment plans involving the lungs. As a result, isodose prescription lines and subsequent target coverage typically selected for treatment plans calculated with the Ray-Tracing algorithm may be different from comparable treatment plans calculated with the Monte Carlo algorithm. These differences may need to be evaluated on a patient-by-patient basis until a widespread consensus can be reached.

SU-FF-T-554: Commissioning a New Anthropomorphic Spine and Lung Phantom to Be Used for Remote Dose Verification of Spine Radiosurgery

Purpose: The commissioning of a new anthropomorphic spine/lung phantom for use in credentialing institutions for participation in an RTOG protocol comparing fractionated IMRT to radiosurgery for spine tumors. Method and Materials: The spine/lung phantom was constructed to approximate the geometric distribution of anatomy encountered in spinal radiosurgery; these anatomical features include the spinal canal, vertebrae, esophagus, heart,lungs and tumor volume. The remainder of the phantom is water filled. These anatomical features were constructed from materials of different densities to allow for visualization with CT and for heterogeneous dose calculation verification. The tumor is loaded with TLD and gafchromic film in the axial and sagittal planes. To commission this system, planning and irradiation must demonstrate agreement of the measured dose distribution with the planned dose distribution to 5% within 3mm. First, two beams in an anterior‐posterior arrangement delivered 8 Gy to 90% of the tumor volume. More complex plans are being developed to be administered. Results: The anterior‐posterior beam arrangement showed reasonable agreement with the planned dose distribution. The axial and sagittal plans had 87% and 91% agreement, respectively (at the gamma level 5%/3mm). These results validated the tool utilized to register the treatment plan to the measured dose distribution. The results also confirmed the accuracy of the treatment isocenter. However, because of the simple beam arrangement, the prescribed critical structure tolerances could not be achieved. The critical structure tolerances can be achieved with the development of a clinically relevant IMRT plan. Conclusion: The phantom has been validated to register and calculate differences in planned and measured dose distributions with a gamma calculation. The simple anterior‐posterior beam arrangement showed good agreement between planning and measurement. Next, clinically relevant IMRT plans will be administered to the phantom. This work is supported by PHS CA010953 and CA081647, awarded by NCI, DHHS.

Characterization of a novel anthropomorphic plastinated lung phantom

Phantoms are widely used during the development of new imaging systems and algorithms. For development and optimization of new imaging systems such as tomosynthesis, where conventional image quality metrics may not be applicable, a realistic phantom that can be used across imaging systems is desirable. A novel anthropomorphic lung phantom was developed by plastination of an actual pig lung. The plastinated phantom is characterized and compared with reference to in vivo images of the same tissue prior to plastination using high resolution 3D CT. The phantom is stable over time and preserves the anatomical features and relative locations of the in vivo sample. The volumes for different tissue types in the phantom are comparable to the in vivo counterparts, and CT numbers for different tissue types fall within a clinically useful range. Based on the measured CT numbers, the phantom cardiac tissue experienced a 92% decrease in bulk density and the phantom pulmonary tissue experienced a 78% decrease in bulk density compared to their in vivo counterparts. By-products in the phantom from the room temperature vulcanizing silicone and plastination process are also identified. A second generation phantom, which eliminates most of the by-products, is presented. Such anthropomorphic phantoms can be used to evaluate a wide range of novel imaging systems.